CHAPTER XTV. 

 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 divi- 

 sions 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 cra- 

 nium 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 or autonomic ganglia. 

 Third, the nerves or bundles of nerve fibers which connect the central 

 nerve axis with the periphery 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 receptors, 

 the other for transmitting stimuli to other tissues, the effectors. These 

 are properly parts of the nervous system. The peripheral organs for 

 receiving stimuli constitute the sense organs and will be treated in a sepa- 

 rate chapter. 



FUNCTION OF THE NERVE CELL. 



The Nerve Cell. The nerve cell, the neurone, is considered the ana- 

 tomical and physiological unit of the nervous system. Waldeyer introduced 

 the term neurone to designate the nerve cell body and all its constituent proc- 

 esses. Much of the details of structure of types of neurones, both as to the 

 structure of the cell body and all its processes, have already been given in 

 Chapter II., see figures 83 to 106. 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, branches, or arborizations, see figure 365. Some- 

 times these processes are short but complexly branched, sometimes they are 

 exceedingly long as compared with the extent of the cell body. The cell 

 processes may or may not be medullated. In medullated nerves the ad- 

 ventitious structures are subdivided into nodes, but the axis-cylinder process 

 is to be regarded as a continuation of the protoplasm of the cell body. 



In recent years the structure of the cell body and its branches has been 

 very carefully investigated, with the result that we have learned that the 

 intimate structure is very complex. Networks of neurofibrillae have been 



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