VERTEBRATA. 135 



near the ventral surface, but after a short course these roots 

 unite into a common trunk. These roots differ in structure 

 and function. The dorsal root bears a nervous enlargement 

 or ganglion ; the ventral has no such structure. Experiment 

 shows that the dorsal root is concerned in bringing sensa- 

 tions to the central nervous system, and, if it be cut, the 

 parts to which it goes will be without feeling. The ventral 

 root, on the other hand, is motor; i.e., it controls the 

 action of muscles, etc. If this root be cut, the muscles, 

 glands, etc., which it supplies are paralyzed. Hence we 

 may speak of the dorsal roots as afferent, since they bring 

 sensations to the central nervous system ; while the ventral 

 roots are efferent, because they carry nervous impulses in 

 the opposite direction. 



The brain must be recognized as an enlarged and special- 

 ized portion of the central nervous system. The canal of 

 the spinal cord continues into the brain, enlarging them into 

 four cavities or ventricles, connected by narrower portions. 



FIG. 62. Diagram of vertebrate brain, c, cerebrum; cb, cerebellum; 7i, 

 infundibulum ; m, medulla ; o, olfactory nerve ; ol, optic lobes ; s, spinal 

 cord ; 1 4, ventricles. 



In the brain five portions may be distinguished. Beginning 

 in front, these are: (1) the cerebrum, composed of right 

 and left halves or hemispheres, and containing in their in- 

 teriors the first and second ventricles; (2) the smaller 

 'twixt-brain, with thin walls and enclosing the third ven- 

 tricle; (3) the thick- walled optic lobes; (4) the cerebel- 

 lum ; (5) the medulla oblongata, the fourth ventricle being 



