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].
Diagram 42.—To illustrate the Development of the Nervous System.
Diagram 43.—Cross-section of the Spinal Cord, showing how it gives off Nerves.
Diagram 44.