The readiness with which a physical change, induced in one part of the centre, is propagated to others, vvhether above or below it, is due no doubt to the vesicular matter. An experiment made by Van Deen illustrates this statement. If, in an animal poisoned by strychnine, the cord be divided in its entire length along the median line, leaving only a slight bridge of grey matter, stimuli applied to any part of the surface will exhibit as extensive reactions as if the cord were entire. It is evident that the.only medium of communication between the oppo site halves must be the small portion of vesi cular matter left undiVided.
Impressions conveyed to the cord by the pos terior roots of any of its nerves, may be reflected to the corresponding motor nerves, and cause movement, or may extend irregularly along the posterior horns of grey matter and stimulate the nerves implanted in them, and thus give rise to new sensations, which may be referred to other and even distant parts of the body or to new motions.
The hypothesis under consideration affords us an explanation, more satisfactory than any other, of the pamlytic state of the sphincter ani in brain disease, already referred to, as well as in that of the spinal cord. This muscle is certainly chiefly under the influence of the will. In ordinary cases of diseased brain, where the lesion is confined to one side, the centre of volition is not sufficiently irnpaired to affect its influence upon the sphincter. In graver lesions, however, although the will may still continuE to exert its control upon one side of the body; it loses its power over the sphincter, which it not excitable by any stimulus. In disease oi the spinal cord, there is paralysis of the sphinc. ters if the lesion involve a sufficient portion atilt cord's substance,in whatever region of the cord i may exist. Even when the lesion is situate higl up in the neck, or in the dorsal region, leaviN the lumbar portion perfectly whole,the sphincter will nevertheless be paralysed. In the former instances, the centre of volition in the cranium is diseased ; in the latter, the defect consists in the destruction of the communication of the brain with that portion of the cord in which the nerves of the sphincter muscle are im planted.
An examination of the action of the sphinc ter will show, as has been already noticed, that the anus is kept closed ordinarily by the passive contraction of the muscle itself; but that its active contractions -are mainly excited by vo luntary influence, allowance being made for some slight action evhich may be produced by the stimulus of sudden distension, as in other circular muscles. Now, as a stimulus to sen
tient nerves constitutes no necessary part of any of these actions, it is probable that the motor nerves of the sphincter have little or no connection with the sentient ones; and, conse quently., that muscle is not usually excitable to contraction by a stimulus appled to a sentient surface. lIence, whenever the influence of the will upon the lumbar portion of the cord is suspended, this muscle ceases to act, whether a mental or a physical stimulus be exerted.
We have remarked before that all that is shown by Dr. Hall's experiments on the horse and on the turtle is that the spinal cord influ enced the sphincter only whilst it was in a state of irritation consequent upon its division. There probably was no real reflex action at all, and the closure of the anus on the! application of a stimu lus was probably only apparently due to that cause, frequent contractions taking place in the muscle in effect of the irritated state of the cord.
On the same principle, animals will exhibit movements of voluntary character for some time after decapitation, the continued irritation of the cord acting as a stimulus. A bird thus treated will fly for some distance, and with considerable energy, and will flap its wings if the cut surface of the cord be irritated. A fly decapitated pursues its course for some way immediately after the removal of the head ; and Walchenaer observed a singular fact respecting the Ccrccris ornata, a wasp which attacks a bee that inhabits holes : " at the moment that the in sect wa.s forcing its way into the hole of the bee, Walckenaer decapitated it; notwithstanding which, it continued its motions, and, when turned round, endeavoured to resume its posi tion and enter the hole." The change in the vesicular matter of the ganglia necessary for the movements of the wasp in pursuit of its prey, had already been excited by a powerful stirnulus of volition, which continued even after the removal of the centre from which it had emanated. Actions at first voluntary, which by frequent repetition become habitual and involuntary, are, no doubt, to be accounted for by the persistence of that condition of the vesicular matter which the will at first induced, and to which the frequency of repetition gives a. character of permanence. Thus Ilabit is due, as it were, to the fixation of a certain state of vesicular matter—it is the conversion of a nien tal into a physical nervous action by frequent repetition.