Viewed by the light of this and other allied experiments, the variation found in the state of nutrition of paralyzed limbs is easily accounted for. In cerebral paralysis-the muscles are still subject to contraction in obedience to reflected stimuli through the spinal cord, while in the complete spinal palsy and that arising from disease of the nerves, they are never excited to action ; whence their firmness in the former compared with their impoverishment in the latter case. In the paralysis of the lower limbs so graphically described by Pott, and resulting from disease propagated to the cord from the vertebra:, the early symptoms are those of irritation, and consist rather of irregular con tractions, probably in part reflex, and which the patient is unable to control, than of any diminution of actual power in the limbs ; and it is constantly remarked that in this stage there is no loss of size in the affected parts, but rather that in the midst of a general emaciation consequent on the patient's confinement, these limbs retain their fullness, and even appear hypertrophied. Should the malady advance to disorganization of the cord, the muscles cease to be excited. They become dead to all stimuli except such as are topically applied, and being never so stimulated, soon become flabby and wasted. Thus it would appear that the spinal cord in cerebral paralysis serves to keep up contractility in the muscles, not by supplying them with it, as from a source, but by exhaust ing them through the contractions it excites. It is not a charger but an exhauster through its nerves ; and as exhaustion alternating with re accumulation is necessary for healthy nutrition, and healthy nutrition induces contractility, it becomes in such cases an important though indirect agent in the maintenance of that pro perty in the muscles. There can be little doubt that if muscles completely cut off from the nervous centres were submitted to galvanic agency at frequent intervals, they would not decrease in size, and might, if already atrophied, be even augmented in bulk and power ; and perhaps some of the vaunted successes obtained by galvanism and electricity may be explained in this manner.
There appears to be no argument nor esta blished fact on the other side which invalidates the experiment of Dr. Reid, or which does not admit of being explained on the ground which that experiment substantiates ; and the whole question is still further cleared by the singular circumstance that has been often adduced, that foetuses born without brain or cord may have their muscular system developed and active.
If, to what has now been advanced, there be added the evidence before adduced, that this is a property inherent jn the very structure of muscle, and that it is capable of being exerted therein independently of all communi cation with other tissues, it will probably no longer remain doubtful that it is a property belonging to muscle as a tissue, and that it only requires for its perfection that nutrition should be perfect. Whatever interrupts nutri tion interferes with it, and it matters little whether such interruption arise from the want of its own exercise or from deficiency of arterial supply, arising from causes either local or general. Inertness of a muscle, whether the
consequence of diseased nerves or otherwise, will be attended with more or less atrophy and weakness, according to its degree, and to that alone.
For full information concerning the varieties in the intensity of this power, and in its dura bility in muscles after systemic death or after their removal from connexion with the nervous and vascular systems, the reader is referred to the article IRRITABILITY.
I would merely remark in corroboration of the views there maintained, that in the animal series the size of the elementary fibres and the consequent amount of their vascular supply, independently of the more or less arterial quality of the blood, is accurately proportioned to their irritability. Thus Birds, whose irrita bility is most exalted and most evanescent, have the smallest fibres and the most richly supplied with blood, while Reptiles, Fish, and Crustacea, in which the irritability is most enduring, have fibres of large dimensions and provided with a vascular web of small com parative size (fig. 286, art. MuscLn). The same is true as regards the heart compared with the voluntary muscles.
b. Of the stimuli of musele.—The stimuli which induce contraction have been classed into remote and immediate. Properly speaking, the remote stimuli are stimuli to the nerves and not to the muscles : they cause a change in the motor nerves, which are thus made to excite contraction by their immediate and topical action on the muscles. Of these the chief are volition, emotion, and impressions carried by the afferent nerves to the nervous centres and involuntarily reflected thence ; but various diseases and injuries of the motor nerves, either at their origin or in their course, and pressure, heat, chemical substances, electricity, gal vanism, &c. applied to their texture, are to be ranged under the same head.
The nature of the change thus induced in the motor nerves is entirely unknown. There seems, however, no ground for believing that it differs with the particular stimulus which induces it, and certainly a clear distinction ought ever to be drawn between it and its exciting cause. The nerves, when this change is induced in them, occasion the muscles to which they are distributed to contract. The stimulus they give is an immediate one, and is termed the vis nervosa or the nervous stimulus of muscle. It acts topically on the muscular fibre. The other immediate stimuli are physi cal and chemical; they are rarely exerted in the living body, except in the case of the hollow muscles. It has already been stated that trustworthy experiments have lately shown these to be tinder the influence of motor nerves derived from the spinal marrow, but it seems probable that some at least are normally excited to contract by direct stimulation, to one form of which, that of stretching or distension, they are peculiarly liable from their arrangement as investments to cavities. All muscles, however, may be made to contract by physical and chemical stimuli applied to their fibres.