All these facts in the physiological history of the spinal cord lead unequivocally to the following conclusions respecting its office:— 1. that the spinal cord (that term being used in its simple anatomical sense, the intra-spinal mass) in union with the brain is the instrument of sensation and voluntary motion to the trunk and extremities; 2. that the spinal cord may be the medium for the excitation of rnovements independently of volition or sensation in parts supplied by spinal nerves, either by direct jrritation of its substance, or by the influence of a stimulus conveyed to it from some surface of the trunk or extremities by its nerves distri buted upon that surface.
(ff the physical nervous actions of the cord.— We must pause here to make a more extended reference to those actions of the spinal cord which are capable of being excited by peri pheral stimulation, and which are independent of mental change. There is no point in the physiology of the nervous system of more inte rest or importance than this, inasmuch as these actions are not limited to the cord, but take place in other portions of the cerebro-spina centre, in which nerves are implanted, and even in ganglions from which nerves take their rise.
The existence of a class of actions like thesei has long been known to physicians and physic)" logists. By the name of sympathetic actions they excited great interest as to the mode o their production. And anatomists explored the frequent and often intricate anastomoses of nerves in their peripheral distribution with the hope of finding in them some clue to the explanation of these phenomena.
To these actions I prefer to apply the name physical nervous actions to mark their peculiar characteristic, namely, independence of the mind, and to denote that they are the result of a physical change produced by a physical im pression, and therefore, in their causation, wholly independent of mental influence. The term es cito-motory has been applied to them by Dr. Hall. To this term, however, there appear to me to be several serious objections. First, this ten-n implies that the excitation of motion takes place in no other way than by a mechanism similar to that by which these movements are produced. Secondly, it denotes the existence of a peculiar excito-motory power different from the ordinary vis nervosa, the agent in all ner vous phenornena. As if this force were not capable of being roused into action at one time by a mental stimulus, at another by a physical stimulus, or at a third by a mental and phy sical stimulus united. Persons get into the habit of using the terms "excito-motory power," " excito-motory phenomena," as if this power, or these phenomena were something gime pe culiar, quite sui generis, and limited to a spe cial part of the nervous system, losing sight of the real truth that they differ froin voluntary actions only in their mode of excitation, that is, by a physical and not by a mental stimulus.
Thirdly, it limits the reflecting power of the nervous centres (i. e. the propagation of the change induced by the application of a physical stimulus at the periphery) to reflection frozn sensitive to motor nerves. Now there are many facts which shew that reflection may take place from a sensitive to another sensitive nerve, and many of the phenomena of sympathy admit of no other explanation excepting on this prin ciple. And I am by no means prepared to affirm that reflection may not take place from motor to sensitive nerves, or even from motor to other motor nerves, under circumstances of an exalted polarity of the nerves and the centres. Fourthly, some of these so-called excito-motory phenomena have nothing to do with muscular action. Take, for example, erection of the penis: it has not been shown that muscular fibres take any part in the production of this phenomenon, or that the stimulus vvhich gives rise to it does more than create a change in the vessels of the penis, which seems due to muscular relaxation rather than to muscular contraction. The exci tation of a gland to secrete by stimulating some surffice connected with it, as the mammary gland by stimulating the nipple, is no doubt a phenomenon of the same kind, but not one in which muscular fibres are excited to contract.
The term " reflex actions," in accordance with Prochaska's view of the reflecting power of the nervous centre, is objectionable inas much as it fails to denote fully the physical character of the phenomena ; and, moreover, it is applicable only to a class of the actions in question, those, namely, in which the excitation of a motor or sensitive nerve takes place through the primaty excitation of another motor or sensi tive nerve. Either this term, however, or that which I have proposed, may be used without in convenience to science because they involve no particular theory-, and yet sufficiently express some leading feature of the phenomena, reflec tion at the centre, in the one case—a physical exciting cause of a phenomenon purely physical in the other. It may be objected to the term " physical nervous action " that the actions produced by the mental stimulus are equally physical in their intrinsic nature. When, how ever, the term is habitually used in contrast with " mental nervous action," all pmctical difficulty or objection vanishes--both are physical pheno mena,—but one is physical in its essence and also in its exciting cause ; the other is physical in its essence, but mental in its cause. The term physical nervous actions may be regarded as a generic expression for all those nervous phenomena in which the mind takes no neces sary share ; reflex actions being a specific term denoting those physical nervous actions of which reflexion at the centre is a prominent character. In this sense I shall use these terms respectively.