Let us now summarize the knowledge we have acquired of the functions of the central nervous system in man and the higher animals.
The spinal cord consists of a series of masses of ganglionic nerve-cells, disposed longitudin ally and showing a bilateral arrangement. The masses are so close together as to be more or less fused, and are, besides, intimately connected with one another by communicating fibres. One or more ganglia presides over the functions of a part of its own lateral half of the body. With their own part the ganglia are connected, by afferent fibres carrying impulses to the ganglia from that part, and by means of efferent fibres impulses are discharged from the ganglia to that part, which regulate the changes in muscle, vessels, &c., in the part, the ganglia being by this means reflexly excited to action. By the commissural fibres impulses conveyed by afferent fibres to one ganglion may extend to other ganglia, and lead to changes affecting other and more distant parts. The spinal cord, further, contains fibres which carry afferent impulses upwards to the brain, and these are in some sort of communication with the afferent nerves of the ganglion, so that the afferent impulse, besides rousing a reflex action, may reach the brain and become a conscious percep tion, may give rise to some kind of sensation.
The cord also contains fibres which descend from centres in the brain, conveying efferent impulses to the ganglia, which excite them, and thus changes are brought about initiated by cells in the brain, not involving a act.
The brain is partly a development of the same system that exists in the cord, having ganglionic masses with related afferent and efferent nerves, specially connected with the head and face and organs of speech. In it are developed, besides, ganglia related to the special senses of vision, hearing, taste, and smell. There also are developed the nervous mechanisms associated with feeling, thinking, willing, &c., and certain portions of its structure are more or less directly connected, by strands of fibres, with the afferent and efferent parts of the cord. By means of these strands the brain becomes a controlling influence to the reflex centres in the cord, consciously perceiving an afferent im pulse which otherwise would only unconsciously excite a reflex act, and consciously initiating movements and other changes in the body, through the medium of the centres in the cord, which otherwise could only be called into action by a reflex stimulus.