I QO ANIMALS 



cerebral and cerebellar regions. Hence, the surface of the brain 

 is largely .composed of the gray matter, while the fibre tracts 

 are wholly beneath the surface. In the human body there are 

 forty-three pairs of nerves, twelve connecting with the brain 

 and thirty-one with the spinal cord. A typical spinal nerve 

 is connected with the spinal cord by two "roots," one dorsal 

 and one ventral. On the dorsal root, not far from the spinal 

 cord, there is a ganglion, and immediately beyond the two roots 

 unite to form the spinal nerve. The latter then divides into 



FIG. 103. Diagram of a cross section of the spinal cord and the roots of the 

 spinal nerves. C, Central canal; df, dorsal fissure; dr, dorsal root of spinal nerve, 

 arising from the dorsal horn of the gray matter (g) ; gn, ganglion on the dorsal 

 root; n, spinal nerve; a/, ventral fissure; vr, ventral root of the spinal nerve, 

 arising from the ventral horn of the gray maater; w, white matter. (From 

 Galloway, by Folsom.) 



three main trunks; one passes into the body cavity to connect 

 with the sympathetic system which supplies the viscera, one 

 passes up to the muscles and skin of the dorsal portion of the 

 body, and the last turns downward to the muscles and skin 

 of the ventral portion of the body. Both dorsal and ventral 

 branches contain both receptor and effector fibres, but at the 

 juncture of dorsal and ventral roots the two classes of fibres 

 separate, the effectors pass over the ventral root and the recep- 

 tors pass over the dorsal root. The cells of the receptors lie 

 in the spinal ganglion of the dorsal root, while the cells of the 

 effectors lie in the ventral horn of the gray matter in the cord. 



