The anatomy of the nervous system, from the standpoint of development and function . here are interposed between the primary sensory and motor elementsone or more intermediate neurons. These, when restricted to one side of thecord, are known as association neurons; when their axons cross the medianplane, as many of them do through the anterior white commissure, they arecalled commissural neurons. When the circuit is complete within a single neural Q2 TIIK XKRVOUS SYSTEM segment it may be said to be intrasegmental (Fig. 66); if it extends through twoor more such segments it is an intersegmental
The anatomy of the nervous system, from the standpoint of development and function . here are interposed between the primary sensory and motor elementsone or more intermediate neurons. These, when restricted to one side of thecord, are known as association neurons; when their axons cross the medianplane, as many of them do through the anterior white commissure, they arecalled commissural neurons. When the circuit is complete within a single neural Q2 TIIK XKRVOUS SYSTEM segment it may be said to be intrasegmental (Fig. 66); if it extends through twoor more such segments it is an intersegmental reflex arc. Intersegmental Reflex Arcs.—Impulses entering the spinal cord through agiven dorsal root may be transmitted to the primary motor neurons of anothersegment in one of two ways: (1) by way of the ascending and descending branchesof the dorsal root fibers, and (2) along the fibers of the fasciculi proprii (Fig. 67).A full account of these two pathways will be presented in the next chapter,but a word of explanation is required here. The fibers of the dorsal root divide,. Fig. 67.—Diagram of the spinal cord, showing the elements concerned in a diffuse unilat-eral reflex: a, Spinal ganglion cell; b, motor cell in anterior column; c, association neuron.(Cajal.) soon after their entrance into the cord, into long ascending and shorter descend-ing branches, which together form the greater part of the posterior funiculusand give off many collaterals to the gray matter of the successive levels of thecord (Fig. 67). Many of the ascending branches reach the brain; but the othersterminate, as do the descending branches and all the collaterals, in the graymatter of the cord (Fig. 68). The fasciculi proprii immediately surround thegray columns (Fig. 68) and consist of ascending and descending fibers, whicharise and terminate within the gray substance of the cord. Most of thesefibers remain on the same side as association fibers concerned in unilateral re- THE SPINAL « i li. fle
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