Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/30

20 PHYSIOLOGY of an electrical nature. We cannot indeed distinctly prove that any chemical change does really accompany nervous activity ; but from many considerations it is extremely probable that a chemical change, an explosive decomposition of more complex into more simple substances, is the basis of a nervous impulse. The energy, however, which is set free by this decomposition is not discharged from the nervous substance to so great an extent as is the case in the muscle-fibre, but is largely consumed in conveying or con ducting the decomposition from one particle of the nervous substance to succeeding ones, that is to say, in carrying out that which is the essential work of the nervous sub stance. Moreover, even in a muscle, while the explosion of inogen and the consequent shortening and thickening of the muscle-substance travel along the fibre from particle to particle in the form of a wave starting from the junction of the nerve -fibre with the muscular fibre, or from some other point of stimulation, this wave of visible con traction is preceded by invisible molecular changes also travelling along the fibre in the form of a wave, changes which manifest themselves by no massive explosions, which are indicated by electrical phenomena chiefly, and which are exceedingly like the nervous impulses of proper nervous structures. In the very substance of the muscular fibre there appears to be a material which is not inogen, but which is capable of undergoing changes, probably of the nature of an explosive decomposition, and it is these which in turn induce the more massive decomposition of the inogen. It is possible, indeed probable, that the con stituent particles of inogen are not able to communicate their explosions to each other, so that the presence in the muscular fibre of an impulse -carry ing material is a neces sity. Be this as it may, a change antecedent to the explosion which is the cause of the actual contraction does occur in every particle of the muscle which contracts, and, as we have urged, the change is probably one taking place in a special substance. This substance may be the verit able protoplasm itself of the fibre, but considerations analogous to those urged before would lead us to suppose that it too is a katastate, but a katastate different in qualities from inogen ; and we may furthur suppose that a very similar katastate is manufactured by nervous proto plasm, and by its decomposition gives rise to nervous energy. This katastate is, as it were, the fuse or trigger whose action fires the massive charge of the muscular gun, and might receive the name of "apheter." If we accept this view as to the nature of the simple nervous impulses which sweep along nerve-fibres and in this respect motor and sensory nerves would seem wholly alike there is no great difficulty in extending the con- nervous ception to the more complicated processes taking place in system, the central nervous system. An ordinary reflex act, so called, is perhaps one of the simplest labours of that system, and we have evidence that in a reflex act sensory impulses arriving along a sensory fibre at the protoplasm of a nerve- cell induce in that protoplasm changes which, though in certain respects differing from, are fundamentally analogous to, those changes in motor and sensory nerve-fibres which constitute their respective nervous impulses. The chief difference is that, whereas along sensory and motor fibres the impulses pursue an even course, possibly undergoing some augmentation, but one which is not sufficiently marked to be beyond doubt, in the nerve-cell, on the contrary, an unmistakable augmentation accompanied by a systematic dispersion takes place. So great is the augment ation in some cases that a gentle short series of sensory im pulses reaching one of a group of nerve-cells along a single sensory fibre may throw the whole group of nerve -cells into such profound agitation that repeated series of even violent impulses may be discharged along a multitude of Meta bolic changes in cen- motor fibres. Allowing for this increase in the energy set free, the changes in the nerve-cells do not seem to differ fundamentally from those in the nerve-fibres (which indeed differ to a certain extent among themselves), and may, like them, be regarded as due essentially to the decomposi tion of some katastate or katastates. Further, just as the apheter of the muscular fibre, that which inaugurates the explosion of the contractile inogen, differs from the apheter of the nerve -fibre, so we may suppose that in the various nerve-cells of different parts of the central nervous system difference of function, while partly due to the mere arrangement and distribution of nervous impulses of the same kind, is also and more largely due to difference in the kind of impulses brought about by difference in the composition and mode of decomposition of the nervous material. For instance, certain changes in the nervous system are accompanied by distinct changes of consciousness, while others are not. And, while we may justly refuse to attempt any explanation of consciousness, Con- it is nevertheless within our right to suppose that, in that scions- nervous substance which is the highest development of ness protoplasm and to whose service the whole body minis ters, amid the many substances of increasing complexity and dignity which enter into its composition there should be a substance or some substances the changes in which are, or may be, accompanied by consciousness. The doc trine of evolution compels us to admit that consciousness must be potentially present in the simple protoplasm of the amoeba, and must be similarly present in all the tissues of the highly-developed animal, instead of being confined to some limited portion of the nervous system. Evolution refuses to admit a sharp line of demarcation between a " conscious " and a " non-conscious " part, and this decision is increasingly supported as our knowledge of the nervous system advances. But a great deal of the earlier part of this article was directed to show that all the powers of the complex animal are the outcome of the differentiation of a primordial protoplasm, while the discussion concerning the molecular changes of tissues in which we are now engaged is simply an attempt to trace out how that differentiation has taken place. And, as far as we can see, there are no just reasons why the differentiation which sets apart the nervous tissue from other parts of the body should not obtain in the nervous tissue itself, and the obscure rudiments of consciousness present in all nervous material become by differentiation developed, in some particular kinds of nervous substance, into consciousness more strictly so called. In the case, then, of secreting cells, of muscular tissue, and of the various forms of nervous tissue the tendency of inquiries into the molecular processes taking place in them is to lead us to regard the varied activities of these tissues as due to molecular disruptive changes in their several katastates, these being various stages of the down ward metabolism or katabolism of protoplasm. Similar considerations might be extended to other tissues of the body which are neither nervous nor muscular, and, though engaged in chemical work, are not distinctly secretory or excretory, such, for instance, as the hepatic cells engaged in the elaboration of glycogen. They might also be extended to those tissues in which the katastates are not exploded and discharged, but retained and massed up in the body for mechanical or other purposes, to car tilage, for instance, the chondrigenous basis or ground- substance which many considerations show to be a product or katastate of protoplasm. We are thus led to the con ception, brought forward in an earlier part of this article, that all over the body protoplasm is continually building itself up out of the pabulum supplied by food, and con

tinually breaking down, giving rise in different tissues and