178 



PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



animals the greater mental development is associated with a greater 

 complexity and richness in the connections of the neurons. As 

 shown in Figs. 78 and 79, the number of processes, particularly 

 the dendritic processes, is much greater in the cortical cells 

 of the higher animals; or, to put this fact in another way, the 

 number of cells in the cortex of the higher animals is much less 

 for an area of the same size than in lower animals. The amount 

 of in-between substance or the richness of the network of processes 



Fig. 78. 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 Ramdn y Cajal.) 



is increased. This anatomical fact would indicate that the greater 

 mental activity in the higher animals is dependent, in part, upon 

 the richer interconnection of the nerve cells, or, expressed physio- 

 logically, our mental processes are characterized by their more 

 numerous and complex associations. A visual or auditory stim- 

 ulus that, in the frog, for instance, may call forth a comparatively 

 simple motor response, may in man, on account of the numerous 

 associations with the memory records of past experiences, lead to 



