CHAPTER XII 



THE AUTONOMIC NERVOUS SYSTEM. NERVOUS FATIGUE. 

 HORMONES OF THE NERVOUS SYSTEM 



The Brain Stem (Medulla and Midbrain). If our attention 

 had been called to the matter when the courses of the various 

 afferent and efferent pathways of the cerebrum and cerebellum 

 were being described, we should have noted that the brain stem 

 forms 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 the brain stem do not pass 

 directly through, 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 nucleus ter- 

 minate 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 mid- 

 brain, then, are strategically located for concentrating into small 

 areas influences from all the receptors of the Body. This region has 

 also its own efferent pathways. 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. Ex- 

 amples of such activities are the beating of the heart, breathing, 

 the secretion of sweat. 



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

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

 that are truly automatic are subject to constant reflex influence. 



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