164 THE HUMAN BODY 



The Medulla and Midbrain. If our attention had been called 

 to the matter when the courses of the various afferent and ef- 

 ferent pathways of the cerebrum and cerebellum were being de- 

 scribed, we should have noted that the medulla and midbrain 

 form a great highway through which pass virtually all impulses 

 on their way to or from the higher brain structures. Moreover, 

 most of the nerve tracts leading through these structures do not 

 pass directly through them, but suffer interruption in one or the 

 other of the many nuclei which occur therein. Wherever a nerve 

 tract is interrupted by a nucleus the axons leading into the nu- 

 cleus terminate in synaptic connection with new neurons by 

 which the tract is continued. There is always the possibility, 

 where such connections are being formed, of a certain amount of 

 diversion from the main channel into side channels. The medulla 

 and midbrain, then, are strategically located for concentrating 

 into small areas influences from all the receptors of the Body. 

 This region has also its own efferent pathway in the tract from 

 the red nucleus. It affords, therefore, an additional field for the 

 establishment of reflex arcs, but, as we shall see, of a somewhat less 

 specialized sort than are afforded by the cerebrum and cerebellum. 



There are a number of so-called " vital processes" going on in 

 the Body. These are activities whose continuance is essential 

 to the maintenance of life, and which must, therefore, go on quite 

 independently of the will; they are of a sort, however, to require 

 modification in accordance with the demands of the Body. Such 

 activities are the beating of the heart, breathing, the secretion of 

 sweat, and some others. 



Most of these so-called "vital" activities are really as purely 

 reflex as any of the ordinary reflex acts of the Body, and the few 

 that are truly automatic are subject to constant reflex influence. 

 Their immediate control is vested in certain centers located in 

 the medulla. This location for the centers insures that they shall 

 never be wholly free from sensory stimulation, for no matter how 

 quiet the surroundings of the Body may be the processes going 

 on within it give rise to sensory stimuli, and, as we have seen, 

 whatever impulses are aroused are sure to pass through the 

 medulla. Detailed consideration of the various centers of the 

 medulla is not necessary here as each will be treated in connec- 

 tion with the vital process with which it is related. 



