ANATOMY OF THE NERVOUS SYSTEM 147 



or convolutions being deeper and more numerous in the brain of 

 man than in that of the animals nearest allied to him; and in the 

 human species more marked in the higher than in the lower races. 

 It should however be added that some species of animals which 

 are not markedly intelligent have much convoluted cerebral 

 hemispheres. 



The brain, like the spinal cord, consists of gray and white nervous 

 matter, but somewhat differently arranged, for while the brain, like 

 the cord, contains gray matter in its interior, a great part of its 

 surface is also covered with it. By the external convolutions of the 

 cerebellum and the cerebral hemispheres the surface over which 

 this gray substance is spread is very much increased (see Fig. 59). 



The Spinal Nerves. Thirty-one pairs of spinal nerve-trunks 

 enter the neural canal of the vertebral column through the in- 

 tervertebral foramina (p. 57). Each divides in the foramen into 

 a dorsal and ventral portion known respectively as the dorsal 

 and ventral roots of the nerve (6 and 5, Fig. 57), and these again 

 subdivide into finer branches which are attached to the sides of 

 the cord, the dorsal root at the point where the dorsal and lateral 

 white columns meet, and the ventral root at the junction of the 

 lateral and ventral columns. Although the nerve-trunks contain 

 both sensory and motor neurons these are completely separated 

 in the roots; the dorsal root contains only sensory neurons; the 

 ventral only motor. At the lines on which the roots are attached 

 there are superficial furrows on the surface of the cord. On each 

 dorsal root is a spinal ganglion (6', Fig. 57), placed just before 

 it joins the ventral root to make up the common .nerve-trunk. 

 This spinal ganglion contains the cell-bodies of the bipolar sensory 

 neurons. Immediately after its formation by the mixture of 

 fibers from both roots, the trunk divides (D, Fig. 57), into a dorsal 

 primary, a ventral primary, and a communicating branch. The 

 branches of the first set go for the most part to the skin and mus- 

 cles on the back; from the second the nerves for the sides and 

 ventral region of the neck and trunk and for the limbs arise; the 

 communicating branches form part of the sympathetic system. 



The various spinal nerves are named from the portions of the 

 vertebral column through the intervertebral openings of which 

 they pass out; and as a general rule each nerve is named from the 

 vertebra in front of it. For example, the nerve passing out be- 



