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A
Rectal mucosa Rectum
Rectal circular muscle Terminal branches
(superior rectal veins)
Longitudinal muscle
Submucosal venous plexus
Levator ani
Ischio-anal fat
Puborectalis
Anal column
External anal sphincter
Anal valve
Conjoint longitudinal coat
Dentate line
Inferior rectal vein
Anal gland
Internal anal sphincter Perineal skin
Fibromuscular septum
Anal verge Intersphincteric groove
Lower anal mucosa
B Tip of coccyx
Terminal branches
(superior rectal veins)
Anococcygeal raphe
Puborectalis
Anorectal junction Attachment to anococcygeal
ligament
Longitudinal muscle
Inferior rectal vein
Attachment to transverse
perineal muscles Posterior external anal sphincter
Decussating fibres of upper
external anal sphincter
External anal sphincter
Fig. 66.44 A, A coronal section through the anal canal; glandular and vascular structures are shown unilaterally for clarity. B, A sagittal section through
the anal canal; the anal canal is angled posteriorly and so anterior sphincteric structures appear to be lower than posterior structures. The glandular,
vascular and fibromuscular structures have been omitted for clarity.
is lined by hairbearing keratinizing, stratified squamous epithelium, The welldefined muscularis mucosae of the rectum continues into
which is continuous with the perianal skin. Dilated veins in the sub the upper canal. Some fibres from the conjoint longitudinal muscle of
epithelial tissue below the dentate line contribute to an external haem the anal canal traverse the internal anal sphincter and turn cranially to
orrhoidal venous plexus (see Fig. 66.44); this can be seen on clinical merge with the muscularis mucosae, forming the submucosal muscle
examination as a circumferential ring at the anal margin if the subject of the anal canal (also called the musculus submucosae ani, or muscu
is asked to bear down. lus canalis ani, or Treitz’s muscle). Attachments of the longitudinal


The junction between the columnar epithelium above and the squa fibres to the submucosal muscle of the anal canal are particularly dense
mous epithelium below is referred to as the anal transition zone (ATZ). at the dentate line (the mucosal suspensory ligament (of Parks)),
It is variable in length and position but often extends about 1 cm causing the mucosa to be tethered more firmly at this level and approxi
proximal to the dentate line (Fenger 1987). Nerve endings, including mately marking the watershed between the portal and systemic venous
thermoreceptors, exist in the submucosa around the upper anal transi drainage of the haemorrhoidal plexuses. The attachments of fibres from
tion zone; they probably play a role in continence by providing a the longitudinal muscle layer, the muscularis mucosae of the anal canal,
highly specialized ‘sampling’ mechanism to identify the contents of the also create ‘spaces’ within the submucosa: a proximal submucous space,
lower rectum when the upper anal canal relaxes (Duthie and Gairns a marginal space extending from the dentate line to the intersphincteric
1960). groove, and a perianal space distally.


Anal canal
1155
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RETPAHC
Fig. 66.45 A, A mid-coronal MRI endocoil image of
the anal canal. B, An anterior coronal MRI endocoil
section in a woman showing the transverse perineii
(TP) joining the external anal sphincter anteriorly
(between arrows). Other abbreviations: EAS,
external anal sphincter; IAS, internal anal sphincter;
PR, puborectalis.
PR
PR
TP
IAS
EAS
A B
Internal anal sphincter
The internal anal sphincter is the specialized, white, thickened terminal
part of the inner circular muscle of the large intestine. It is thicker in
males and in patients with chronic constipation. The muscle fibre
arrangement is not truly circular, but rather a tight spiral that shortens
COIL and widens with relaxation. It commences at the anorectal junction and
ends above the anal verge; its lower border is palpable at the inter
PR sphincteric groove, which corresponds to the proximal limit of the
subcutaneous part of the external anal sphincter. It is traversed by
Lm
fibres passing medially from the conjoint longitudinal muscle into the
Ias submucosa.
Eas
At rest, it is tonically contracted but it relaxes as a consequence of
Tp reflex activity, predominantly during defecation. Transient relaxation of
the upper internal anal sphincter occurs in response to rectal distension
Cs Eas (the rectoanal inhibitory reflex) and postprandial rectal contractions
(the sampling reflex). Relaxation allows the passage of distal rectal
5 cm Bs contents into the upper anal canal, enabling a conscious or subcon
scious perception of their physical nature; this is accompanied by sus
tained contraction of the distal internal anal sphincter and contraction
of the external anal sphincter to maintain continence. The rectoanal
inhibitory reflex is primarily mediated by the enteric nervous system,
although spinal pathways may have a modulatory role. It is absent in
Fig. 66.46 An MRI endocoil mid-sagittal view of the anal canal in a man. patients with Hirschsprung’s disease.
Abbreviations: Bs, bulbospongiosus; Cs, corpus spongiosum; Eas,


external anal sphincter; Ias, internal anal sphincter; Lm, longitudinal Vascular supply and innervation
muscle; PR, puborectalis; Tp, transverse perineii. The internal anal sphincter is supplied by terminal branches of the
superior and inferior rectal vessels, and innervated extrinsically by auto
nomic nerves. Sympathetic fibres originate in the upper two lumbar
spinal segments, and parasympathetic fibres originate in the second to
fourth sacral spinal segments, both being distributed via the inferior
hypogastric plexus. Stimulation of cholinergic muscarinic receptors
(parasympathetic) causes internal anal sphincter relaxation and
longitudinal anal muscle contraction, while αadrenergic receptor stim
ulation (sympathetic) causes contraction of both the internal anal
sphincter and longitudinal muscle. Activation of nitrergic nerves also
mediates internal anal sphincter relaxation and is the rationale for using
topical nitroglycerine and other drugs that promote nitric oxide release
in the treatment of pathological conditions associated with increased
resting anal tone.
Conjoint longitudinal muscle
The conjoint longitudinal muscle of the anal canal is a direct continu
ation of the outer longitudinal smooth muscle of the rectum, descend
Fig. 66.47 The endoscopic appearance of the anorectal junction viewed ing between internal and external anal sphincters, and augmented in
internally from above. The endoscope is visible in the upper left corner of its upper part by striated muscle fibres from the medial aspect of levator
the image. ani (Lunniss and Phillips 1992). The muscle is particularly prominent
in the fetus, where it is actually thicker than the internal anal sphincter.
With advancing age, there is gradual replacement of muscle by connec
MUSCLES OF THE ANAL CANAL tive tissue, such that the layer becomes thin in the elderly and few
muscle fibres are seen in its distal part. As it passes down the anal canal,
The anal canal is surrounded by internal and external anal sphincters, muscle fibres peel off in three directions: internally, through the internal
separated by the conjoint longitudinal muscle layer, the whole arrange anal sphincter to reach the anal submucosa; inferiorly, through the


ment being referred to as the anal sphincter complex (Figs 66.48– striated muscle of the lower subcutaneous part of the external anal
66.49) (AlAli et al 2009). The anatomy of this region has been clarified sphincter to insert into the perianal skin (some of these fibres encircling
in recent years using endoanal sonography and MRI. the anal orifice); and outwards, through the upper part of the external


LARgE inTEsTinE
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nOiTCEs
S
las
Lm
las
PR
Eas
PR
A B C
UUrr
TTpp
Vag
PPPRRR llaass
SSccEEaass
SSppEEaass
E F
AAccll
D
Upper
Mid
Upper
Lower
Mid
Lower
G
Fig. 66.48 A–C, Axial views of the anal canal at three levels on endoanal ultrasound in a woman. The endoanal ultrasound probe is the central black
structure. A, The upper anal canal. The ‘U’ shape of puborectalis is visible. B, The middle anal canal. The external anal sphincter is now a complete
ring anteriorly (arrowhead). C, The lower anal canal. Below the termination of the internal anal sphincter, the longitudinal layer extends through the
subcutaneous external anal sphincter (between arrowheads). D–F, An MRI of the anal canal. D, At upper anal canal level, the sling of puborectalis
extends anteriorly to the pubic bones. E, At mid anal canal level, the transverse perineii fuse into the external anal sphincter anteriorly. The superficial
(middle) external anal sphincter is attached either side of the anococcygeal ligament. F, The low anal canal level, below the internal anal sphincter. G, A
key for levels of the anal canal. Abbreviations: Acl, anococcygeal ligament; Eas, external anal sphincter; Ias, internal anal sphincter; Lm, longitudinal
muscle; PR, puborectalis; S, subepithelial tissues; ScEas, subcutaneous (lower) part of the external anal sphincter; SpEas, superficial (middle) external
anal sphincter; Tp, transverse perineii; Ur, urethra; Vag, vagina.
anal sphincter (see Fig. 66.44). The lowermost fibres create a honey 1984) and is central to the development of haemorrhoids. Its exten
comb arrangement in the subcutaneous fat and separate a superficial sions provide pathways for the spread and containment of infection.


perianal space from the deeper ischioanal fossa. Additional ramifica
External anal sphincter
tions of this muscle beyond the external anal sphincter have been
described, emphasizing its central role in anorectal stability (Courtney The external anal sphincter forms the bulk of the anal sphincter
1950). complex. It is an oval tube of striated muscle composed mostly of
The conjoint longitudinal muscle is innervated by autonomic nerves type I slow twitch muscle fibres adapted for prolonged contraction. The
that share their origin with those supplying the internal anal sphincter length and thickness of the external anal sphincter is less in females
(see above). Its contraction during defecation shortens and widens the (Rociu et al 2000). The historical concept of the muscle having three
anal canal, and everts the anal orifice. Degeneration of the muscle and parts (deep, superficial and subcutaneous) is no longer valid, but upper
its gradual replacement by connective tissue occurs with age (Haas et al (deep) and lower (superficial or subcutaneous) parts are described by


Anal canal
1157
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RETPAHC
Bulbocavernosus
Bulbospongiosus
Levator ani
Conjoint
Superficial transverse
longitudinal perineal muscle
muscle
External anal sphincter
(lower part)
External anal
sphincter
(upper part)
Internal anal
sphincter
Conjoint
longitudinal Anal orifice
muscle
External anal
sphincter
*
* (lower part)
Fig. 66.50 A dissection of the male external anal sphincter in a fresh
cadaver, viewed from below. (Courtesy of Dr Robert P Myers.)
Fig. 66.49 A transilluminated 2.5 mm thick coronal section of a male
pelvis processed for epoxy resin E12 sheet plastination, showing the
entire length of the anal canal and its muscular layers: namely, the
external anal sphincter, conjoint longitudinal muscle and internal anal entered surgically to provide access in a variety of operations (e.g. inter
sphincter. Note the continuity of the external anal sphincter with levator sphincteric excision of the rectum and intersphincteric approaches to
ani superiorly, consisting of two parts: the upper part and the lower part. fistulae). Within the space lie the intersphincteric anal glands, the
The upper part of the conjoint longitudinal muscle is continuous with the source of most anal fistulae (Parks 1961). There is an average of twelve
longitudinal muscular layer of the rectum and, inferiorly, its fibres traverse intersphincteric anal glands within the adult anal canal, evenly distrib
the lower part of the external anal sphincter to blend with or form uted around the circumference. Their function is unknown, but they
fibro-fatty, honeycomb-like compartments around the perianal region secrete mucin (different in composition from that secreted by rectal
(asterisks). The internal anal sphincter ends above the lower part of the mucosa) and they communicate with the anal lumen via ducts (lined
external anal sphincter. (Courtesy of Dr Saad Al-Ali, with permission from by epithelium similar to that of the anal transition zone), which cross


Al-Ali S, Blyth P, Beatty S, Duang A, Parry B, Bissett IP. (2009)
the internal anal sphincter to open at the level of the anal valves imme
Correlation between gross anatomical topography, sectional sheet
diately above the dentate line (SeowChoen and Ho 1994). Retrograde
plastination, microscopic anatomy and endoanal sonography of the anal
passage of bacteria from the anal canal to the gland is understood
sphincter complex in human males. J. Anat. 215; 212–220.)
to cause infection; inflammatory occlusion of the duct prevents spon
taneous drainage back into the lumen of the anal canal. Sepsis may
then spread along a variety of routes to cause abscesses and fistulae
some authors. The upper part surrounds the internal anal sphincter
within various spaces (Parks et al 1976).
while the lowermost part encircles the anal canal inferior to the internal
anal sphincter (Fritsch et al 2002, AlAli et al 2009).
The upper part of the external anal sphincter is attached to the ano
VASCULAR SUPPLY AND LYMPHATIC DRAINAGE
coccygeal ligament posteriorly and to the perineal body anteriorly;
OF THE ANUS
some muscle fibres on each side of the sphincter decussate to form a
commissure in the anterior and posterior midline. The uppermost fibres
Arteries
blend with the lower medial fibres of puborectalis and attach to the
anococcygeal raphe posteriorly and the transverse perineal muscles The anal canal is supplied by terminal branches of the superior rectal
anteriorly. The lower part of the external anal sphincter extends below artery and the inferior rectal branch of the internal pudendal artery,
the internal anal sphincter and is traversed by the terminal fibres of the together with a small contribution from the median sacral artery. The
conjoint longitudinal muscle (see above). Anteriorly, it is attached to arterial supply to the epithelium of the lower anal canal in the


bulbospongiosus and bulbocavernosus (Fig. 66.50) (Shafik et al 2007). midline, particularly posteriorly, is relatively deficient (Klosterhalfen
Like the levator ani and internal anal sphincter, the external anal et al 1989); this is further diminished if the internal anal sphincter is
sphincter is tonically contracted at rest (the postural reflex). hypertonic (Schouten et al 1996). The epithelium is more firmly teth
ered to underlying structures in the midline, which may also be a
Vascular supply and innervation focal point of pressure in the anal canal. Collectively, these factors are
The external anal sphincter is supplied by the inferior rectal vessels, with thought to predispose to the occurrence of acute and chronic anal fis
a small contribution from the median sacral artery. It is innervated sures, which are most commonly found in the midline, especially
bilaterally by the inferior rectal branch of the pudendal nerve (contain posteriorly.
ing contributions from the second, third and fourth sacral spinal
Veins
nerves). The pudendal nerve also carries afferent fibres from the lining
of the anal canal and perianal skin. The upper external anal sphincter The venous drainage of the anal canal parallels the arterial supply. The
may also receive motor fibres from the nerve to levator ani (ventral rami upper canal is drained predominantly by the superior rectal veins, tribu
of predominantly the third and fourth sacral spinal nerves). taries of the inferior mesenteric vein and the portomesenteric venous
system; some blood returns to the internal iliac veins via the middle
Intersphincteric space and anal glands rectal veins. The lower part of the anal canal, including the external
The intersphincteric ‘space’ is a potential space between the conjoint haemorrhoidal venous plexus, drains via the inferior rectal branches of
longitudinal muscle layer and the external anal sphincter. It can be the internal pudendal vein into the internal iliac vein.


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Haemorrhoids and the external anal sphincter, which are tonically contracted at rest,
Haemorrhoids develop when the supporting connective tissues of the can be further contracted voluntarily to move stool back up from the
anal cushions degenerate, causing downward displacement of the cush distal rectum. If the defecatory urge is associated with a conscious
ions and abnormal venous dilation (Lohsiriwat 2012). They are associ decision to evacuate, distal progression of colonic highamplitude
ated with laxity of the anal canal submucosa and enlargement of the propagating complexes, rectal contractions, raised intraabdominal
terminal branches of the superior rectal artery supplying the anal pressure from voluntary straining (the efficiency of which is influ
cushions. enced by posture), relaxation of pubor ectalis (which straightens the
anorectal angle) and the anal sphincters all combine to enable defeca
Lymphatic drainage tion. Relaxation of the internal anal sphincter is a reflex response to
Lymph from the upper anal canal drains cranially into the submucosal rectal distension, whereas relaxation of the external anal sphincter is
and intramural lymphatics of the rectum. Lymphatics around puborec voluntary. During defecation, contraction of the conjoint longitudinal
talis drain to internal iliac lymph nodes. Lymph from the lower anal muscle shortens and opens the anal canal and flattens the anal cush
canal and external sphincter drains to inguinal lymph nodes. This ions. Both rectal and a variable quantity of colonic contents are evacu
pattern of drainage is particularly important when considering lym ated. After defecation, the external anal sphincter contracts (the
phatic spread of malignant tumours from the lower rectum and anal closing reflex), the internal anal sphincter gradually recovers its resting
canal. Blockage of lymph drainage along normal routes can lead to tone, and the postural reflex is reactivated. The conjoint longitudinal
unusual patterns of dissemination. muscle relaxes, permitting the anal canal to elongate and the anal
cushions to reexpand.
Problems with defecation may result from impaired colonic transit


DEFECATION or from a variety of functional or mechanical rectal disorders (Lunniss
et al 2009). Functional causes include inadequate or ineffective expul
Defecation is the act of voiding stool from the anus and involves the sive effort, paradoxical contraction of the pelvic floor and external anal
coordinated function of the colon and rectum, pelvic floor and anal sphincter (dyssynergia), and lack of normal sensation (rectal hyposen
sphincter. It is influenced by multiple factors, including colonic transit, sitivity). Mechanical causes include impedance to evacuation when
stool volume and consistency, posture, age, gender, psychology and straining (rectal intussusception, obstructing masses or anal stenosis),
behaviour, culture and lifestyle (Palit et al 2012). Social behaviour misdirection of force vectors during straining (rectocele), or dissipation
entrains defecation to occur at a time of convenience rather than of evacuatory forces (ballooning perineum).
simply in response to a defecatory urge. The urge to defecate is pre
ceded by highamplitude propagated contractions in the colon, which Anal continence and incontinence
result in the antegrade propulsion of intraluminal contents to the As long as anal pressure exceeds rectal pressure, faecal continence will
rectum; these contractions increase in frequency and intensity just be maintained. Anal resting tone is derived mainly from sympatheti
before expulsion. The response to rectal filling involves intact rectal cally mediated tonic contraction of the internal anal sphincter, aug
afferent nerves, reflex rectal contractions and normal rectal wall bio mented by contraction of the external anal sphincter. The anal cushions
mechanics. The rectum acts as a reservoir and is able to relax to contribute by providing an effective seal. Anal continence involves
accommodate faecal material and gas (adaptive relaxation). Periodic central, spinal and peripheral pathways, somatic, autonomic and enteric
sampling within the upper anal canal helps to determine the nature of nerves, intact sacral sensorimotor reflexes, and structurally and func
the rectal content. If the defecatory urge is inconvenient, puborectalis tionally intact smooth and striated musculature.
Intestinal glands
Columnar absorptive cell (crypts) Mucosa


Neuroendocrine cell
and adjacent capillary
Submucosa
Part of
muscularis
externa
Lymphoid follicle
Goblet cell
Fig. 66.51 The microstructure of the colonic wall and its epithelial cells. Note the aggregations of lymphocytes (blue) and undifferentiated epithelial cells
(white).


1159
66
RETPAHC
Key references
Sphincteric causes of faecal incontinence may be structural (disrup
MICROSTRUCTURE OF THE LARGE INTESTINE
tion or atrophy of part of the sphincter musculature) or neuropathic
(damage to the nerve supply to the sphincters), or a combination of
both. The most common causes of sphincter disruption are obstetric
injury, anal surgery (for haemorrhoids, fistula or fissure) and trauma. Available with the Gray’s Anatomy e-book
Pudendal neuropathy is most commonly associated with childbirth
(prolonged second stage) and chronic straining. The layers of tissue in the large intestinal wall (Fig. 66.51) resemble
Although the anus constitutes the final barrier to faecal inconti those in the small intestine (Ch. 65), except that villi and circular folds
nence, suprasphincteric factors are also essential in maintaining faecal are absent and the glands (crypts) are longer.
continence, particularly the rate at which the stool is delivered to the
rectum, rectal sensation and compliance, and adaptive relaxation. Thus,
even in the presence of structurally and functionally intact sphincter
mechanisms, faecal incontinence may occur: for instance, in diarrhoeal
states or conditions where there is loss of rectal reservoir function (Scott
Bonus e-book image
and Lunniss 2007). Conversely, chronic rectal distension with retained
faeces may result in passive (overflow) leakage, possibly as a result of a
chronically relaxed internal anal sphincter consequent on a persistently
activated rectoanal inhibitory reflex, together with blunted rectal sensa Fig. 66.29 A CT scan (coronal reformat) showing both right colonic
tion causing diminished conscious contraction of the external anal and sigmoid diverticulosis.
sphincter.
KEY REFERENCES
Brookes SJ, Dinning PG, Gladman MA 2009 Neuroanatomy and physiology Lunniss PJ, Gladman MA, Benninga MA et al 2009 Pathophysiology of
of colorectal function and defaecation: from basic science to human evacuation disorders. Neurogastroenterol Motil 21:31–40.


clinical studies. Neurogastroenterol Motil 21: 9–19. A review of the spectrum and classification of rectal evacuatory disorders.
A contemporary review of the neuroanatomy and physiology of colorectal
Palit S, Lunniss PJ, Scott SM 2012 The physiology of human defecation. Dig
motor function.
Dis Sci 57:1445–64.
Fenlon HM 2002 Virtual colonoscopy. Br J Surg 89:1–3. A detailed review of contemporary understanding of the normal physiology
A description and review of the use of multislice CT to image the colon. of defecation.
Fisher DF Jr, Fry WJ 1987 Collateral mesenteric circulation. Surg Gynecol Parks AG 1961 The pathogenesis and treatment of fistula in ano. Br Med J
Obstet 164:487–92. 1:463–9.
A review of collateral mesenteric circulations that develop during disease An early, full description of the relationship between the anatomy of anal
processes. glands and cryptoglandular sepsis.
Heald RJ, Husband EM, Ryall RDH 1982 The mesorectum in rectal cancer Scott SM, Lunniss PJ 2007 Risk factors in faecal incontinence In: Ratto C,
surgery: the clue to pelvic recurrence? Br J Surg 69:613–16. Doglietto GB (eds) Fecal Incontinence. Diagnosis and Treatment.
The original description of the mesorectal plane and its relevance to the London: Springer, pp. 43–66.
surgical excision of rectal tumours. A review of both congenital and acquired risk factors in faecal incontinence
with descriptions of the underlying pathophysiologies.
Klosterhalfen B, Vogel P, Rixen H et al 1989 Topography of the inferior rectal
artery: a possible cause of chronic primary anal fissure. Dis Colon Thomson WH 1975 The nature of haemorrhoids. Br J Surg 62:542–52.
Rectum 32:43–52. An anatomical and clinical study directed at understanding the nature of
A detailed postmortem angiographic study demonstrating the arrangement haemorrhoids.
of anal arterial supply.


Large intestine
1159.e1
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RETPAHC
Mucosa Lamina propria
The lamina propria is composed of connective tissue that supports the
The mucosa is pale, smooth and, in the colon, raised into numerous epithelium, forming a specialized pericryptal myofibroblast sheath
crescentshaped folds between the sacculi. It is thicker, darker, more around each intestinal gland. Solitary lymphoid follicles within the
vascular, and more loosely attached to the submucosa in the rectum. lamina propria, similar to those of the small intestine, are most abun
dant in the caecum, appendix and rectum, but are also present scattered
Epithelium
along the rest of the large intestine; efferent lymphatic vessels originate
The luminal surface of all but the anorectal junction is lined by colum within them. Lymphatic vessels are absent from the intercryptal lamina
nar cells, mucous (goblet) cells and occasional microfold (M) cells that propria.
are restricted to the epithelium overlying lymphoid follicles.
Muscularis mucosae
Columnar (absorptive) cells Columnar (absorptive) cells are the The muscularis mucosae of the large intestine is essentially similar to
most numerous of the epithelial cell types. Although there is some that of the small intestine.
variation in their structure, they all bear apical microvilli which are
shorter and less regular than those on enterocytes in the small intestine. Submucosa
Typical junctional complexes around their apices limit extracellular
diffusion from the lumen across the intestine wall. The submucosa of the large intestine is similar to that of the small
intestine.
Mucous (goblet) cells Mucous cells have a similar structure to
those of the small intestine but they are more numerous. They are Muscularis externa
outnumbered by absorptive cells for most of the length of the colon
but they are equally frequent towards the rectum, where their numbers
The muscularis externa has outer longitudinal and inner circular layers
increase.
of smooth muscle. The longitudinal fibres form a continuous layer that
is aggregated into macroscopically visible longitudinal bands or taeniae
Microfold (M) cells Microfold cells are similar to those of the small


coli (see Figs 66.3 and 66.13). Between the taeniae coli, the longitudinal
intestine.
layer is much thinner, less than half the thickness of the circular layer.
intestinal glands (crypts) The circular fibres constitute a thin layer over the caecum and colon,
and a thicker layer in the walls of the rectum; they form the internal
Intestinal crypts are narrow, perpendicular tubular glands that are
anal sphincter in the anal canal. Interchange of fascicles between circu
longer, more numerous and closer together than those of the small
lar and longitudinal layers occurs, especially near the taeniae coli.
intestine. Their orifices lend a cribriform appearance to the mucosa in
Deviation of longitudinal fibres from the taeniae coli to the circular
surface view. The glands are lined by low columnar epithelial cells,
layer may, in some instances, explain the haustrations of the colon.
mainly goblet cells, augmented by columnar absorptive cells and
neuroendocrine cells. The latter are situated mainly at the bases of
Serosa
the glands, and secrete basally into the lamina propria. In general, the
glands lack Paneth cells but some may be present in the caecum. Stem
cells located at or near the bases of the intestinal glands (crypts) are the The serosa or visceral peritoneum is variable in extent. Small, fatfilled
source of the other epithelial cell types in the large intestine. They appendices epiploicae are most numerous on the sigmoid and trans
provide cells that migrate towards the luminal surface of the intestine; verse colon but generally absent from the rectum.
their progeny differentiate, undergo apoptosis and are shed after
approximately 5 days.


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SUBSECTION: Abdominal viscera
CHAPTER
67
Liver
The liver is the largest of the abdominal viscera, occupying a substantial of proteins and clotting factors; the metabolism of amino acids; and
portion of the upper abdominal cavity. It occupies most of the right bile production. It is involved in a plethora of other biochemical reac-
hypochondrium and epigastrium, and frequently extends into the left tions. Since the majority of these processes are exothermic, a substantial
hypochondrium as far as the left anterior axillary line (Fig. 67.1). As part of the thermal energy production of the body, especially at rest, is
the body grows from infancy to adulthood, the liver rapidly increases provided by the liver. The liver is populated by phagocytic macrophages,
in size. This period of growth reaches a plateau around 18 years and is components of the mononuclear phagocyte system capable of remov-
followed by a gradual decrease in liver weight from middle age. The ing particulates from the blood stream. It is an important site of hae-
ratio of liver to body weight decreases with growth from infancy to mopoiesis in the fetus.
adulthood. The liver weight is 4–5% of body weight in infancy and An account of the more common eponyms relating to the anatomy
decreases to approximately 2% in adulthood. The size of the liver also and surgery of the liver is provided in Stringer (2009).
varies according to sex, being smaller in females, and body size, enlarg-
ing with fat deposition. It has an overall wedge shape, which is, in part,
determined by the form of the upper abdominal cavity into which it SURFACES OF THE LIVER
grows. The narrow end of the wedge lies towards the left hypochon-
drium, and the anterior edge points anteriorly and inferiorly. The supe- The liver is usually described as having superior, anterior, right, poste-
rior and right lateral aspects are shaped by the anterolateral abdominal rior and inferior surfaces, and has a distinct inferior border (Figs 67.2–
and chest wall, as well as the diaphragm. The inferior aspect is shaped 67.3). However, the superior, anterior and right surfaces are continuous


by the adjacent viscera. The capsule is no longer thought to play an and no definable borders separate them. It is more appropriate to group
important part in maintaining the shape of the liver; it is notable that them as the diaphragmatic surface, which is mostly separated from the
it allows expansion when the liver hypertrophies in response to disease, inferior, or visceral, surface by a narrow inferior border. At the infraster-
surgical resection or contra lateral embolization of the portal vein or nal angle, the inferior border is adjacent to the anterior abdominal wall
hepatic artery. and accessible to examination by percussion, but not usually palpable,
Throughout life, the liver is reddish brown in colour, although this except on deep inspiration. In the midline, the inferior border of the
can vary, depending on the fat content. Obesity is the most common liver is near the transpyloric plane. In women and children, the border
cause of excess fat in the liver (steatosis); the liver assumes a more yel- often projects a little below the right costal margin.
lowish tinge as its fat content increases and gains a bluish tinge with
venous obstruction. The texture of the organ is usually soft to firm, Superior surface The superior surface is the largest and lies imme-
although it depends partly on the volume of blood it contains and on diately below the diaphragm, separated from it by peritoneum, except
its fat and fibrous tissue content. for a small triangular area where the two layers of the falciform ligament
The liver performs a wide range of metabolic activities required for diverge. The majority of the superior surface lies beneath the right
homeostasis, nutrition and immune defence. For example, it is impor- dome, but there is a shallow cardiac impression centrally that corre-
tant in the removal and breakdown of toxic, or potentially toxic, materi- sponds to the position of the heart above the central tendon of the
als from the blood; the regulation of blood glucose and lipids; the diaphragm. The left side of the superior surface lies beneath part of the
storage of certain vitamins, iron and other micronutrients; the synthesis left dome of the diaphragm.
Stomach (fundus)
Inferior vena cava
Right cupola of diaphragm
Oesophagus
Right crus of
diaphragm
Right suprarenal gland
Right kidney
Hepatic flexure
Duodenum


1160 Fig. 67.1 The ‘bed’ of the liver. The outline of the liver is shaded green. The central bare area is unshaded.


Liver
1161
76
RETPAHC
A B
Inferior vena cava
Bare area
Superior layer of
Posterior and coronary ligament
anterior layers Posterior
of left triangular
Superior ligament Porta hepatis
Inferior layer of coronary
ligament
Right triangular ligament
Right lobe Left lobe Left lobe Right lobe
Fissure for ligamentum
C D
venosum
Superior
Inferior border
Inferior
Falciform Attachment of lesser
Right Anterior ligament omentum
Gallbladder Ligamentum Gallbladder
teres
Fissure for ligamentum
teres
Inferior border
Fig. 67.2 The surfaces and external features of the liver. A, Superior view. B, Posterior view. C, Anterior view. D, Inferior view.
Spinal area
A B
Oesophageal area
Gastric area
Left dome Suprarenal area
of diaphragm
Cardiac area
Left dome D
Right dome
of diaphragm Spinal area Renal area
C
Oesophageal area
Area related to Suprarenal area
right lower
lobe of lung
Gastric area
Area related to Colic area
costodiaphragmatic
recess Duodenal area
Fig. 67.3 Relations of the liver. A, Superior view. B, Posterior view. C, Anterior view. D, Inferior view.
Anterior surface The anterior surface is approximately triangular the diaphragm and the seventh and eighth ribs. The diaphragm, the
and convex, and is covered by peritoneum, except at the attachment of costodiaphragmatic recess lined by pleura, and the ninth and tenth ribs
the falciform ligament. Much of it is in contact with the anterior attach- lie lateral to the middle and lower thirds of the right surface. Lateral to
ment of the diaphragm. On the right, the diaphragm separates it from the lower third, the diaphragm and thoracic wall are in direct contact.
the pleura and sixth to tenth ribs and cartilages, and on the left, from Rarely, the hepatic flexure and proximal transverse colon lie on a long
the seventh and eighth costal cartilages. mesentery over the right, anterior and superior surfaces of the liver; this
may lead to misdiagnosis of pneumoperitoneum on a radiograph or


Right surface The right surface is covered by peritoneum and lies very rarely cause symptoms (Chilaiditi’s syndrome) (see p. 1144).
adjacent to the right dome of the diaphragm, which separates it from
the right lung and pleura and the seventh to eleventh ribs. The right Posterior surface The posterior surface is convex, wide on the right
lung and basal pleura lie above and lateral to its upper third, between but narrow on the left. A median concavity corresponds to the forward


LivER
1162
8
NOiTCES
convexity of the vertebral column close to the attachment of the liga- of the falciform ligament with no subdivisions. It is substantially
mentum venosum. Much of the posterior surface is attached to the thinner than the right lobe, having a thin apex that points into the left
diaphragm by loose connective tissue, forming the triangular ‘bare area’. upper quadrant.
The inferior vena cava lies in a groove or tunnel in the medial end
of the ‘bare area’. To the left of the caval groove, the posterior surface of Quadrate lobe The quadrate lobe is visible as a prominence on the
the liver is formed by the caudate lobe, and covered by a layer of peri- inferior surface of the liver, to the right of the groove formed by the
toneum continuous with that of the inferior layer of the coronary liga- ligamentum teres (and thus is incorrectly said to arise from the right
ment and the layers of the lesser omentum. The caudate lobe is related lobe, although it is functionally related to the left hemi-liver). It lies
to the diaphragmatic crura and the right inferior phrenic artery above anterior to the porta hepatis and is bounded by the gallbladder fossa
the aortic hiatus, and separated by these structures from the descending to the right, a short portion of the inferior border anteriorly, the fissure
thoracic aorta. for the ligamentum teres to the left, and the porta hepatis posteriorly.
The fissure for the ligamentum venosum separates the caudate lobe Like the caudate lobe, its morphology varies between individuals (Joshi
from the left lobe. The fissure cuts deeply in front of the caudate lobe et al 2009).
and contains the two layers of the lesser omentum. The posterior surface
of the left lobe bears a shallow impression near the upper end of the Caudate lobe The caudate lobe is visible as a prominence on the
fissure for the ligamentum venosum that is caused by the abdominal inferior and posterior surfaces to the right of the groove formed by the
oesophagus. The posterior surface of the left lobe to the left of this ligamentum venosum; it lies posterior to the porta hepatis. To its right


impression is related to the fundus of the stomach. Together, these is the groove for the inferior vena cava. Above, it continues into the
posterior relations make up what is sometimes referred to as the ‘bed’ superior surface on the right of the upper end of the fissure for the liga-
of the liver (see Fig. 67.1). mentum venosum. In gross anatomical descriptions, this lobe is said to
arise from the right lobe but it is functionally separate.
Inferior surface The inferior surface is bounded by the inferior edge
of the liver. It blends with the posterior surface in the region of the
origin of the lesser omentum, the porta hepatis and the inferior layer FUNCTIONAL ANATOMICAL DIVISIONS
of the coronary ligament, and is marked near the midline by a sharp
fissure that contains the ligamentum teres (the obliterated fetal left Current understanding of the functional anatomy of the liver is based
umbilical vein). The gallbladder usually lies in a shallow fossa but this on Couinaud’s division of the liver into eight (subsequently nine, then
is variable; it may have a short mesentery or be completely intrahepatic later revised back to eight) functional segments, based on the distribu-
and lie within a cleft in the liver parenchyma. The quadrate lobe lies tion of portal venous branches in the parenchyma (Couinaud 1957).
between the fissure for the ligamentum teres and the gallbladder. Further understanding of the intrahepatic biliary anatomy, especially of
The inferior surface of the left lobe is related inferiorly to the fundus the right ductal system, was enhanced by contributions from Hjortsjö
of the stomach and the upper lesser omentum. The quadrate lobe lies (1951) and Healey and Schroy (1953), who used the biliary system as
adjacent to the pylorus, the first part of the duodenum and the lower the main guide for division of the liver (Fig. 67.4).
part of the lesser omentum. Occasionally, the transverse colon lies The liver is divided into four portal sectors by the four main branches
between the duodenum and the quadrate lobe. To the right of the of the portal vein. These are right lateral, right medial, left medial and


gallbladder, the inferior surface is related to the hepatic flexure of the left lateral (sometimes, the term posterior is used in place of lateral,
colon, the right suprarenal gland and right kidney, and the first part of and anterior in place of medial). The three main hepatic veins lie
the duodenum (see Fig. 67.1). between these sectors as intersectoral veins. These intersectoral planes
are also called portal fissures (or scissures). Each sector is subdivided
into segments (usually two), based on their supply by tertiary divisions
SUPPORTS OF THE LIVER
of the vascular biliary (Glissonian) sheaths.
The liver is stabilized and maintained in its position in the right upper
Fissures of the liver
quadrant of the abdomen by both static and dynamic factors. A three-
tier classification of supporting structures has been proposed: the sus-
pensory attachments at the posterior abdominal wall to the inferior Knowledge of the fissures of the liver is essential for understanding liver
vena cava, hepatic veins, coronary and triangular ligaments (primary surgery. Three major fissures (main, left and right portal fissures), not
factors); the support provided by the right kidney, right colic flexure visible on the surface, run through the liver parenchyma and contain
and duodenopancreatic complex (secondary factors); and the attach- the three main hepatic veins. Three minor fissures (umbilical, venous
ment to the anterior abdominal wall and diaphragm by the falciform and fissure of Gans) are visible as physical clefts of the liver surface. The
ligament (tertiary factors) (Flament et al 1982). fissure of Gans is also known as Rouvière’s sulcus or the incisura hepatis
The surgical implications of these different factors are important for dextra. Accessory fissures are rare.
understanding the pathophysiology of blunt liver trauma and when
considering the stability of transplanted liver grafts. The inferior vena Main portal fissure The main fissure, sometimes called Cantlie’s
cava and the hepatic veins, especially the right hepatic vein, appear to line, extends from the midpoint of the gallbladder fossa at the inferior
be the most important anatomical structures that support the bulk of margin of the liver back to the midpoint of the inferior vena cava, and


the liver. Other factors that influence the position of the liver within contains the middle hepatic vein. It separates the liver into right
the abdominal cavity include positive intra-abdominal pressure and the and left hemi-livers. Segments V and VIII lie immediately to the right,
movement of the diaphragm during respiration. and segment IV immediately to the left, of the fissure.
Left portal fissure The left portal fissure divides the left hemi-liver
GROSS ANATOMICAL LOBES
into medial (anterior) and lateral (posterior) sectors. It extends from
the midpoint of the inferior edge of the liver between the falciform
Historically, the liver has been divided on the basis of its external
ligament and the left triangular ligament to the point that marks the
appearance into right, left, caudate and quadrate lobes, which are, in
confluence of the left and middle hepatic veins. It contains the left
part, defined by peritoneal ligamentous attachments. Additional liver
hepatic vein and separates the left medial (anterior) and left lateral
lobes have been reported but are rare.
(posterior) sectors; segment III lies immediately anteriorly and segment
Right lobe The right lobe is the largest in volume and contributes to II posteriorly.
all surfaces of the liver. It is divided from the left lobe by the falciform
ligament anteriorly and superiorly and the ligamentum venosum and Right portal fissure The right portal fissure divides the right hemi-
fissure for the ligamentum teres inferiorly. On the inferior surface, to liver into lateral (posterior) and medial (anterior) sectors. The plane of
the right of the grooves formed by the ligamentum teres and ligamen- the right fissure is the most variable of the portal fissures and runs
tum venosum, there are two prominences separated by the porta approximately diagonally through the anatomical right lobe from the
hepatis; the caudate lobe lies posterior, and the quadrate lobe lies lateral end of the inferior border to the termination of the right hepatic
anterior. The gallbladder lies in a shallow fossa to the right of the quad- vein. The fissure divides the right anterior sector to its left (segments V
rate lobe. and VIII) from the right posterior sector to its right (segments VI and


VII), and contains the right hepatic vein. The right fissure traverses the
Left lobe The left lobe is the smaller of the two main lobes, although thickest portion of liver parenchyma that is commonly transected
it is nearly as large as the right lobe in young children. It lies to the left during liver resection.


Liver
1163
76
RETPAHC
Fig. 67.4 The fissures and sectors of
the liver. (Right lateral = right
posterior; right medial = right
anterior.)
II
I
VII VIII
IV
III
V
VI
Umbilical fissure
Left lateral sector
Left portal fissure
Main portal fissure Left medial sector
Right lateral sector
Right portal fissure Right medial sector
A B Fig. 67.5 Segments of the liver (after
Couinaud). A, Superior view. B, Posterior
view. C, Anterior view. D, Inferior view.
The segments are usually referred to by
VII
II II I number (name): I (caudate); II (left lateral
VII superior); III (left medial inferior);
IV (left medial superior) (sometimes
III
VIII III subdivided into superior and inferior
IV IV parts); V (right medial inferior); VI (right
lateral inferior); VII (right lateral superior);
V VI VIII (right medial superior).
C D
II
VII
VII
III II I
VIII
IV VI
III
VI V
IV
V
Umbilical fissure The umbilical fissure (or fissure for the ligamen- Venous fissure The venous fissure (or fissure for the ligamentum
tum teres) separates segment III from segment IV within the left medial venosum) is in direct continuity with the umbilical fissure on the
sector and contains a major branch of the left hepatic vein (the umbili- undersurface of the liver and contains the ligamentum venosum (the
cal fissure vein). It is marked anteriorly by the attachment of the falci- obliterated ductus venosus). It lies between the caudate lobe and
form ligament and inferiorly by the ligamentum teres, where it may be segment II.
covered by a bridge of liver tissue extending between segments III and
Fissure of Gans The fissure of Gans (or Rouvière’s sulcus) lies on
IV. This liver bridge is usually avascular and can be divided safely with
the undersurface of the right lobe of the liver behind the gallbladder
diathermy during surgery. The umbilical fissure also contains the umbil-


fossa. It often marks the variable site of division of the portal pedicle
ical portion of the left portal vein, segmental bile ducts converging to
to the right posterior sector.
form the left hepatic duct, and the terminal branches of the left branch
of the hepatic artery. The umbilical portion of the left portal vein offers
Sectors and segments of the liver
direct surgical access to the left portal vein; this is important for mobi-
lization of the left portal vein in surgery for hilar cholangiocarcinoma
and in operations to restore intrahepatic portal blood flow after portal Sectors
vein occlusion. A knowledge of the arrangement of the vascular and The sectors of the liver are made up of between one and three segments:
biliary structures within the umbilical fissure is also essential when right lateral sector = segments VI and VII; right medial sector = segments
splitting the liver for an adult and paediatric recipient and for live donor V and VIII; left medial sector = segments III and IV (and part of I);
liver transplantation for a child recipient. left lateral sector = segment II (Fig. 67.5). Segments are numbered


LivER
1164
8
NOiTCES
portion), while in the Far East, right and left subdivisions are recog-
nized. The Glissonian sheaths to segment I arise from both right and
left main sheaths; the segment therefore receives vessels from both the
II left and right branches of the portal vein and hepatic arteries. The
IV venous drainage of the caudate lobe is directly into the inferior vena
cava by multiple small tributaries that nearly always arise from the
lower, and occasionally from the middle, third of the segment, but
L
rarely from the upper third. The bile ducts draining the segment are
VIII M closely related to the confluence of the right and left hepatic ducts (see
Chapter 68), such that excision of bile duct tumours affecting the hilum
R
of the liver usually requires removal of segment I.
Segment II Segment II lies posterolateral to the left portal fissure and
VII
is the only segment in the left lateral sector of the liver. It often has only
one Glissonian sheath and drains into the left hepatic vein. Rarely, a
separate vein drains directly into the inferior vena cava.
Segment III Segment III lies between the umbilical fissure and the
A left portal fissure, and is supplied by one to three Glissonian sheaths;
it drains into the left hepatic vein. The vein of the falciform ligament
can provide an alternative drainage route for segment III. Very rarely,
the venous drainage is to the middle hepatic vein and reconstruction is
required following a right hepatectomy involving resection of the
IV middle hepatic vein and in split liver transplantation (Dar et al 2008).
II
Segment IV Segment IV lies between the umbilical fissure and the
LPV
main portal fissure, immediately anterior to segment I. Segment IV is
PV supplied by three to five Glissonian sheaths, most of which arise in the
umbilical fissure; their origins are often close to those that supply seg-
ments II and III. Occasionally, segment IV is supplied by branches from
VIII I the main left pedicle. The main venous drainage of segment IV is into
the middle hepatic vein but the segment can also drain into the left
IVC
hepatic vein through the vein of the falciform ligament. Segment IV has


been divided into IVa superiorly and IVb inferiorly; this is relevant to
transverse hepatectomy, in which segments IVb, V and VI are removed
along a transverse portal plane (Sugarbaker 1990).
VII
Segment V Segment V is the inferior segment of the right medial
sector and lies between the middle and the right hepatic veins. Its size
B is variable, as are the number of Glissonian sheaths that supply it from
the right anterior sheath. Venous drainage is usually into the right and
middle hepatic veins, but may be direct into the inferior vena cava via
an inferior right hepatic vein.
III
Segment VI Segment VI forms the inferior part of the right lateral
IV
sector posterior to the right portal fissure. It is often supplied by two to
three branches from the right posterior Glissonian sheath, and occa-
sionally the Glissonian sheath to segment VI can arise directly from the
right pedicle. Venous drainage is normally into the right hepatic vein
but, like in segment V, may be via an inferior right hepatic vein directly
V I into the inferior vena cava.
RPV
Segment VII Segment VII forms the superior part of the right lateral
sector and lies behind the right hepatic vein. The sheath to segment VII
is often single. The venous drainage is into the right hepatic vein; occa-
sionally, the segment can drain directly into the inferior vena cava.
VI
Segment VIII Segment VIII is the superior part of the right medial
sector. The right medial (anterior) sectoral sheath ends in segment VIII
C and supplies it, after giving off branches to segment V. Venous drainage
is to the right and middle hepatic veins.
Fig. 67.6 Segments of the liver seen on axial computed tomography (CT)
scan. A, A contrast enhanced CT shows the left (L), middle (M) and right Peritoneal attachments and ligaments
(R) hepatic veins at the superior aspect of the liver, marking the left, main
of the liver
and right portal fissures. B, Inferior to this, the caudate lobe (segment I)
lies between the inferior vena cava (IVC) and the portal vein (PV). The left
portal vein (LPV) separates segment II superiorly from segment III The liver is attached to the anterior abdominal wall, diaphragm and


inferiorly. C, The right portal vein (RPV) divides segments V and VI other viscera by several peritoneal ligaments.
inferiorly (C) from segments VII and VIII superiorly (B).
Falciform ligament and ligamentum teres
The liver is attached to the anterior abdominal wall by the falciform
clockwise from below, starting with segment I and ending with segment ligament, derived from the ventral mesogastrium in the embryo. The
VIII (see Figs 67.4–67.5; Fig. 67.6). two layers of this ligament pass posteriorly and slightly to the right from
the posterior surface of the anterior abdominal wall and the undersur-
Segment I Segment I corresponds to the anatomical caudate lobe and face of the diaphragm in the midline, and attach to the anterior and
lies posterior to segment IV, with its left half directly adjacent to segment superior surfaces of the liver. On the dome of the superior surface, the
II and its medial half wrapped around the retrohepatic segment of the right leaf runs laterally and is continuous with the upper layer of the
inferior vena cava to a variable extent. In European literature, it is sub- coronary ligament. The left layer of the falciform ligament turns medi-
divided into three parts (caudate process, Spiegel lobe and paracaval ally and is continuous with the anterior layer of the left triangular


Liver
1165
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RETPAHC
ligament. The ligamentum teres is the obliterated remnant of the left dividing the lesser omentum because an aberrant or accessory left
umbilical vein of the fetus; it runs in the lower free border of the falci- hepatic artery may run within it; when present, it invariably extends to
form ligament and continues into a fissure on the inferior surface of the liver at the base of the umbilical fissure, and may be identified by
the liver. In fetal life, the left umbilical vein opens into the left portal a pulsation in the lesser omentum close to the umbilical fissure.
vein. It is supposed to be obliterated in adult life but frequently remains
partially patent; in reality, its lumen is usually closed rather than obliter- Ligamentum venosum
ated and it can be dissected at the umbilicus, dilated and used to access The ligamentum venosum represents the obliterated venous connection
the left portal vein in more than half of individuals. The ligamentum that existed in the fetus between the left branch of the portal vein and
teres may reopen in conditions such as portal hypertension, when it the termination of the left hepatic vein (the ductus venosus). It is used
forms a sizeable collateral channel. as a guide to gain control of the left hepatic vein outside the liver during
The ligamentum teres is important in abdominal surgery for several surgery. By dividing the ligament close to its insertion into the left
reasons. It is often divided in upper abdominal surgery to optimize hepatic vein and retracting it laterally, the angle between the left and
access to the upper abdominal viscera or as the first step in mobilization the middle hepatic veins may be accessed.
of the liver. It is vascularized by numerous small arterial branches This can be helpful in left liver resection with preservation of the
(mainly from the artery to segment IV) and para-umbilical veins, and caudate lobe, but if the caudate lobe is to be removed, the left hepatic
these anastomose with branches of the superior epigastric artery; it is vein (or, more usually, the confluence of the left and middle hepatic


therefore important to ligate or coagulate the ligament during its divi- veins with the inferior vena cava) can be approached more posteriorly
sion. The ligamentum teres is a guide to the segment III hepatic duct without division of the ligamentum venosum.
in hepaticojejunostomy formation, and to the left portal vein lying in
the umbilical fissure during mesenterico-portal bypass (Vellar et al Porta hepatis, hepatoduodenal ligament
1998, di Francesco et al 2014). and hilar plate
Coronary ligament
The porta hepatis is a deep transverse fissure on the inferior surface of
The coronary ligament is formed by the reflection of the peritoneum
the liver. It is situated between the quadrate lobe anteriorly and the
from the diaphragm on to the superior and posterior surfaces of the
caudate process posteriorly, and contains the portal vein, hepatic artery
right lobe of the liver. A large triangular area of liver devoid of peritoneal
and hepatic nervous plexuses as they ascend into the parenchyma of
covering, the so-called ‘bare area’ of the liver, lies between the two layers
the liver, and the right and left hepatic ducts and some lymph vessels
of the coronary ligament. Here, the liver is attached to the diaphragm
that emerge from the liver. The hepatic ducts usually lie anterior to the
by areolar tissue, which is in continuity inferiorly with the anterior
portal vein and its branches, and the hepatic artery with its branches
pararenal space. On the right, the two layers of the coronary ligament
lies between the two (Figs 67.7–67.8). However, the right hepatic artery
converge laterally to form the right triangular ligament. On the left, the
sometimes lies anterior to the common hepatic duct; this variation is
two layers become closely applied, and form the left triangular liga-
important during bile duct reconstruction by hepaticojejunostomy
ment. The upper layer of the coronary ligament is reflected superiorly
on to the inferior surface of the diaphragm and inferiorly on to the right
and superior surfaces of the liver. The lower layer of the coronary liga-
Anterior
ment is reflected inferiorly over the right suprarenal gland and right
kidney, and superiorly on to the inferior surface of the liver. Surgical
Right lobar nerves


division of the right triangular and coronary ligaments allows the right Left hepatic duct
lobe of the liver to be brought forwards, and exposes the lateral aspect Right hepatic duct
Left hepatic artery
of the inferior vena cava behind the liver.
Right hepatic artery
Left lobar lymphatics
Triangular ligaments
The left triangular ligament is a double layer of peritoneum that extends Left portal vein Right portal vein
for a variable length over the superior border of the left lobe of the liver.
Medially, the anterior leaf is continuous with the left layer of the falci- Peritoneal envelope
form ligament, and the posterior layer is continuous with the left layer
of the lesser omentum. The left triangular ligament lies in front of the Posterior
abdominal part of the oesophagus and part of the fundus of the Fig. 67.7 A cross-section of the structures at the porta hepatis viewed
stomach. Division of the left triangular ligament allows the left lobe of from above.
the liver to be mobilized for exposure of the abdominal oesophagus
and crura of the diaphragm. The left triangular ligament is an important
stabilizing factor for the left lobe after excision of the right lobe of the Common hepatic duct
liver. Its division will result in the left lobe being unstable, to the extent
that it can rotate and displace into the space created under the right
hemidiaphragm, which, in turn, can compromise the venous outflow
of the left lobe with consequent liver dysfunction. If it is divided, the
left lobe should be stabilized by reattaching the falciform ligament to
the anterior abdominal wall.
The right triangular ligament is a short structure that lies at the right
lateral limit of the ‘bare area’ of the liver and represents the convergence
laterally of the two layers of the coronary ligament.
Lesser omentum
The lesser omentum is a double layer of peritoneum that extends from
the lesser curvature of the stomach and proximal duodenum to the
inferior surface of the liver. Its free margin contains the portal triad (Ch.
63). Its attachment to the inferior surface of the liver is L-shaped. The
vertical component follows the line of the fissure for the ligamentum
venosum, the fibrous remnant of the ductus venosus. More inferiorly,


the attachment runs horizontally to complete the L in the porta hepatis.
At its upper end, the left layer of the lesser omentum is continuous with
the posterior layer of the left triangular ligament, and the right layer is
continuous with the coronary ligament as it encloses the inferior vena
cava. At its lower end, the two layers of the lesser omentum diverge to
surround the structures of the porta hepatis. A thin, fibrous condensa-
tion of fascia usually runs from the medial end of the porta hepatis into
the fissure for the ligamentum teres. This fascia is continuous with the Portal vein Inferior vena cava Hepatic artery
lower border of the falciform ligament where the ligamentum teres Fig. 67.8 An axial CT of the porta hepatis. The hepatic ducts lie
re-emerges at the inferior border of the liver. Care should be taken when anteriorly, the portal vein posteriorly, and hepatic artery between the two.


LivER
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(Michels 1966). All these structures are enveloped within a perivascular A Medial (anterior) division
sheath of loose connective tissue that surrounds the vessels and bile
VIII
ducts as they course through the liver parenchyma, and is continuous III
II
with the fibrous hepatic capsule (of Glisson). The dense aggregation of VII I
vessels, supporting connective tissue, and liver parenchyma just above IV Left hepatic artery
the porta hepatis is often referred to as the ‘hilar plate’ of the liver.
Understanding the concept of the hilar plate is important in surgical
approaches to the hilar structures; division or lowering of the hilar plate
is essential for optimum surgical access to the left hepatic duct.
The hepatic artery, bile duct and portal vein extend between the VI V Left gastric
porta hepatis and the upper border of the duodenum in the free edge
artery
of the hepatoduodenal ligament, which forms the anterior boundary Lateral (posterior) division
Splenic artery
of the epiploic foramen. Rapid control of the vessels entering the porta
Right hepatic artery
hepatis (the hepatic pedicle) can be obtained by compressing them in
the free edge of the lesser omentum (a ‘Pringle’ manœuvre); this is Cystic artery
conveniently done by dividing the lesser omentum immediately to the Common hepatic artery
Hepatic artery ‘proper’
left of these structures and passing a tape around them through the
epiploic foramen. Gastroduodenal artery Right gastric artery
The left hepatic duct is extrahepatic as it descends obliquely along
the undersurface of segment IV (the quadrate lobe) to the confluence
B Cystic duct Caudate lobe,
of the hepatic ducts. Access to this extrahepatic segment of the left
Cystic artery Portal vein anterior surface
hepatic duct is particularly useful when performing a biliary-enteric
bypass procedure to treat a stricture of the hepatic duct confluence. The Liver, right lobe
right hepatic duct is more intrahepatic and extension of a hepatico- Diaphragm,
right crus
jejunal anastomosis on to the right hepatic duct necessitates incision of


the liver parenchyma in the gallbladder fossa.
Glissonian sheaths
Left gastric
Glisson’s capsule of the liver becomes condensed as Glissonian sheaths artery
around the branches of the portal triad structures as they enter the liver
Inferior vena
parenchyma and subdivide into segmental branches. Thus, each bile cava
duct, hepatic artery and portal vein is surrounded by a single fibrous Common
sheath, which Couinaud called the ‘Valoean sheath’ (after Valoeus, an hepatic artery
anatomist from the Middle Ages who first described the liver capsule). Right gastric
Within each sheath, the portal vein is surrounded by loose areolar con- artery
nective tissue, making dissection of the portal vein relatively easy. The Gastro-
fibrous tissue around the bile ducts and hepatic artery is tougher, and duodenal
artery
dissection of these structures is more difficult. This sheath arrangement
facilitates surgical control of the right and left vasculobiliary pedicles
of the liver, as well as sectoral and segmental divisions in complex liver
resections (Yamamoto et al 2012).
Gallbladder, Bile Superior Right gastro-
VASCULAR SUPPLY AND LYMPHATIC DRAINAGE fundus duct pancreatico- epiploic artery
duodenal
artery
The blood vessels connected with the liver are the portal vein, hepatic
Fig. 67.9 The hepatic artery. A, Branches. B, Usual relations of the
artery and hepatic veins. The portal vein and hepatic artery ascend in
hepatic artery, bile duct and portal vein to each other in the lesser
the lesser omentum to the porta hepatis, where each usually bifurcates.
omentum: anterior aspect, portion of the liver removed.
The common hepatic duct and lymphatic vessels descend from the
porta hepatis alongside the portal vein and hepatic artery (see Fig.
67.8). The hepatic veins leave the liver via its posterior surface and run
directly into the inferior vena cava. well to the left of the porta hepatis. The main significance of an early
division is that the right branch may pass behind the portal vein
Hepatic artery
(Lanouis and Jamieson 1993). The segmental arteries of the liver are
macroscopically end arteries, although some collateral circulation


In adults, the common hepatic artery is intermediate in size between occurs between segments via fine terminal branches.
the left gastric and splenic arteries. In fetal and early postnatal life, it is Anatomical variants of the normal arrangement of the hepatic artery
the largest branch of the coeliac trunk. The hepatic artery gives off the are found in about one-third of individuals and are important to
right gastric and gastroduodenal arteries, as well as branches to the bile re cognize because they are relevant to surgical and interventional radio-
duct and gallbladder from its right hepatic branch (Fig. 67.9A). After logical procedures (Covey et al 2002, Michels 1966, López-Andújar
originating from the coeliac trunk, the hepatic artery passes anteriorly et al 2007, Saba and Mallarini 2011). An artery that supplies part of the
and laterally above the upper border of the pancreas to the upper aspect liver in addition to its normal artery is defined as an accessory artery. A
of the first part of the duodenum. It is subdivided into the common replaced hepatic artery is an artery that does not originate from an
hepatic artery, from the coeliac trunk to the origin of the gastroduode- orthodox position and provides the sole supply to that part of the liver.
nal artery, and the ‘hepatic artery proper’, from that point to its bifurca- The most common anatomical variants are a replaced or accessory
tion. It ascends anterior to the portal vein and medial to the bile duct left hepatic artery that arises from the left gastric artery, or a replaced
within the free margin of the lesser omentum in the anterior wall of or accessory right hepatic artery that arises from the superior mesenteric
the epiploic foramen. It divides into right and left branches at a variable artery, both occurring in 10–20% of individuals.
level below the porta hepatis. The right branch of the hepatic artery Variations in the intrahepatic arteries are common and may be surgi-
usually crosses posterior (occasionally anterior) to the common hepatic cally important. For example, the segment IV artery most commonly
duct (Fig. 67.9B). This close proximity often means that the right arises from the left hepatic artery, but in up to 30% of cases, it arises


hepatic artery is involved in bile duct cancer earlier than the left hepatic from the right hepatic artery or the hepatic artery proper (Onishi et al
artery. Occasionally, the right hepatic artery crosses anterior to the 2000). The segment IV artery never arises to the right of the common
common hepatic duct and is more vulnerable to injury in biliary hepatic duct; thus, if the right hepatic artery is divided to the right of
surgery. It almost always divides into an anterior branch supplying seg- the common hepatic duct, this arterial supply to segment IV is not
ments V and VIII, and a posterior branch supplying segments VI and endangered. Failure to recognize this variation may compromise the
VII. The anterior division also often supplies a branch to segment I and blood supply to segment IV following right hepatectomy, and is espe-
the gallbladder. The hepatic artery proper sometimes divides at a low cially important during right lobe donation for live donor liver
level close to its origin and, occasionally, it divides at a higher level, transplantation.


Liver
1166.e1
76
RETPAHC
A replaced hepatic artery proper may arise from the superior artery arises from the left gastric artery, entering the liver through the
mesenteric artery (Fig. 67.10) and can be suspected at surgery by a rela- umbilical fissure; this artery provides a source of collateral supply in
tively superficial portal vein (reflecting the absence of a hepatic artery cases where the arteries at the porta hepatis are occluded, but it may
that would normally ascend in front of the vein). More commonly, a also be injured during mobilization of the stomach, as it lies in the
replaced or accessory right hepatic artery arises from the superior upper portion of the lesser omentum. Rarely, an accessory left or right
mesenteric artery (see Fig. 67.10; Fig. 67.11). In such cases, the variant hepatic artery may arise from the gastroduodenal artery or directly from
artery runs in the lesser omentum behind the portal vein and bile duct, the aorta. The presence of replaced arteries can be life-saving in patients
and can usually be identified at surgery by palpable pulsation behind with bile duct cancer; because they are further away from the bile duct
the portal vein. An accessory right hepatic artery may be injured during they tend to be spared from infiltration by the cancer, making excision
resection of the pancreatic head because the artery lies in close proxim- of the tumour feasible. Knowledge of these variations is also essential
ity to the portal vein. Occasionally, a replaced or accessory left hepatic when performing whole and split liver transplantation.
Replaced left hepatic artery: 10–20% Replaced hepatic artery proper: 4% Fig. 67.10 Common hepatic artery
variants.
Replaced right
hepatic artery: 10–15% Left gastric artery
Superior mesenteric artery Superior mesenteric artery or aorta
Left hepatic artery Fig. 67.11 Hepatic
arteriograms. A, A
selective hepatic
arteriogram shows
normal left hepatic
artery branches and
small right hepatic
artery branches. B, The
right hepatic artery is
arising from the origin
of the superior
mesenteric artery.
A
Gastroduodenal artery
Right hepatic artery
B


Liver
1167
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RETPAHC
Veins The main extrahepatic tributaries of the portal vein are the left gastric
(coronary) vein, which ends in the left margin of the portal vein, and
the posterior superior pancreaticoduodenal vein near the head of the
The liver has two venous systems. The portal system conveys venous
pancreas. The portal vein divides into right and left branches at the
blood from the majority of the gastrointestinal tract and its associated
hilum of the liver (Fig. 67.13). The left portal vein has a longer extra-
organs to the liver (p. 1039). The hepatic venous system drains blood
hepatic course (4–5 cm) than the right portal vein, tends to lie more
from the liver parenchyma into the inferior vena cava.
horizontal, and is often smaller in calibre. It has a horizontal portion
Portal vein that runs along the inferior surface of segment IV and invariably gives
The portal vein is formed behind the neck of the pancreas, usually from branches to segment I and sometimes to segment IV. The left branch of
the convergence of the superior mesenteric and splenic veins (Fig. the portal vein continues within the liver, giving off a segment II branch
67.12; see also Figs 70.8b, 59.8). Its origin lies in the transpyloric plane laterally before taking a more anterior and vertical course in the umbili-
between the lower border of the body of the first lumbar vertebra and cal fissure. Here, it gives off branches to segments III and IV, and receives
the upper border of the body of the second lumbar vertebra (Mirjalili the obliterated left umbilical vein (ligamentum teres). The majority of
et al 2012). The portal vein is approximately 8 cm long and ascends the portal venous supply to segment IV comes from the left portal vein,
obliquely to the right behind the first part of the duodenum, the and only occasionally from the right branch of the portal vein or its
common bile duct and gastroduodenal artery, and anterior to the infe- branches to segment V or VIII. The right branch of the portal vein is


rior vena cava. It enters the right border of the lesser omentum and only 2–3 cm in length and usually divides into a right medial (anterior)
ascends anterior to the epiploic foramen to reach the right end of the sectoral division supplying segments V and VIII, and a right lateral
porta hepatis, where it divides into right and left main branches, which (posterior) sectoral division supplying segments VI and VII. The medial
enter the liver. In the lesser omentum, the portal vein lies posterior to division may give a branch to segment I.
both the bile duct and the hepatic artery. It is surrounded by the hepatic Portal vein variations usually involve the right branch (Covey et al
nerve plexus and accompanied by numerous lymphatics and some 2004). If the latter is absent, which occurs in about 10–15% of livers,
lymph nodes. The portal vein contains smooth muscle in its wall and, the portal vein usually trifurcates into left portal, right medial and right
in experimental animals at least, has well-developed spontaneous con- lateral sectoral veins. This has implications for split liver and live donor
tractions with frequencies between 0.01 and 1 Hz (Burt 2003). It is liver transplantation, where its presence might be considered as a rela-
typically valveless. tive contraindication. The right lateral sectoral portal vein may arise
from the portal vein, or the right medial sectoral portal vein may origi-
nate from the left portal vein, a variant that it is important to remember
during left-sided liver resection. Rarely, the portal bifurcation is absent,
Portal vein in which case the portal vein enters the liver, giving off the right sectoral
branches, and then turns left to supply the left lobe, which presents an
added complexity in major liver surgery (Chaib 2009).
Porto-systemic shunts
Increased pressure within the portal venous system may result in dila-
tion of portal venous tributaries and reversed flow at sites of porto-
systemic anastomoses. Common sites of porto-systemic shunts are
listed in Table 67.1.
Hepatic veins
The liver drains by three major hepatic veins into the suprahepatic part
of the inferior vena cava and via numerous minor hepatic veins that
drain into the retrohepatic inferior vena cava. The adult retrohepatic


inferior vena cava is 6–7 cm long and surrounded, to a variable extent,
by segment I (Camargo et al 1996). The three major veins are located
between the four sectors of the liver (see Fig. 67.6A; Figs 67.14–67.15).
Thus, the right hepatic vein lies between the right medial and lateral
sectors, the middle hepatic vein lies between the right and left hemi-
livers, and the left hepatic vein lies between the left medial and lateral
sectors. During hepatic resection, the surgeon should transect the liver
Superior mesenteric vein parenchyma slightly to the left or right of the particular fissure that is
Fig. 67.12 A coronal CT of the portal vein and superior mesenteric vein. being opened to avoid the main trunk of a hepatic vein.
Fig. 67.13 The main portal vein and its intrahepatic
branches. (Right lateral = right posterior; right medial =
II right anterior.)
I
VII VIII
IV
III
V
VI
Right lateral sectoral vein Left portal vein
Right medial sectoral vein Right portal vein


LivER
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NOiTCES
inf.HV1 RHV MHV8 LHV4 LHV
Middle hepatic vein Inferior vena cava
Fig. 67.14 A sagittal ultrasound of the middle hepatic vein. The middle inf.HV2 MHV5 MHV5 MHV4 MHV4 MHV4
hepatic vein is seen draining into the inferior vena cava. Fig. 67.15 Modern CT analysis techniques are helpful in demonstrating
variations in hepatic venous anatomy and this is very useful for planning
live donor surgery for liver transplantation. Abbreviations: inf. HV1, inf.
HV2, right inferior hepatic veins; LHV, left hepatic vein; LHV4, left hepatic
Table 67.1 Common sites of porto-systemic anastomoses in portal hypertension and
associated clinical implications vein branch to segment 4; MHV4, middle hepatic vein branches to
segment 4; MHV5, middle hepatic vein branches to segment 5; MHV8,
Portal vein tributaries Systemic veins Clinical middle hepatic vein branches to segment 8; RHV, right hepatic vein.
presentations (Courtesy of MeVis Medical Solutions AG, Bremen, Germany.)
Left gastric vein Distal oesophageal veins Oesophageal and
draining into azygos and gastric varices
hemiazygos veins
Superior rectal veins Middle and inferior rectal Rectal varices the right side of the middle hepatic vein and is sometimes large enough
veins draining into internal
to be mistaken for the middle hepatic vein. Anteriorly, the middle
iliac and pudendal veins
hepatic vein drains some of segment V; the sizes of the tributaries drain-
Persistent tributaries of left Periumbilical branches of ‘Caput medusae’
ing segments V and VIII are variable. Intrahepatic venous anastomoses
branch of portal vein in epigastric and intercostal
ligamentum teres veins between the middle and right hepatic veins, particularly in segment
Tributaries of right branch of Retroperitoneal veins Dilated retroperitoneal VIII, have been reported in up to one third of adult livers (Hribernik
portal vein overlying ‘bare draining into azygos, veins at risk during and Trotovšek 2014).
area’ of liver hemiazygos, lumbar, surgery or interventional


intercostal and phrenic veins procedures Left hepatic vein
Omental and colonic veins near Retroperitoneal veins near May be problematic The left hepatic vein lies between the left medial and left lateral sectors
hepatic and splenic flexures hepatic and splenic flexures during surgery
of the liver and drains segments II, III and, occasionally, IV. Small veins
draining segment II and, occasionally, the superior part of segment IV
may drain directly into the inferior vena cava. Usually, a major tributary
of the left hepatic vein, the umbilical fissure vein, runs between
Right hepatic vein
segments III and IV and contributes to their drainage. Occasionally,
This is the longest and largest hepatic vein. It is usually single, but
the vein draining segment III ends separately in the confluence of
occasionally remains as two trunks until it terminates by draining into
the left and middle hepatic veins. These variations in venous drainage
the inferior vena cava. The right hepatic vein runs in the right portal
are of significance in split liver transplantation and live donor liver
fissure between the right medial and lateral sectors. It drains the whole
transplantation.
of segments VI and VII, and variable proportions of segments V and
VIII, depending on the extent to which these segments drain into the Minor hepatic veins
middle hepatic vein. The right hepatic vein is formed anteriorly near
Segment I veins drain directly into the inferior vena cava and vary in
the inferior border of the liver and lies in a coronal plane through most
number from one to five. Since this segment has an independent venous
of its course. It drains into the inferior vena cava near the upper border drainage from the rest of the liver, in patients with Budd–Chiari syn-
of the caudate lobe. Of the three major hepatic veins, the right hepatic
drome, in which the major hepatic veins are blocked, segment I often
vein is most variable in its size, not only due to the variable contribu-
continues to drain effectively and undergoes compensatory hypertro-
tion of the middle hepatic vein to the drainage of segments V and VIII
phy. There may be an accessory inferior or middle right hepatic vein, as
but also due to the existence of an accessory right inferior (30%) and/
well as several smaller ‘retrohepatic’ veins that drain the right lobe


or middle hepatic vein (10%) (Fang et al 2012). The hepatic venous
directly into the inferior vena cava. When present, they are of surgical
anatomy is particularly important in right lobe living donation and can
importance, especially if greater than 5 mm in diameter; they drain
be assessed preoperatively by computed tomography (CT) or magnetic
segments V and VI independently of the three major hepatic veins and,
resonance imaging (MRI) and/or by intraoperative ultrasound.
therefore, a tumour involving the latter can be resected safely as long
as venous drainage from the accessory veins is preserved. In live donor
Middle hepatic vein
and split liver transplantation, larger accessory veins must be individu-
The middle hepatic vein lies in the main portal fissure between the right
ally anastomosed to the recipient inferior vena cava to ensure adequate
and left hemi-livers. It usually joins the left hepatic vein and terminates
venous drainage.
in the inferior vena cava as a short common trunk (about 5 mm long);
it ends as a single trunk in the inferior vena cava in fewer than 10% of Transjugular intrahepatic porto-systemic shunt
individuals. The middle hepatic vein drains the central part of the liver
(TiPS) procedure for portal hypertension
and receives constant tributaries from segments IV, V and VIII. The vein
from segment IV lies in a sagittal plane and enters the middle hepatic
Available with the Gray’s Anatomy e-book vein on its left side. The vein from segment VIII runs transversely into


Liver
1168.e1
76
RETPAHC
In extreme cases of chronic portal hypertension, a large-calibre anasto-
mosis between the portal and systemic circulations may be created
within the liver parenchyma by inserting a stent between a large portal
and hepatic vein within the liver. The stent is introduced through a
catheter inserted into the internal jugular vein and guided into the liver
under radiological control. This large shunt relieves the severe portal
hypertension but tends to worsen incipient hepatic encephalopathy.


Liver
1169
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RETPAHC
Segmental anatomy of the liver in relation area’ and superior surface; distension or disruption of the liver capsule
to hepatic resection causes quite well-localized, sharp pain.
Hepatic plexus
The hepatic plexus receives preganglionic parasympathetic fibres from
Available with the Gray’s Anatomy e-book
the anterior and, to a lesser extent, the posterior vagus, and postgangli-
onic sympathetic fibres via the coeliac and superior mesenteric plexuses.
Lymphatic drainage Nerve fibres accompany the branches of the portal triad into the liver,
penetrating as far as the hepatocytes. Aminergic, cholinergic, peptider-
gic and nitrergic nerve fibres have been identified and appear to be
Lymph from the liver is rich in protein and is mostly a product of the
involved in hepatic metabolism and the control of sinusoidal blood
hepatic sinusoids (Ohtani and Ohtani 2008). It passes, via deep and
flow (McCuskey 2004); however, the transplanted, denervated liver
superficial pathways, to nodes above and below the diaphragm (Trut-
indicates that this role is not essential. Multiple fine branches from the
mann and Sasse 1994). Obstruction of hepatic venous drainage
plexus supply the extrahepatic bile ducts and gallbladder; the vagal
increases the flow of lymph in the thoracic duct.
fibres are motor to the muscle of the bile ducts and gallbladder, and
Superficial hepatic lymphatics inhibitory to the sphincter of the bile duct. Branches from the plexus
Superficial lymphatics run in subserosal areolar tissue over the surface also run inferiorly with the right gastric artery to contribute to the
of the liver and drain in four directions. Lymphatics from most of the supply of the pylorus; with the gastroduodenal artery and its branches
posterior surface, including the caudate lobe, drain into nodes along- to reach the pylorus, proximal duodenum and pancreas; and with the
side the inferior vena cava; a few lymphatics from the posterior surface right gastroepiploic artery to provide a small contribution to the nerve
of the left lobe pass towards the oesophageal hiatus and nodes around supply of the stomach.


the cardia. Lymphatics in the coronary and right triangular ligaments
Referred pain
may pass directly to the thoracic duct. Lymphatics from most of the
inferior, anterior and superior surfaces drain into hepatic nodes at the Pain arising from the parenchyma of the liver is poorly localized. In
porta hepatis. A few lymphatics from the right superior surface accom- common with other structures of foregut origin, pain is referred to the
pany the inferior phrenic artery across the right diaphragmatic crus to epigastrium. Stretch or irritation of the liver capsule by inflammation
drain into coeliac nodes. or neoplasia produces well-localized ‘somatic’ pain. Pathology involv-
ing the diaphragmatic surface of the liver may be referred via the phrenic
Deep hepatic lymphatics
nerve to the right shoulder region (C3,4,5 dermatomes).
Fine lymphatics within the portal triads and around interlobular veins
merge to form larger vessels. Some ascend through the parenchyma to
pass through the vena caval opening in the diaphragm and drain into MICROSTRUCTURE
inferior mediastinal nodes, but most drain to lymph nodes at the porta
hepatis. The liver is essentially an epithelial-mesenchymal outgrowth of the
caudal part of the foregut, with which it remains connected via the
biliary tree (see Ch. 60). Most of the surface of the liver is covered by
INNERVATION a typical serosa, the visceral peritoneum. Beneath this, and enclosing
the whole organ, is a thin (50–100 µm) capsule of connective tissue,
The liver has a dual innervation. The parenchyma is supplied by nerves from which extensions pass into the liver as septa and trabeculae.
arising from the hepatic plexus, which contains sympathetic and para- Branches of the hepatic artery and hepatic portal vein, together with
sympathetic (vagal) fibres; they all enter the liver at the porta hepatis. bile ductules and ducts, run within these connective tissue trabeculae,
The capsule is supplied by fine branches of the lower intercostal nerves, which are termed portal tracts. The combination of the two types


which also supply the parietal peritoneum, particularly around the ‘bare of vessel and a bile duct is termed a portal triad (Fig. 67.19); these
A B Fig. 67.19 The structural organization of human
liver tissue. A, Lobules bordered by delicate
connective tissue septa (arrows), in which run
branches of the hepatic portal vein, hepatic artery
and bile duct, grouped as portal triads. A central
vein drains each lobule. B, A portal triad
containing branches of the hepatic portal vein (PV;
generally the largest profile), the hepatic artery (A)
PV and a bile duct (B), with typical round epithelial
nuclei. Other small bile ductule and arteriolar
branches are also visible. C, A functional view, in
which the territories of the hepatic lobules are
A shown as regular hexagons (unlike their real
B appearance, which is highly variable). Functionally,
the liver microarchitecture is better considered in
terms of acini extending between adjacent central
venules and divided into three zones as shown.
(A, With permission from Dr JB Kerr, Monash
C Central vein Portal triad: branches of Terminal branches of hepatic University, from Kerr JB 1999 Atlas of Functional
(tributary of hepatic vein) hepatic artery, bile duct arteriole and portal vein
and hepatic portal vein Histology. London: Mosby. B, Courtesy of Mr
Peter Helliwell and the late Dr Joseph Mathew,
Department of Histopathology, Royal Cornwall
Hospitals Trust, UK.)
1
2
3
Classic hepatic lobule Portal lobule Liver acinus Metabolic zonation
within liver acinus