NERVE 



181 



association with these previous impressions, leads to their 

 renewal, leads to our recollecting the past experiences. 



1. Structural Development. 



In the lower vertebrata the differentiation of the cortex 

 from the basal ganglia is incomplete, and it is only in the 

 higher mammals — monkeys and man — that the cortex 

 reaches full physiological importance. 



Fig. 91. — The passage of fibres from the nuclei of the thalamus to the 

 cortex cerebri of a primitive mammal. Th.Op., thalamus ; L.P., lobus 

 pyriformis ; /.r., iissura rhinica ; F///'", auditory area; //'", visual 

 area; i/"., hippocampus. (Elliot Smith.) 



The cerebrum originally developed as a ganglion in 

 connection with the organ of smell, and in the osmatic 

 mammals — those in which smell plays a great part in 

 guiding their actions — a large part of the cerebrum remains 

 specially connected with the nose. This may be called the 

 rhinencephalon (fig. 61, p. 135). 



The cortex cerebri, or neopallium, is a secondary develop- 

 ment from the thalamus, with which it remains closely 

 associated by outgoing and ingoing neurons (fig. 91). 



In the human foetus at four months, two layers are visible in 

 the cortex — (1) an outer molecular layer of fibres, and under 

 this (2) layers of undifferentiated cells (fig. 90). 



By the sixth month this second layer has become divided 

 into two by a well-developed layer of small cells, the 



