146 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



they divide the white matter into three longitudinal tracts, the 

 funiculi (formerly called column), dorsal, lateral and ventral in posi- 

 tion (fig. 154). The location of the roots of the spinal nerves, to be 

 described later, still further emphasizes the limits of the funiculi. 



As the white matter is composed of longitudinal fibres, these 

 funiculi serve as tracts by which the different parts of the central 

 nervous system are connected with each other. Farther, there is a 

 differentiation of the fibres. Those in the dorsal funiculus carry 



Fig. 154. — Diagram of spinal cord and nerve roots; gray matter shaded. A, L, 

 P, ventral, lateral and dorsal funiculi; Ap, fasciculus cerebrospinalis anterior; B, 

 fasciculus cuneatus (Burdach); cc, central canal; ca, d, cp, anterior, lateral and pos- 

 terior columns of gray matter; Dr, dorsal root of spinal nerve; Fa, ventral fissure, Fp, 

 dorsal septum; G, fasciculus gracilis (Goll); g, ganglion of dorsal root; Lp, fasciculus 

 cerebrospinalis lateralis; sn, spinal nerve. 



impulses toward the brain — are ascending tracts ; while the ventral 

 funiculus of either side is a descending tract and the lateral funiculi 

 consist of fibres of both kinds and carries impulses in both directions. 



The fibres in each of these funiculi are grouped in smaller bundles or 

 fasciculi, each with its name, some of which are indicated in figure 154. As some 

 of these fibres coming from the brain are distributed at different levels along 

 the cord, and others, going to the brain, are added to the funiculi at different 

 places, it follows that the size of the funiculi decreases with the distance from 

 the brain, and that some of the bundles may disappear in the more distal parts 

 of the cord. 



In the higher groups the spinal cord is approximately cylindrical, but in the 

 lower it is flattened dorso-ventrally, the flattening being greatest in the cyclos- 

 tomes. In the lower groups there is also a difference in the shape of the gray 

 matter, the H shape being less distinct (fig. 169). 



In the fishes the cord tapers pretty regularly from the brain to the posterior 

 end, but with the development of legs and an increase of the musculature, the 



