Hitherto we have not dealt at great length with the development of the organs whose functions we have been describing, either from the point of view of the embryologist or the evolutionist. Nor have we spent much time upon their gross anatomy. With the nervous system we must proceed rather differently; for to understand how its higher functions can be performed they must be traced from their origin step by step, while their complexity is largely vested in the structure of special organs. The way in which the nervous system was evolved is shown in Diagram 5. Originally, no doubt, the cells of the outer layer, when the latter was in its simplest form—that is to say, only one cell thick, not several, as it is in our skin—would, when influenced in any way directly call forth the activity of the motor cells lying beneath them. (See Diagram 40, Fig. 1.) In Fig. 2, however, we see one cell of the outer layer becoming specialized. It has thrown out a process above the surface of the skin the more readily to catch impressions, and has sent another down into the body the better to distribute them. Diagram 41, Fig. 1 shows the nerve cell at a further stage. The principle is the same, but the cell is removed to a safer place. In Fig. 2 it is not exposed to the outside world at all, but by receiving its impulses second-hand from several cells the same work is done with greater economy and uniformity. Some of the special sense organs are still developed in this way. [Illustration: DIAGRAM 40.—SHOWING ORIGIN OF A NERVE CELL.] [Illustration: DIAGRAM 41.—SHOWING THE DEVELOPMENT OF A NERVE CELL.] Once the nerve cell is developed and safely shifted into the interior of the body, it is clothed with a protecting feltwork of connective tissue, and the nerve fibres are also surrounded by connective-tissue cells which secrete around them the fatty substance which makes nerves look white. Such is the nerve cell or intermediary between the world and the muscles; but thence to harmonious movement in a body with complex organs capable of varied actions is a long step. To obtain precision and uniformity throughout the body, all the impressions received must be collected and balanced, and stimuli, the correct outcome of this balancing, must be transmitted to the muscles, glands, etc., whose activity circumstances require. The way in which cells of the outer layer become enclosed to form a central nervous system is shown in Diagram 5; but its development will be better seen in the figures of Diagram 42. [Illustration: DIAGRAM 42.—TO ILLUSTRATE THE DEVELOPMENT OF THE NERVOUS SYSTEM.] [Illustration: DIAGRAM 43.—CROSS-SECTION OF THE SPINAL CORD, SHOWING HOW IT GIVES OFF NERVES.] [Illustration: DIAGRAM 44.]