are situated in the central portion of each half, and the two masses of gray matter thus formed are connected by a narrow isthmus of gray matter. The outer part of the cord consists chiefly of white fibers. The white matter is thus on the outside of the cord (Fig. 107). The brain, unlike the cord, has the gray matter on the outside and the white matter on the inside. For microscopic study of the spinal cord, see Fig. 108.
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Fig. 107.—Cross-section of Spinal Cord, showing area of gray matter (dark).
The Work of the Spinal Cord.—There are two functions of the cord: reflex action and transmission of impulses from the body to the brain. Reflex action is action that takes place without the aid of the will.
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Fig. 108.—Section of Spinal Cord, showing nerve cells (large black spots) with their branches (black dots and lines). Five bundles of nerve fibers are shown near upper margin. (Peabody.)
Reflex action never begins in the cord, but at the outer end of a sensory fiber, usually located in the skin. The impression goes to the cord along a sensory fiber. It is received in a sensory cell and transferred by dendrons to a motor cell which sends back an impulse along a motor fiber to a muscle; the muscle contracts and the action is complete. At least two nerve cells are necessary for reflex action. The actions of the lowest animals are almost entirely reflex.
