CHAPTER VII. 



THE NERVE ELEMENTS. 



Histological basis First form of central system Description of 

 cell Human types Chemical characters Various shapes- 

 Outgrowths Construction of cell Changes during develop- 

 ment Volume of neuroblast Changes in the size of the 

 elements with age Nerve fibres The medullary sheath 

 Size of young cell-bodies and fibres Calculation of the 

 average size of nerve elements and of their number in man. 



IN this chapter the size, shape, number, and growth of 

 the nerve elements will be considered. The central 

 system is a mass of such elements embedded in the 

 structures which support and nourish them, and the 

 changes observed in the whole mass must be therefore 

 the sum of the changes taking place in all the con- 

 stituents, but since in the matter of enlargement the 

 subsidiary structures play but a minor part, attention 

 will be given principally to the nerve elements. 



At an early stage in any developing mammal the 

 central nervous system is represented by a tube, the 

 walls of which are formed by ectodermal cells, and by 

 two strands of similar cells coextensive with the tube 

 and lying on either side of it. These latter give rise to 

 the ganglionic portion of the nervous system, which it 

 is convenient to distinguish from the medullary portion, 

 formed by the tube alone. By flexures of this tube, 

 and by variation in the thickness of its walls due to 



H' 



