ELECTRIC MANIFESTATIONS OF MUSCLE AND NERVE. 345 Electricity by its action on the electrical organs, just as it produces Motion by its action on the muscles. 424. It is another interesting point of analogy between the action of Muscles and that of Electrical organs, that the former, like the latter, is attended with a change of electric state. In any fresh vigorous muscle, there is a continual current from the interior to exterior, which appears to depend upon the fact that the actions connected with the nutrition and disintegration of its tissue go on more energetically in the interior of the muscle, than they do near its surface, where the proper muscular fibres are mingled with a large proportion of areolar and tendinous substance. During the contraction of a muscle, this current is diminished in intensity, or is even entirely suspended; but it is renewed again, so soon as the muscle relaxes. An electric current has been found to exist also in Nerves; and its conditions are in most respects similar to those of the muscular current. CHAPTER X. FUNCTIONS OF THE NERVOUS SYSTEM. 425. We have now completed our consideration of the Functions of Organic or Vegetative Life; those changes, namely, in the Animal body, which are concerned in the maintenance of its own fabric; and which, although performed in a different mode, and having different objects to fulfil, are essentially the same in character with those which take place in Plants. The first and most striking difference of mode results, as we have seen, from the nature of the food of Animals, which requires that they should possess a cavity for its reception, and a chemical and mechanical apparatus for its digestion (or reduction to the fluid form), in order that it may be prepared for absorption into the vessels. In regard to the absorption of the aliment, and its circulation through the system, there is but little essential difference between Plants and the lower Animals; but in the higher tribes of the latter, we find that a muscular organ having the action of a forcingpump is appended to the system of tubes in which the fluid 346 FUNCTIONS OF ORGANIC AND ANIMAL LIFE. circulates, in order to drive it through them with the requisite certainty and energy. The respiration of Animals, again, is essentially the same with that of Plants; the chief difference being that, in order to secure the active performance of this important function, the higher Animals are provided with a complex apparatus of nerves and muscles, by which the air or water in contact with the aerating surface is continually renewed. And in regard to the functions of secretion and excretion, we have seen that, though there is a wide difference in the form of the organs by which they are executed, they are the same in essential structure; and that the difference in their mode of operation consists chiefly in this, that their products in the Animal are destined to be carried out of the body, instead of being retained within it, as in Plants. 426. In regard to the immediate objects of these functions, also, there is but little essential difference; for in both instances it is the conversion of alimentary materials into living organized tissue. But the ultimate purpose of this tissue is far from being the same in the two kingdoms. Nearly all the nourishment taken-in by Plants is applied to the extension of their own fabric; and hence there is scarcely any limit to the size they may attain. There is very little waste or decay of structure in them, the parts once formed (with the exception of the leaves and flowers) continuing to exist for an indefinite time; this is a consequence of the simply physical nature of the functions of the woody structure, which has for its chief object to give support to the softer parts, and to serve as the channel for the movement of the fluid that passes towards and from them.-The case is very different in regard to Animals. With the exception of those inert tribes which may be compared with Plants in their mode of life, we find that the whole structure is formed for motion; and that every act of motion involves a waste or decay of the fabric which executes it. An energetic performance of the nutritive actions is required, therefore, in the more active Animals, simply to make good the loss which thus takes place; we find, too, that their size is restrained within certain limits; so that, instead of the nourishment taken into the body being applied, as in Plants, to the formation of new parts, it is employed for the most part in the simple repair of the old. Thus we may say that, whilst the ultimate object of Vegetable Life is to build up a vast GENERAL PURPOSES OF NERVOUS SYSTEM. 347 fabric of organized structure, the highest purpose of the Organic Life of Animals is to construct, and to maintain in a state fit for use, the mechanism which is to serve as the instrument of their Functions of Animal Life,-enabling them to receive sensations, and to execute spontaneous movements, in accordance with their instincts, emotions, or will. 427. This mechanism consists of two kinds of structure,the Nervous and Muscular,-which have entirely different offices to perform. The Nervous system is that which is the actual instrument of the Mind. Through its means, the individual becomes conscious of what is passing around him; its operations are connected, in a manner we are totally unable to explain, with all his thoughts, feelings, desires, reasonings, and determinations; and it communicates the influence of these to his muscles, exciting them to the operations which he determines to execute. But of itself it cannot produce any movement, or give rise to any action; any more than the expansive force of steam could set a mill in motion, without the machinery of the Steam-engine for it to act upon. The Muscular System is the apparatus by which the movements of the body are immediately accomplished; and these it effects by the peculiar power it possesses of contracting upon the application of certain stimuli, of which Nervous agency is the most powerful. 428. Although the presence of a Nervous System is the most distinguishing attribute of Animals, yet we do not encounter it by any means universally. For among certain of those classes which possess on other grounds a title to be ranked in the Animal kingdom, it seems beyond a doubt that no nervous system exists; and there are many others in which, if it be present at all, its condition is so rudimentary, that it can take little share in directing the general operations of the organism. The life of such beings, in fact, is chiefly vegetative in its nature; their movements are not dissimilar in kind to those that we witness in Plants; and their title to a place in the Animal kingdom chiefly rests upon the nature of their food, and the mode in which they appropriate.it (§§ 7, 8). This is the case with the Protozoa generally (§§ 128-137), and in a less degree with Zoophytes (§§ 121-127). 429. In proportion as we ascend the Animal series, however, we find the Nervous System presenting a greater and GENERAL PURPOSES OF NERVOUS SYSTEM. 348 greater complexity of structure, and obviously acquiring higher and yet higher functions; so that in Vertebrated animals, and more especially in Man, it is evidently that portion of the organism to whose welfare everything else is brought into subordination (§ 73). And we observe this to be the case, not merely in virtue of its direct instrumentality as the organ of Mind, but also in the intimacy of its relation to the Organic functions, which are placed in great degree under its control. Thus we find that the inlets and outlets to the Digestive apparatus, the mechanism by which food is brought to the mouth and conveyed into the stomach, and that by which indigestible matters are voided from the large intestine, are subjected to its influence; although the act of digestion itself, and the passage of the aliment from one end of the canal to the other, are performed independently of it. So, again, the movements of Respiration, by which the air within the lungs is renewed as fast as it becomes vitiated, are not only effected through its instrumentality, but are placed, for the purposes of Vocalization, as far under the control of the Will as would be consistent with a due regard to the safety of life. Yet among many of the lower tribes of Animals, the ingestion of food and the aeration of the circulating fluid are provided-for by ciliary action alone (§ 45), in which we have every reason to believe that nervous agency has no participation whatever. 430. If, taking the Nervous System of Man as the highest type of this apparatus, we analyse in a general way the actions to which it is subservient, we find that they may be arranged under several distinct groups, which it is very important to consider apart, whether we are studying his psychical 1 functions or those of the lower animals.-1. The simplest mode of its action is that in which an impression made upon an afferent nerve excites, through the ganglionic centre in which it terminates, an impulse in the motor nerve issuing from it, which, being transmitted by it to the muscular apparatus, calls forth a respondent movement. Of this action, which is called reflex, or "excito-motor," and which may be performed without any consciousness either of the impression or of the motion, we have already seen examples in the movements of Deglutition This term is used to designate the sensorial and mental endowments of Animals, in the most comprehensive sense. MODES OF ACTION OF NERVOUS SYSTEM. 349 (§ 194) and Respiration (§ 340).-2. If the ganglionic centre to which the impression is conveyed, should be one through which the consciousness is necessarily affected, sensation becomes a necessary link in the circular chain; and the action is distinguished as consensual, or "sensori-motor." The closing and opening of the pupil of the eye, in accordance with the amount of light that falls upon the retina, together with other remarkable adjustments which are involuntarily made in the working of that wonderful organ, are characteristic examples of this class. In the foregoing operations no mental change higher than simple consciousness of impressions-that is to say, Sensation, with which may be blended the simple feelings of pleasure and pain-is involved. Such would appear to be the condition of the Human infant on its first entrance into the world, before the self-education of its higher faculties has commenced; and such is probably the state of Invertebrated animals generally, whose instinctive actions seem to be referable to one or other of the foregoing classes. 431. But Sensation is the very lowest form of purely Mental action. When the outness or externality of the objects which give rise to our sensations has been recognised by perception, we begin to form ideas respecting their nature, qualities, &c.; and it is in the various processes of association, comparison, &c., to which these ideas are subjected, that our Reasoning faculty consists. Now these processes may go on in great degree automatically, that is, without any control or guidance on our own part, as happens in the states of Dreaming, Reverie, and Abstraction; and they may express themselves in action, as we see in the movements of a Somnambulist, who may be said to be acting his dreams. This form (3) of Nervous activity, which may be termed ideo-motor, seems to be the ordinary mode in such of the lower animals as are governed by Intelligence rather than by instinct (Chap. XIV.); but it is abnormal and exceptional in Man.-With ideas are associated feelings of various kinds, which constitute Passions and Emotions; and these (4), when strongly excited, may become direct springs of action, so powerful as even to master the control of the Will, producing emotional movements. 432. In the well-regulated mind of Man, however, the Will (5) possesses supreme direction over the whole current of thought, feeling, and action: regulating the succession of the |