CHAPTER XXII 



THE NERVOUS SYSTEM 



The nervous mechanism in man consists of two distinct though 

 associated systems, the cerebrospinal nervous system and the sympa- 

 thetic nervous system. Each of these systems is composed of a central 

 portion which is its center of nervous activity, and of a peripheral 

 portion which serves to place the center in connection with the organs 

 which it controls. In the cerebro-spinal system the central portion 

 consists of the cerebro-spinal axis, or brain and spinal cord. The 

 peripheral portion is formed by the cranial and spinal nerves. The 

 central portion of the sympathetic system consists of a series of 

 ganglia from which the sympathetic nerves take origin. These 

 latter constitute its peripheral portion. The cerebro-spinal axis, or 

 brain and spinal cord, constitutes the central nervous system. The 

 cranial and spinal nerves and the sympathetic system constitute 

 the peripheral nervous system. , 



HISTOLOGICAL DEVELOPMENT AND GENERAL 



STRUCTURE 



The beginning differentiation of the nervous system appears very 

 early in embryonic life. It is first indicated by a longitudinal median 

 thickening of the outer embryonic layer or ectoderm, to' form the 

 neural plate. The sides of the plate become elevated to form the 

 neural folds, leaving between them the neural groove. By the dorsal 

 union of these folds the neural groove is converted into the neural 

 tube. The lumen of the neural tube corresponds to the central canal 

 of the cord and the ventricles of the brain in the adult, and from the ec- 

 todermic cells which form the walls of this tube practically the entire 

 nervous system is developed. The caudal portion of the tube is of 

 nearly uniform diameter^ — the spinal cord. At the cephalic end, the 

 neural plate, even before its closure, is wider and forms, when closed, 

 an expanded portion of the tube, the brain. In their further develop- 

 ment, the walls of the brain form three expansions — the three brain 

 ''vesicles" known as the forebrain {prosencephalon), midbrain {mes-^ 



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