Integrative Systems 



153 



DORSAL NUCLEUS 

 (CLARKE3 COLUMN^ 



DORSAL ROC 



GROUND BUNDLE/ 



SUBSTANTIA GELATI 



DORSOMEDIAL 



Fig. 144. A diagram of a cross section of the mammalian spinal cord, showing 

 the fiber tracts or fasiculi, and the arrangement of nuclei in the gray matter. 

 (After Sobotta. Courtesy, Neal and Rand: "Chordate Anatomy," Philadelphia, 

 The Blakiston Company.) 



it above, and a ventral fissure which cuts up into it below (Figs. 

 143, 144). 



The somewhat greater part of the nervous substance of the cord 

 consists of nerve-fibers, most of which are medullated (see p. 319). 

 Cell-bodies of neurons constitute the rest of it. Some of these cells be- 

 long to the motor fibers which emerge from the cord in the spinal 

 nerves. Others are cells of association-neurons, one or more of which 

 may be interpolated between the primary afferent neuron and the ulti- 

 mate efferent neuron of a reflex arc. And some are commissural neurons 

 connecting neuron systems of right and left sides of the cord. The vast 

 numbers of fibers which extend lengthwise of the cord are more or less 

 definitely massed into a thick external layer of "white matter" 

 white because of the fatlike substance of the medullary sheaths. The 

 deeper "gray matter" consists of the cell-bodies and the adjacent re- 

 gions of their related fibers which, so far as they lie within the "gray" 

 substance, are usually not medullated. 



The "gray matter," as seen in a cross section of the cord (Figs. 

 143, 144), appears as a doroventrally elongated mass on each side of the 

 cord, the two lateral masses being connected by a transverse mass, the 

 gray commissure, which lies near the center of the cord and sur- 

 rounds the small central canal. The dorsal and ventral extensions of 

 the gray region are known, respectively, as dorsal and ventral gray 

 columns. A lateral column, intermediate between dorsal and ven- 

 tral columns, is sometimes recognized. 



The fibers in the outer "white matter" are disposed in bundles or 

 tracts, each consisting of fibers of similar origin and function. Groups 

 of tracts form fasciculi. A fasciculus may contain both sensory tracts 



