All these points afford the highest degree of probability that there is no direct dependence of muscle upon the nervous centres for the developement of its proper force ; and that this force is the result of the nutrient actions of muscle. The only way in which the nervous system can be said to have an influence upon the muscular force is by promoting the actions of the muscles, and thereby their nutrition. If a muscle have its nerves divided, and be left to itself, its nutrition fails after a certain period, and its contractility with it ; but if it be exer cised daily by galvanic stimulation, its nutri tion remains unimpaired, and its contractility likewise.
The tone of a muscle is nothing but the effect of the continuous developetnent of the muscular force resulting from the natural changes in the muscle ; it is this state of ten sion which denotes that these changes are actively proceeding, and that a uniform degree of attraction is being exerted between all the parts of the muscular fibre, in a degree propor tionate to their masses, arid that by this the muscles are maintained in a uniform state of tension so long as they are undisturbed by sti muli conveyed to them through the nervous system, or from some other source.
It seems, therefore, as reasonable as any pro position in physiology, to affirm that the passive contraction or tone of muscles is due to a pro perty inherent in the muscular tissue itself, and depentient solely on its proper nutrition, and that it is not derived from any other tissue. And if this be true, it is clear that the spinal cord cannot be the source of the tone of the muscular system.
This statement is confirmed by the result of the experiment of removing the whole spinal cord in frogs or other animals. When this has been done, the limbs of the animal fall quite flaccid, the °muscles being no longer capable of preserving that degree of active contraction which is necessary to maintain attitude. A decapitated frog will continue in the sitting.
posture through the influence of the spinal cord, but, immediately this organ has been removed, the limbs fall apart from the loss of the controlling and co-ordinating influence of the nervous centres. And careful exatnination of the muscles in such a case as this will show that the molecular phenomena which charac terise passive contraction continue in the mus cular tibres. The state of rigor mortis, which is analogous to that of tone, comes on just as readily in animals which have been deprived of the bmin and spinal cord, as in those in which these centres ha% e been undisturbed before death. In short, healthy nutrition sup plies all the conditions necessary' for the main tenance of tone or passive contraction ; nor is the spinal cord (although itself healthy) able to preserve the tense condition of the muscles, if they are not well nourished.
These remarks apply equally to Dr. IIall's doctrine, that the spinal cord is a direct source of irritability to the muscular system. The same arguments which prove that tone is not derived from it are of equal weight with refe rence to irritability.
It cannot be admitted as an argument in favour of the view which derives muscular irri tability from the spinal cord, that muscles lose their firmness and waste, when they have been for some time separated from their proper ner vous connections. They suffer, in this Nvay, merely for Nvant of a proper amount of exercise, which they cannot obtain in consequence of the influence of the will being cut off from the limb. If, however, the paralysed limb be ex ercised artificially, as by the galvanic current, their nutrition and their plumpness may be preserved. For this important observation we are indebted to Dr. John Reid, who likewise called attention to the confirmatory fact, that, in those palsies with which there is combined more or less of irritation of the nervous centre, the muscles do not suffer so much in their nutrition, in consequence of the exercise they undergo in the startings so frequently excited in them by the central irritation. This is not unfrequently seen in cases of paraplegia from irritant disease of the spinal cord.
The supposition that the spinal cord rnig,ht be the source of irritability to the muscles led Dr. llall to the very extraordinary inference, that in hemiplegic paralysis, in which the in fluence of the brain is cut off from certain muscles, while that of the cord retnained, the irritability of those muscles becomes augmented. Ile arrives at this conclusion by the following line of argument: assuming the cord to be the source of the irritability of the muscles, the brain may then evidently be looked upon as the exhauster of that irritability in the volun tary actions; if, then, the influence of the brain be cut off, it naturally follows that, as the great agent of exhaustion has lost its power, the irritability, which is ever, as it were, flow ing from the cord, will accumulate in the muscles. From numerous experiments I am enabled to state that in nearly all the cases of hetniplegic paralysis from cerebral lesion there is no evidence of any augmentation of the irritability of the muscles of the palsied limbs. If the readiness with which they will respond to the galvanic stimulus be taken as a test, it may on the other hand be stated very confidently that there is evidence of the di minution of the irritability of the paralysed muscles, for in nearly all these cases the same current being passed through both sound and palsied limbs at the same time, the latter have contmcted either not at all or with very little power as compared with the healthy limbs.