On the mechanism of the functions of Me cord.—Ilaving shown that the spinal cord is concerned in voluntary motions and in sensa tion, (mental nervous actions,) and in certain reflex actions as well as in certain org,anic functions, (physiall nervous actions,) it is portant to ascertain what is the mechanism by which these various actions take place.
The most convenient way to discuss this point will be to examine into the value of cer tain hypotheses which have been framed to explain it. We shall find it necessary in this discussion to keep before us two propositions in favour of which sufficient evidence has al ready been adduced. These are, 1. That the brain or some part of it is essential to the production of mental nervous actions; in other words, that acts of volition and sensation cannot take place without the brain : and, 2. That the vesicular is the truly dynamic nervous matter, that which is essential to and the source of the developement of all nervous power.
The first hypothesis which we shall notice is one of so much ingenuity that one is tempted thereby to adopt it, and would gladly do so if it were found sufficient to explain the pheno mena, and if it were consistent with that sim plicity which characterises the mechanism of the body. It originated with Dr. Marshall Hall, and has been advocated by him with great zeal and ability; it may be distinguished as the hypothesis of an excito-motery system of nerves, and of a trite spinal cord, the centre of all physical nervous actions.
Thts hypothesis may be stated as follows.* The various muscles and sentient surfaces of the body are connected with the bmin by nerve fibres which pass from the one to the other. Those fibres destined for. or proceeding from the trunk to the bmin pass along the spinal cord, so that that organ is in great part no more than a bundle of nerve fibres going to and from the brain. • These fibres are specially for sensation and volition —sensori-volitional.
But, in addition to these, there is, according to Dr. another class of fibres proper to the spinal cord and to its intracranial ation, which form a connection with the grey matter of the cord. Of these fibres some are afferent or incident, others efferent or re flex, and these two kinds have an immediate but unknown relation to each other, so that each afferent nerve has its proper efferent one, the former being excitor, the latter motor.
'Die aggregate of these fibres, together with the grey matter, constitutes the true spinal cord of Dr. Hall, which is not limited to the spinal canal, but passes up into the cranium as far as the erttra cerebri. (Its extent, indeed, is intich the same as that which has been assigned by Prochaska to his sensorium commune.) These fibres are quite independent of those of sensa tion and volition and of the sensorium com mune, using that term as indicating the centre of intellectual actions. Although bound up
with sensitive and motor fibres, they are not affected by them, and they maintain their sepa rate course in the nerves, as well as in the centres.* 2. A second hypothesis is that which accords with the views of Miiller and many other phy siologists of the present day, and likewise pro bably with those of Whytt. It assumes that the fibres of sensation and volition proceed to and from some part or parts of the intracranial nervous mass,—that every nerve-fibre in the body is continued into the brain. Those which are distributed to the trunk and extremities pass along the spinal cord, separating from it with the various roots of the nerves, and in their course within the spine mingling more or less with the vesicular matter of the cord. There are, accord ing to this hypothesis, no other fibres but these, (save the commissural,) and they are sufficient to manifest the physical as well as tile mental acts. Nerves of sensation are capable of ex citing nerves of motion which are in their vici nity; and they may produce this effect even when the spinal cord has been severed from the brain, for their relation to the grey matter of the cord is such that their state of excitement is readily conveyed to it.
3. According to a third hypothesis, it is as sumed that all the spinal and encephalic nerves, of whatever function, are implanted in the grey matter of the segrnents of the cerebro spinal cenne with which they are severally con nected, and do not pass beyond them. The several segments of the cerebro-spinal axis are connected with each other through the conti nuity of the grey matter from one to another, and through the medium of commissural fibres which pass between them. Through these means, motor or sensitive impulses may be propagated from segment to segment ; and a stimulus conveyed to any segment from the periphery may either simultaneously affect the brain and cause a sensation, or it may be re flected upon the motor nerves of that segment and stimulate their muscles to contract. Or both these effects may take place at the same moment, as a result of one and the same sti mulus. According to this hypothesis, each segment of the cord, so long as it retains its proper commissural connection with the brain (by commissural fibres and continuous grey matter), is part and parcel of the centre of voli tion as well as of that of sensation, and the mind is as directly associated with each seg ment of the cord as it is with any portion of the encephalon. Let that commissural con nection be dissolved, and the mind will imme diately lose its hold upon the cord ; but the various segments of that organ may nevertheless still be acted upon by physical impulses, and may still continue to evolve the nervous force in connection with the natural changes which may take place within.