Leaving, for the time, our consideration of the cord with this general understanding of the plan of its structure and of its functions, let us turn to the brain. Here again light is thrown on the subject by comparative anatomy and the anatomy of development. Even in some of the least-developed organisms there is a marked difference in size between the ganglion nearest the anterior extremity or head of the animal and the succeeding ganglia. Fig. 91 (p. 141) shows this, where the first pair of ganglia (o) are very large, and are so connected with the succeeding pair (SO as to form a ring. • Through this ring the gullet of the animal passes. This special development in this region is due to the necessity of nervous arrangements for the regulation of the means of introducing food into the body and of special organs, organs of sight, of hearing, tactile organs, and so on, and in these animals this ganglion is compar able in many ways to the brain of man and the higher animals. The anatomical similarity be tween this ganglion in invertebrate animals and the brain of vertebrates is readily seen when one compares such specially-developed ganglia of the highest invertebrates with the brain of the lowest vertebrates. Still more obvious of the human body be studied, its segmented arrangement becomes quite clear. Thus the study of comparative anatomy and of develop ment would lead one to conclude that the spinal becomes the similarity when one studies the de- ' velopment of the human nervous system. For in a very early stage of human development the nervous system consists of a straight tube of nervous matter miming down the back por tion of the body, that is, a spinal cord merely ; and in the lowest vertebrate— the lancelet or am phioxus—there is no more than this, the anterior end of the cord being slightly swollen, represent ing the barest rudi ment of a brain, being connected with a rudimentary eye and olfactory organ. As the de velopment of the human embryo proceeds, the an terior end of the spinal tube enlarges into a bladder-like growth, which by constriction in two places becomes marked off into three, and these subsequently, by constriction of the first and third, into five little bladders or vesicles. These are the rudiments of what become subsequently developed into the fully-formed brain by ex pansion of parts and by growth of new material and thickenings in their walls (Fig. As growth goes on, the tube is encroached upon until only a fine canal remains, still traceable in the fully-formed brain. At first the nerve tube is quite straight, but as development goes on it becomes bent, and the parts of the brain become folded upon one another, certain parts undergoing such rapid development OA to over lap and cover other parts, so that at last the exceedingly complicated brain is produced, the relations of the parts of which it would be im possible to determ ine unless one were acquainted with its mode of development (Fig. 95). The brain, that is to say, is really an outgrowth from the spinal cord, constructed, to begin with, on the same type, consisting of ganglia with connecting fibres and with their respective in going or afferent and outgoing or efferent nerve tracts. This extension of the spinal cord is made to meet, in the first place, the demand for largely augmented nerve arrangements to supervise the great elaboration of structure and function that takes place within the very limited area of the head, that elaboration being con nected with the varied and delicate movements of the face, of the jaws and tongue, and with the development, to a very high state of per fection, of special organs of sense connected with sight, hearing, smelling, taste. And in deed the cranial nerves, with the exception of the optic and olfactory nerves, are exactly com parable to the nerves coming from the spinal cord, the primitive type of which we have seen to be an afferent and efferent fibre connected with a nerve-cell in a ganglion. The ganglia of the cranial nerves, however, are not arranged in the brain in the same obvious orderly series as those of the spinal nerves, but become scat tered and divided up by the great development of nerve structure that occurs in the brain.
To complete our physiological view of the building up of the brain certain other things must now be taken into account.
The process termed reflex action, which has been explained (p. 132), is sufficient to account for the various movements and changes which occur in the lowest organisms. Certain modi fied structures in or on the external wall of the animal are affected by changes in the medium in which the animal lives, and impressions are produced on them which, transmitted through the medium of nerve-cells, lead to movements of the animal. Similarly, changes in one part of the body of the animal lead by reflex action to changes in another part, and the close con nection of nerve-ganglia with one another by commissural fibres ensures the co-operative action of the body as a whole. This does not imply any conscious perception, on the part of the animal, either of the stimulus applied to a part of the body or the change which it evokes. But in the higher animals this con sciousness exists. Consciousness is undoubtedly a function of nerve structure, requiring a me chanism of nerve-cells, however impossible it is to understand how the activity of nerve-cells becomes transformed into a mode of conscious ness, and it seems certain that some part of the brain is associated with consciousness. A blow on the head, or some disorder of the brain, may deprive a man of consciousness for the time, but still the functions of animal life go on, and reflex actions occur, showing that, while the functions of seine of the higher ganglia are in abeyance, the remainder are unaffected. One
might say that the blow or brain disorder had temporarily reduced the man to the condition of some of the lower animals. Here then, in the higher animals and man, is a new function, provided for by a special development in the brain. Now notice carefully what this implies. If an impression be made on some part of the body of a conscious animal, that impression would affect an afferent nerve and would pass, in the first place, to its connected ganglion, let us say in the spinal cord. If no interference occurred, it would excite a change in the gan glion in the cord, as the result of which an im pulse would be discharged from the ganglion down an efferent nerve, and sonic movement or other change would result. But the animal is conscious of the impression, and the nerve centres for consciousness are situated in the brain. This implies that the impression, be sides reaching its proper ganglion in the cord, and setting agoing there the changes referred to, must have travelled up the cord along affer ent fibres and reached a higher centre—the centre for consciousness in the brain. In the spinal cord of the higher animals and man, then, besides groups of nerve-cells constituting gan glia, and their commissural fibres, with the afferent and efferent nerves of each ganglion, there must be afferent tracts passing upwards to other ganglia in the brain, and these must be related somehow to the afferent nerves pass ing to the cord, and to the ganglia of the cord to which they pass.
Let us go a step further. We have said that reflex action is sufficient to account for the movements made by, and changes occurring in, the bodies of lower organisms. If such organ isms were subject to no external impulses what ever, it is safe to assume they would remain motionless. But in more highly-developed ani mals changes are initiated by the animal itself, apart from any external impression, as well as in consequence of such external impressions. The animal, that is to say, is endowed with volition, or will, in a more or less developed form. This new capacity also requires a ner vous mechanism which is situated in the brain, and by destruction or removal of a certain portion of the brain this capacity is lost, and the animal is reduced to the condition of a lower organism, requiring an external stimulus to set reflex action in operation and produce Movement. For instance, if a frog be deprived of its cerebral hemispheres, "unless disturbed by any form of peripheral stimulus, it will sit for ever quiet in the same spot, and become converted into a mummy. All spontaneous action is annihilated. Its past experience has been blotted out, and it exhibits no fear in circumstances which otherwise would cause it to retire or flee from danger. . . . Surrounded by plenty it will (lie of starvation; but, unlike Tantalus, it has no psychical suffering, no de sire, and no will to supply its physical wants (Ferrier). Now the meaning of this is that, in such more highly-organized animals, certain centres exist in the brain which have the power of initiating impulses leading to movement. This implies that from these centres there pro ceed efferent nerve-fibres which, passing through the brain, descend the spinal cord, conducting impulses which, issuing from the cord, pass to muscles, exciting them to movement. It can be shown that these fibres descending from the brain are in some sort of communication with the ganglionic cells of the spinal cord, through which cells, indeed, the movements are effected. Thus the spinal cord consists of nerve ganglia united by commissural fibres. Each ganglion presides over a limited part of the body, receiv ing impressions from that part by afferent nerves, and exciting changes in that part by discharging impulses down efferent nerves, in response to the excitation brought by the affer ent nerves. But in communication with the afferent nerves are other afferent fibres, which pass up the cord and carry the impression to centres in the brain, so that a consciousness of the impression arises. Also in communication with the ganglia are efferent fibres descending from centres in the brain, associated with voli tion. Thus, from the spinal cord ganglia, motor impulses may issue, either in consequence of an impression brought by an afferent nerve— that is reflexly—or in consequence of an im pulse descending from the brain, as the result of an effort of will, initiated, that is to say, by the animal itself. Finally, to complete our general view, the higher animals are endowed with more or less intelligence, which in man reaches its highest development, including the faculty of memory, judgment, reason, imagina tion. These all require some nervous mechan and, humanly speaking, are inseparable from the activity of nervous structure, of nerve-cells. Only in a very rough way can the region of the brain, which is the seat of these operations, be indicated. Just as we have seen that in lower organisms the manifestation of higher functions is associated with increased complexity of structure, with increase in nerve ganglia and their connected fibres, so are the beginnings of these higher mental operations associated with increased development of brain structure. The gradual evolution of these higher functions to more complete manifestations, as one passes from one animal to another higher in the scale, is accompanied by increase in brain structure and complexity. Upon the degree of development of the cerebral hemispheres de pends the intellectual condition of the animal throughout the whole animal kingdom.