I may now resume the points of this article, and observe, 1st. That the spinal marrow, exclusive of the cerebrum, is the source of the muscular irritability : 2d. That the cerebrum is, in its acts of voli tion, an exhauster of that irritability : 3d. That in muscles separated from their nervous connexion with the brain we have augmented irritability : 4th. That in muscles separated from their nervous connexion with the spinal marrow we have, on the contrary, diminished irritability : 5th. That the degree of the irritability of the muscular fibre of paralytic limbs, compared with that of the muscles of the healthy limbs, will afford us a source of diagnosis between cerebral and spinal paralysis, and especially between 1. Ilemiplegia of the face, and 2. Paralysis of the facial nerve; 3. llemiplegia of the arm or leg, and 4. Disease of the nerves of these limbs ;* 5. Disease of the spinal marrow in the dorsal region, and 6. Disease of the cauda equina in the lumbar region ; &c.
6th. That the greater influence of emotion, of certain respiratory acts, of the principle of tone, &c. on the muscles of certain paralytic limbs than on those of healthy limbs, depends on their augmented irritability : 7th. That the same principle explains the greater susceptibility of the muscles in certain cases of paralytic limbs, to the influence of strychnine : 8th. That, in the conclusions of M. Fouquier, Professor Muller, &c., a sufficient distinction was not made between the influence of the cerebrum and of the spinal marrow, which in this, as in so many other respects, have such different properties: Oth. From these and other experiments and observations, I conclude, too, that sleep restores the irritability of the muscular system, by arresting the acts of volition which exhaust or diminish it; muscular efforts, on the other hand, diminish the irritability and induce fatigue.
Before I conclude, I must beg my reader's attention to some experiments of that able phy siologist, Dr. J. Reid, of Edinburgh, which appear, at first sight, to be contradictory to those which I have just detailed.
Dr. J. Reid's paper is published in the Fourth Report of the British Association for the Advancement of Science, p. 671. It is as follows :— " Although physiologists are still divided in opinion as to the question whether nerves fur nish a condition necessary to the irritation of muscles, (i. e. whether every stimulus which excites a muscle to contraction acts on it through the intervention of nervous filaments,) they have now very generally abandoned the once prevalent theory, that the irritability of muscles is derived from the brain or spinal cord, i. e. that muscles are continually re
ceiving, through their nerves, from those larger masses of the nervous system, supplies of a certain influence or energy, which enables them to contract; and that some of the state ments of Dr. Wilson Philip, in particular, are generally regarded as decisive against this theory.
" Dr. Wilson Philip found by experiment, that the irritability of a muscle of which the nerves were entire, was exhausted by applying a stimulus directly to the muscular fibres (sprinkling salt on them) even more quickly than that of a muscle of which the nerves had been cut, and where all communication with the supposed source of nervous influence or energy had been cut off; and lie states gene rally that a muscle of voluntary motion, if ex hausted by stimulation, will recover its irritabi lity by rest, although all its nerves have been divided.
" But in opposition to this statement, and in support of the old theory of nervous influ ence continually flowing through certain of the nerves into the muscles, it has lately been stated by Mr. J. W. Earle, that when the nerves of the limb of a frog were cut, the skin stripped off, and the muscles irritated by sprinkling salt on their fibres, until they had lost their power of contraction, although they did not lose their power much more quickly than when the nerves were entire, yet they did not regain their power, although left undisturbed for five weeks; while the muscles of the limbs of another frog, simi larly treated, but of which the nerves were left entire, completely recovered their irritability.
" It occurred as a fundamental objection to the experiment of Mr. Earle, that in the case where the nerves had been divided, the mus cles had become inflamed ; being found at the end of the five weeks softer in their texture than natural, a good deal injected with blood, and with some interstitial deposition of fluid in them ;' while in the limb to which the salt had been applied, but of which the nerves were left entire, and where the irritability was recovered, although the colour of the muscles was rather darker than natural, their texture remained un changed, and there was no interstitial deposi tion of fluid in them.' " In these circumstances it might evidently be supposed that it was the inflammation and disorganization of the muscles, not the section of the nerves, which prevented the recovery of the irritability in the case where the nerves had been cut; and it became important to have the experiment repeated, with care to avoid such injury of the limb of the animal as should cause inflammation to succeed the section of the nerves.