Elementary anatomy, physiology and hygiene for higher grammar grades . se organs and passes out as arounded nerve cord, we call it a nerve trunk. Because ofthe insulating material which surrounds the fibers, nervetrunks and nerve tracts are white iii color, and because THE NERVOUS SYSTEM 241 of the protoplasm of which nerve cells are made, thenerve centers have a grayish color. This led the anat-omists and physiologists in times past to classify nervesubstance as gray matter and white matter. 2. THE SPINAL CORD. AND BRAINI. THE SPINAL CORD FiGUEE 56 gives aview of the whole centralnervous syst
Elementary anatomy, physiology and hygiene for higher grammar grades . se organs and passes out as arounded nerve cord, we call it a nerve trunk. Because ofthe insulating material which surrounds the fibers, nervetrunks and nerve tracts are white iii color, and because THE NERVOUS SYSTEM 241 of the protoplasm of which nerve cells are made, thenerve centers have a grayish color. This led the anat-omists and physiologists in times past to classify nervesubstance as gray matter and white matter. 2. THE SPINAL CORD. AND BRAINI. THE SPINAL CORD FiGUEE 56 gives aview of the whole centralnervous system, with thespinal cord as seen frombehind. The nervoussystem may be said tocomprise two organs:first, the brain ; second,the spinal cord. Thespinal cord represents aseries of important nervecenters, around which aregrouped bundles of nervetracts (Fig. 57). Thegrayish outline of thecenters makes a figuresomething like the letterH when the cord is cutacross ; and all the sub-stance outside of the Hrepresents tracts or bun-dles of fibers which arepassing upward toward. Fig. 56. — Brajn and spinal cord, with thethirty-one pairs of spinal nerves. 242 PHYSIOLOGY the brain or downward from the brain. They are simplycut-off fibers in this picture (Fig. 57). The centers for motion in this spinal cord are in thefront horns of the H. Reflex action has been describedin Chapter III. If we make a careful study of Figure 67,we can see how this reflex action is brought about. Sup-pose something irritates the skin. A message is carriedalong the long dendrite reaching from the skin to the
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