CHAPTER XII 



CENTRAL NERVOUS SYSTEM 



THE entire nervous system, for the sake of convenience, is 

 divided into a number of parts : 



1. The central nervous system or the cerebrospinal axis (brain 

 and spinal cord). 



2. The peripheral nervous system (spinal and cranial nerves 

 and ganglia, sympathetic ganglia and nerves). Physiologically 

 considered, such a division of the nervous system has no par- 

 ticular significance; the functions of these parts are intimately 

 related and dependent upon one another, and form such an 

 indivisible unit that any detached group of nervous structures 

 would have no meaning. The essential constituents of the 

 nervous system are separate but contiguous nerve cells or neu- 

 rons. A neuron is meant to include every part of a nerve cell 

 under the control of a given nucleus. It consists of the cell 

 body and all its outgrowths. Of the latter, the axons are of 

 various lengths, some in man spanning almost the entire length 

 of the body. The ends of the axons and of the branches which 

 an axon gives off along its length are divided into fine twigs 

 or terminal arborizations. The remaining branches of the nerve 

 cell do not attain the length of the axon, but very soon divide 

 dichotomously, and they also form, finally, finely divided ter- 

 minal arborizations. The arrangement of neurons is such 

 that the end brush of the axon of one cell is in intimate relation 

 to the end brush of the dendrite of another cell. Therefore, an 

 impulse generated in any one neuron is transmitted to its 

 neighbor, which in turn passes it on to the next neuron. 



It is the function of the central nervous system to bring the 

 body into relation with changes in its environment and to pre- 

 serve the harmonious working of all its organs. Widely separ- 

 ated parts of an organism, therefore, are bound together by 



