but in humans it is not known to have any significant function. reproductive system main article: list of homologues of the human reproductive system genitalia in the internal genitalia of each human sex, there are some residual organs of mesonephric and paramesonephric ducts during embryonic development: gartner's duct epoophoron vesicular appendages of epoophoron paroophoron human vestigial structures also include leftover embryological remnants that once served a function during development, such as the belly button, and analogous structures between biological sexes. for example, men are also born with two nipples, which are not known to serve a function compared to women. in regards to genitourinary development, both internal and external genitalia of male and female fetuses have the ability to fully or partially form their analogous phenotype of the opposite biological sex if exposed to a lack/overabundance of androgens or the sry gene during fetal development. examples of vestigial remnants of genitourinary development include the hymen, which is a membrane that surrounds or partially covers the external vaginal opening that derives from the sinus tubercle during fetal development and is homologous to the male seminal colliculus. some researchers have hypothesized that the persistence of the hymen may be to provide temporary protection from infection, as it separates the vaginal lumen from the urogenital sinus cavity during development. other examples include the glans penis and the clitoris, the labia minora and the ventral penis, and the ovarian follicles and the seminiferous tubules. in modern times, there is controversy regarding whether the foreskin is a vital or vestigial structure. in 1949, british physician douglas gairdner noted that the foreskin plays an important protective role in newborns. he wrote, "it is often stated that the prepuce is a vestigial structure devoid of function ... however, it seems to be no accident that during the years when the child is incontinent the glans is completely clothed by the prepuce, for, deprived of this protection, the glans becomes susceptible to injury from contact with sodden clothes or napkin." during the physical act of sex, the foreskin reduces friction, which can reduce the need for additional sources of lubrication. "some medical researchers, however, claim circumcised men enjoy sex just fine and that, in view of recent research on hiv transmission, the foreskin causes more trouble than it's worth." however, recent canadian studies on circumcision & hiv risk have thrown this conclusion into question the area of the outer foreskin measures between 7 and 100 cm2, and the inner foreskin measures between 18 and 68 cm2, which is a wide range. regarding vestigial structures, charles darwin wrote, "an organ, when rendered useless, may well be variable, for its variations cannot be checked by natural selection." musculature a number of muscles in the human body are thought to be vestigial, either by virtue of being greatly reduced in size compared to homologous muscles in other species, by having become principally tendonous, or by being highly variable in their frequency within or between populations. head the occipitalis minor is a muscle in the back of the head which normally joins to the auricular muscles of the ear. this muscle is very sporadic in frequency—always present in malays, present in 56% of africans, 50% of japanese, and 36% of europeans, and nonexistent in the khoikhoi people of southwestern africa and in melanesians. other small muscles in the head associated with the occipital region and the post-auricular muscle complex are often variable in their frequency. the platysma, a quadrangular (four sides) muscle in a sheet-like configuration, is a vestigial remnant of the panniculous carnosus of animals. in horses, it is the muscle that allows it to flick a fly off its back. face in many lower animals, the upper lip and sinus area is associated with whiskers or vibrissae which serve a sensory function. in humans, these whiskers do not exist but there are still sporadic cases where elements of the associated vibrissal capsular muscles or sinus hair muscles can be found. based on histological studies of the upper lips of 20 cadavers, tamatsu et al. found that structures resembling such muscles were present in 35% (7/20) of their specimens. arm the palmaris longus muscle is seen as a small tendon between the flexor carpi radialis and the flexor carpi ulnaris, although it is not always present. the muscle is absent in about 14% of the population, however this varies greatly with ethnicity. it is believed that this muscle actively participated in the arboreal locomotion of primates, but currently has no function, because it does not provide more grip strength. one study has shown the prevalence of palmaris longus agenesis in 500 indian patients to be 17.2% (8% bilateral and 9.2% unilateral). the palmaris is a popular source of tendon material for grafts and this has prompted studies which have shown the absence of the palmaris does not have any appreciable effect on grip strength. the levator claviculae muscle in the posterior triangle of the neck is a supernumerary muscle present in only 2–3% of all people but nearly always present in most mammalian species, including gibbons and orangutans. torso the pyramidalis muscle of the abdomen is a small and triangular muscle, anterior to the rectus abdominis, and contained in the rectus sheath. it is absent in 20% of humans and when absent, the lower end of the rectus then becomes proportionately increased in size. anatomical studies suggest that the forces generated by the pyramidalis muscles are relatively small. the latissimus dorsi muscle of the back has several sporadic variations. one particular variant is the existence of the dorsoepitrochlearis or latissimocondyloideus muscle which is a muscle passing from the tendon of the latissimus dorsi to the long head of the triceps brachii. it is notable due to its well developed character in other apes and monkeys, where it is an important climbing muscle, namely the dorsoepitrochlearis brachii. this muscle is found in ≈5% of humans. leg the plantaris muscle is composed of a thin muscle belly and a long thin tendon. the muscle belly is approximately 5–10 centimetres (2–4 inches) long, and is absent in 7–10% of the human population. it has some weak functionality in moving the knee and ankle but is generally considered redundant and is often used as a source of tendon for grafts. the long, thin tendon of the plantaris is humorously called "the freshman's nerve", as it is often mistaken for a nerve by new medical students. tongue another example of human vestigiality occurs in the tongue, specifically the chondroglossus muscle. in a morphological study of 100 japanese cadavers, it was found that 86% of fibers identified were solid and bundled in the appropriate way to facilitate speech and mastication. the other 14% of fibers were short, thin and sparse – nearly useless, and thus concluded to be of vestigial origin. breasts extra nipples or breasts sometimes appear along the mammary lines of humans, appearing as a remnant of mammalian ancestors who possessed more than two nipples or breasts. one 2021 report demonstrated that all healthy young men and women who participated in an anatomic study of the front surface of the body exhibited 8 pairs of focal fat mounds running along the embryological mammary ridges from axillae to the upper inner thighs. these were always located in the same relative anatomic sites – analogous to the loci of breasts in other placental mammals – and often had nipple-like moles or extra hairs located atop the mounds. therefore, focal fatty prominences on the fronts of human torsos likely represent chains of vestigial breasts composed of primordial breast fat. behavioral