CHAPTER VIII. 



DEVELOPMENT OF NERVE ELEMENTS. 



Time of development The elements at birth Schiller's observa- 

 tions The frog Observations by Birge Origin of sensory 

 fibres The frog compared with the mammal Significance of 

 medullary substance Regeneration Means by which the 

 brain-weight changes with age Influence of stature on brain- 

 weight Significance of brain-weight in different races In 

 the two sexes Among animals Anatomical relations alone 

 explained. 



THE constructive development of the nerve elements 

 consists in an increase in size, in the formation of the 

 abundant cytoplasm and outgrowths, and in chemical 

 modifications which affect differently the several sub- 

 divisions of the cell. The cells are first clearly charac- 

 terised in the spinal cord, then in the medulla oblongata, 

 and finally in the cerebral hemispheres and cerebellum. 

 But this modification begins in different groups and 

 in different members of the same group at different 

 times. Kaiser, in his valuable observations on the 

 arrangement of the nerve cells in the cervical enlarge- 

 ment of the mammalian cord has made some interesting 

 studies on exactly this point. 1 His table (36) shows the 

 number of developed nerve cells in the similar portions 

 of the cervical spinal cord of man at different ages, 

 beginning with the fcetus at the fifth month. 



1 Kaiser, Die Funktionen der Ganglienzellen des 

 Haag, 1891. 



