RESPONSIVE LIFE OF ORGANISMS 461 



have seen, compose the dorsal root ganglion. The 

 bodies of the efferent neurones lie in the gray matter of 

 the central part of the cord, where 

 they are associated with other 

 intercalary nerve cells. The 

 afferent fibers pass through the 

 white part of the cord up -ward 

 toward the brain by several 

 routes. But the white matter is 

 composed mainly in ourselves 

 of fibers of intermediary neu- 

 rones, or of efferent fibers de- 

 scending from the brain. These 

 fibers are arranged in great 

 tracts or "columns," which are 

 the main lines of communica- 

 tion between the brain and out- 

 lying ganglia. Even the main 

 fiber tracts are too numerous for 

 us to attempt to give an account 

 of them here. 



Lying in the pathway of some 

 of these bundles in the base of 

 the brain are some very impor- 

 tant masses of cells, that are 

 , centers of control over the most 



FIG. 260. The nervous system of 



man. (After Ranke) vitally important processes of 



the body. In the midst of the medulla, for example, lie 

 the centers for the control of heart beat and respira- 

 tion. But the greater part of the cells of the brain occupy 

 the outer layer of gray matter of the cerebellum and cerebral 

 hemispheres. The cerebellum is connected in an important 

 way with the control and coordination of the involuntary 

 movements of the body, and the cerebrum, or upper brain, 



