THE FUNCTIONS OF THE CENTRAL NERVOUS SYSTEM.
To disentangle the fully-developed human nervous system, or that of any of the higher animals, in order to determine the relations and duties of the various cells and fibres of which, as we have said, it is built up, would he an impossible task. The structure is so delicate, the weaving and interweaving of fibres are so complex, the orderly disorder of cells so great, that the destruction of the tissue is the main result of any attempt to unravel it. Nevertheless various methods of inquiry and experiment yield a fairly consistent general view of the relations and functions of the different strands of fibres and groups of cells, though, to render one's know ledge full and exact, innumerable de tails remain yet to be discovered.
The study of the development of the nervous system, from the lowest or ganism possessing any nerve structure at all up to man, and the gradual increase in its com plexity from below upwards, attended, as such increase of complexity always is, by greater elaboration of function, throws much light upon the problem. The study of the development of the nervous system in one kind of animal has of recent years yielded much valuable infor mation, since it is found that certain strands of fibres in the spinal cord reach their full development sooner than others. The exa mination of the spinal cord of an animal in a stage of its development will show certain fibres fully formed, others only in process of formation, and the course of the fully-formed fibres can therefore be traced through the cord and distinguished from those of the less fully formed fibres. Further, the spinal cord can be removed from the body of a newly killed animal, can be hardened by various reagents so as to stand being cut into slices—sections, as they are called—of extreme tenuity, and stained by pigments of various hue. All the fibres are not alike when viewed under the microscope, but vary in size and in other respects, so that by cutting a continuous series of sections, staining them, and mounting them on glass slides in proper order, groups of fibres can be traced by their microscopic characters from one section to another onwards in the cord.
Experiment yields much information. Thus, if the destruction of a limited area of the cord is followed by loss of power in certain groups of muscles, the conclusion is obvious that the fibres in that part of the cord conduct impulses leading to the movement of these muscles; or if stimulation, say by an electric current, of a definite limited region on the surface of the brain in a living animal always leads to the action of the same muscle or group of muscles, the conclusion is obvious that that area of brain is related to the particular movement.
This conclusion would be strongly confirmed if destruction of that particular area were always followed by the loss of that particular movement and by no other obvious effect.
Nature, unfortunately, "so careless of the single life," is prone to experiment by the agency of disease with far greater frequency and freedom than the experimental physiolo gist or pathologist, and the examination, after death, of the brain, cord, and nerves of a per son who has died of some nervous disorder, and the relation of the alterations found in the diseased structure with the symptoms observed during life, have been prolific of information.
By collecting and comparing all the facts derived from these various methods of obser vation and inquiry, our present knowledge of the connections and functions of the nervous system has been built up.
What the nature of that knowledge is an attempt will be made to set forth briefly and with as little elaboration and detail as possible, anything beyond a very broad and general view being impossible in a treatise such as this.
The earliest indication of a nervous struc ture is found in that class of animals to which, among others, the polyps and medusa belong (Ccelenterata). The hydroid polyp is tubular in shape, and consists of an outer and an inner layer of cells, between which is a middle layer of modified cells. If any part of the outside of the animal be touched it responds by mov ing. ThiS response to an external stimulus is not in itself proof of the existence of a nervous structure, for apparently undifferentiated pro toplasm will . so respond. But microscopical investigation has shown that certain of the cells of the outer wall are peculiarly modified on their outer or exposed ends, and that from their inner deep ends processes run in to the middle layer. It is believed that these peculiar cells are fitted to receive an impression from contact with an external body, which impres sion they transmit by the deep processes to the middle layer, the result of which is con traction of the middle layer and the movement of the body.