92 THE NERVOUS SYSTEM 



segment it may be said to be intrasegmental (Fig. 66) ; if it extends through two 

 or more such segments it is an intersegmental reflex arc. 



Intersegmental Reflex Arcs. Impulses entering the spinal cord through a 

 given dorsal root may be transmitted to the primary motor neurons of another 

 segment in one of two ways: (1) by way of the ascending and descending branches 

 of 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 funiculus 

 and give off many collaterals to the gray matter of the successive levels of the 

 cord (Fig. 67). Many of the ascending branches reach the brain; but the others 

 terminate, as do the descending branches and all the collaterals, in the gray 

 matter of the cord (Fig. 68). The fasciculi proprii immediately surround the 

 gray columns (Fig. 68) and consist of ascending and descending fibers, which 

 arise and terminate within the gray substance of the cord. Most of these 

 fibers remain on the same side as association fibers concerned in unilateral re- 



