126 THE HUMAN BODY 



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. 61). 



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

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

 tervertebral foramina (p. 53). 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. 59), 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. 59), 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. 59), 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 

 muscles 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- 



