176 



PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



structure. Modern physiology has recognized clearly that different 

 parts of the cerebrum have different functions, but the differentia- 

 tion in structure which usually accompanies a specialization in 

 function is not very evident. Differences in the thickness of the 

 layers, in the size or shape of the cells, have been pointed out, 

 but it is perhaps something of a disappointment to find so little 

 of an anatomical -distinction between structures whose reaction in 

 consciousness is so widely separated. It would seem that the 



ifr ^f 



Fig. 79. A-D, Showing the phylogenetic development of mature nerve cells in a 

 series of vertebrates : a-e, the ontogenetic development of growing cells in a typical mam- 

 mal (in both cases only pyramidal cells from the cerebrum are shown) ; A, frog; B, lizard; 

 C, rat; D, man; a, neuroblast without dendrites; b, commencing dendrites; c, dendrites 

 further developed; d, first appearance of collateral branches; e, further development of 

 collaterals and dendrites. (From Ramon y Cajal.) 



structural peculiarities must lie chiefly in the ultimate chemical 

 composition and physical properties of the protoplasm. In the 

 third place, the central nervous system throughout the verte- 

 brates is constructed upon the same lines, a mechanism of intercon- 

 necting neurons. There is a vast difference in the mental activity 

 of a frog and a man, but the cortex of the cerebrum shows a funda- 

 mental similarity in structure in the two cases. The chief difference 

 that comparative anatomy is able to show is that in the higher 



