CHAPTER XIV 



THE NERVOUS SYSTEM 



THE nervous system consists of an extremely complex anatomical mass 

 of nerve cells and fibers. It is usually described as made up of two main 

 divisions, the cerebro-spinal system and the sympathetic. These two divisions 

 must be regarded as parts of one great whole, and in no sense coordinate. 

 The gross subdivision of the nervous system may be given as the following: 

 first, the cerebro-spinal axis, which consists of matter within the bony 

 cranium and spinal column, constituting the brain and cord. Second, smaller 

 masses for the most part in the abdominal and thoracic cavities, also in the 

 neck and head, and constituting the sympathetic ganglia. Third, the nerves 

 or bundles of nerve fibers which connect the central nerve axis with the per- 

 iphery and with the sympathetic ganglia. Fourth, there are special peripheral 

 organs in connection with the beginnings and endings of the nerve fibers, the 

 one for receiving nerve stimuli, the other for transmitting stimuli to other tis- 

 sues. These are properly parts of the nervous system. The peripheral 

 organs for receiving stimuli constitute the sense organs and will be treated in 

 a separate chapter. 



I. FUNCTION OF THE NERVE CELL. 



The Nerve Cell. The nerve cell, the neurone, may be considered 

 the anatomical and physiological unit of the nervous system. The details 

 of the structure of the nerve cell, both its body and its processes, have 

 already been given in chapter II. It is sufficient to recall that the types of 

 nerve cells found in various parts of the nervous system vary extremely. 

 The peculiar feature, however, consists in the fact that the cell body has 

 one or more processes. Sometimes these processes are short but com- 

 plexly branched, sometimes they are exceedingly long as compared with the 

 extent of the cell body. The cell processes may or may not be medullated 

 and subdivided into nodes, but the axis-cylinder process is to be regarded as 

 a continuity of the protoplasm of the cell body. In recent years the struct- 

 ure of the cell body and its branches has been very carefully investigated, 

 with the result that we are finding that the intimate structure is very complex. 

 Networks of neurofibrillae have been described not only in the cell body, but 

 extending throughout the course of the processes and, in fact, from cell to cell. 

 We are not in a position at the present time fully to determine what bearing 



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