THE NERVOUS SYSTEM 



503 



visno-psychic , it undergoes no change in congenital blindness, and 

 therefore has no visuo-sensory function. The visuo-psychic layers 

 are not affected in dementia. A third region of the cortex is the 

 prefrontal ; it is the last to reach maturity, and is the region specially 

 affected in mental disease. Its cell layer, No. 2, has been pre- 

 viously referred to as showing marked retrogressive changes in mental 

 affections. Layer No. 2 is the most important of the human brain, 

 and it is the only one which varies in depth in normal individuals. 



Mo! 



eeulcir 



Snpra-grdBuW 

 (pyramidal) 



Granular 



Infra- grcnui lei r 



NORMAL 

 HUMAN ADULT 



I 



n 



m 

 w 



V 



277 



asG 



10ff 



THE MOLE 



241 

 •302 



538 



Total depth 1-892. 



•34* 



338 



Fig. 153. — The Approximate Relative Depths of the Cerebral Cortical 

 Layers in Man and the Mole (Watson). 



The function of the cell-laminae of the cortex, according to the 

 observers previously mentioned, and generally accepted by neuro- 

 logists, is as follows : 



Layers i and 2 carry out the psychic, or associational, functions of 

 the cerebrum ; they represent educability and general intelligence. 

 Hence they are rudimentary in the insectivora, better developed in 

 rodents, again better developed in ungulates and carnivores than in 

 rodents, and markedly more developed in the primates than the 

 carnivores. They have to do ' with all those activities which it is 

 obvious the animal has acquired (or perfected) by individual experi- 

 ence, and with all the possible modifications of behaviour which 



