 the leg from producing any fallacy, the
crural artery should be laid bare, and insulated from surrounding
parts, by passing a thin plate of glass, or sealing wax, between it and
them.

That the flash is the effect of such an increased action of the
vessels, composing the choroid coat, might be somewhat more difficult
to prove. It is however known to every one, that a blow, and that
pressure upon the eye, are capable, as I have before observed, of
producing a similar effect. And the following case, which Bonetus
quotes from Hermannus Cummius, if it may be credited, affords an almost
positive proof, that vision depends upon the stimulus given to the
retina by the activity of blood vessels in some part of the eye.
'Quando theologus, plaga dolorifica, a rupta instrumenti musici chorda
accepta, nocte subsequenti jam adulta, e somno evigilans, cuncta clare,
ac si de die esset, vidit, adeo, ut minimos picturarum et tapetum
tractus observare, characteresque ex libro legere posset. Oculo vero
laeso clauso, tenebras densissimas adesse ille percepit, eodemque iterum
aperto, conclave illustratum visum est, lucem tamen candelae allatae
solisque splendorem de die, aegre tulit oculus affectus, quod per
aliquot dies duravit, tandemque sensim remisit.'

Haller speaks of such cases as by no means uncommon, and quotes the
names of several authors, who have related similar ones.

The direction of this influence, when suffered to pursue its natural
course, appears to be the same with that of most other stimuli, i. e.
from the place at which it first affects a nerve, onwards to the part,
in which that nerve terminates. I have repeatedly caused electrical
sparks to be passed into my own ulnar nerve at its passage over the
inner condyle of the humerus, but both the sensations and the
contractions produced by them have been entirely confined to the hand
and fore arm.

It appears too, both from the experiments of Dr Monro, and of Dr
William Alexander of Halifax in Yorkshire, that when no communication
is left between the trunk and posterior extremities of a frog, except
by its sciatic nerves, a strong solution of opium, injected under the
skin of its posterior extremities, deprives them both of their
sensibility and of their contractile power; but that it does not in the
least affect the trunk of the body. If, on the contrary, it be applied
to the trunk, it exhausts both the trunk and the extremities.

M. Galvani is said to have observed the effects of the influence, which
he discovered, diffused over the whole body of a frog, when the metals
were applied to a nerve merely laid bare, without being either divided
or separated from surrounding parts. If we are allowed to infer this
diffusion of the influence from the restlessness expressed by the
animal, M. Galvani's observation may be just. If from the contractions
produced, I suspect it is by no means so; since, in every experiment
which I have made upon the subject, the contractions have been confined
to those parts to which the nerve touched by the metals was distributed.

That this influence, however, may pass in a direction contrary to the
course of nerves, is evident from some of the experiments which I have
related relative to its effects upon the senses, but is still more
clearly demonstrated by the following.

If, after having divided at the pelvis a frog recently killed, the
sciatic nerves be freed from cellular membrane up to their origin from
the spine, and all the parts below this, except themselves, be cut
away, the muscles on each side of the spine, for some little way up,
may be brought into contraction by touching the nerves alone with the
two metals in contact. This experiment has not always succeeded with
me, and never unless the frog had been recently killed. So long as the
hind legs remain undivided from the nerves, it never succeeded; the
only contractions produced being in the legs.


                         OF THE BLOOD VESSELS.

We are told by Dr Valli, that no contractions are excited by arming the
blood vessels; but as he has not told us whether his experiments were
made upon them while the blood still continued to flow through them, or
after they had been deprived of their blood, I determined to make the
following experiment.

Having laid bare, and separated from surrounding parts and from each
other, the crural artery, and nerve, in the thigh of a full grown frog,
I cut out the whole of the nerve between the pelvis and the knee. I
then insinuated beneath the artery a thin plate of sealing wax, spread
upon paper, and broad enough to keep a large portion of the artery
completely apart from the rest of the thigh. The blood still continued
to flow, through the whole course of the artery, in an undiminished
stream. The artery, thus partially insulated, was touched with silver
and zinc, which were then brought into contact with each other; but no
contraction whatever was produced, in any muscle of the limb. This
experiment was frequently repeated upon several different frogs, both
in whom the nerve was, and in whom it was not, divided. The result was
uniformly the same. But vivid contractions were produced in the whole
limb, when an electrical spark, or even a full stream of the aura, was
passed into the artery.

It, however, by no means follows from this experiment, that the
sanguiferous system of animals bears no relation whatever to the
influence discovered by Galvani. I have already shewn, that the heart
may be affected by it, and have given reason to believe, that the
smallest arteries of the body are not exempted from its action. Should
it ever be proved to be an exclusive property of animals, it is not
impossible but that even its origin may be traced to their sanguiferous
system.

-----

Footnote 11:

  I have not been at the pains to inform myself, who first was the
  author of this doctrine; but its adoption by Caldani, by Haller, and
  by Fontana, and by all upon the faith of experiment, was certainly
  sufficient to give it currency, in opposition to that of Willis,
  Lower, Kaau, Boerhaave, Laghi, and even of the ingenious Whytt.

Footnote 12:

  Mem. Med. Soc. Lond. vol. iii.

------------------------------------------------------------------------


                              SECTION IV.

    _An attempt to investigate the Source from which the respective
            Powers of Nerves, and of Muscles, are derived._


As yet, the question whence the nerves and muscles of animals derive
their respective properties, remains in a state of doubt. By many, the
brain has been considered as the source not only of the several
energies exerted by nerves, whether appropriated to sensation, to the
excitement of muscles subservient to the will, or distributed to organs
exempted from its influence; but likewise of that unascertained power,
by which muscles contract on the application of a stimulus.

By others again, these several properties are supposed to be derived
from the arteries, which may either supply the materials and
construction of that exquisite and peculiar organization, which fits
nerves and muscles for performing their respective functions, or may
furnish, from the blood, some subtile principle, such as that believed
by M. Fontana, to exist there, or such as that we are now examining,
which differently modified in different parts, may be the latent cause
of all the phenomena exhibited by animals.

The advocates for the first opinion observe, that whenever the brain is
considerably injured, or its free communication, by means of nerves,
with moving parts is interrupted, a deprivation both of sense and
motion is the uniform consequence: and, further, that the several
organs, both of sense and of motion, appear to suffer detriment from
the over strained exertions of the brain in thinking, equal to that
which they experience from their own exertions.

The second opinion is countenanced by facts and observations not less
important. From experiments of Haller; some which are recorded in one
of the early volumes of the Philosophical Transactions, and others, it
appears that a paralysis of the posterior extremities of animals was
induced by tying their aorta.

Both Dr Monro and Dr Alexander of Halifax have remarked, that when all
the blood vessels, supplying the posterior extremities of frogs, had
been divided, and a solution of opium injected under the skin of these
extremities, they became, in less than half an hour, both motionless
and insensible; whereas, the fore part of the body was not observably
affected six hours afterwards; and, in Dr Monro's experiments, the
frogs lived till the day following. Hence Dr Monro concludes, 'that
concomitant arteries, somehow or other, tune the nerves, so as