THE NERVOUS SYSTEM 



499 



thickening of the cortex in mammals is correlated with separation of the 

 cells, which, however, retain connexion with one another by means of 

 elongated dendritic processes, the number of interconnexions with adjacent 

 neurons increasing with the multiplication of dendrites. In general, the 

 higher the animal, the longer and more numerous the dendrites, and 



NEOCORTEX - 



MEDIAL AREA 



EPISTRIATuM 



A LEPIDOSIREN. 



A IXPIDOSIREN 



HIPPOCAMPUS 



EPISTRIATUM 



B. RANA. VENTRAL C. LACERTA. 



Fig. 414. — Cross sections of the left cerebral hemisphere of A-A ', a fish (Lepidosiren), 

 B, an amphibian (Rana), and C, a reptile (Lacerta), showing the increasing relative 

 importance of the epistriatum. The epistriatum arises dorsal to the striatum as a local 

 thickening of the ventro-lateral wall of the hemisphere. (Redrawn from Plate, after 

 Kuhlenbeck.) 



consequently the greater the possible number of interrelationships 

 between cellular elements. A correlated increase appears in the number 

 of associational fiber tracts which connect the gyri or folds of the cortex. 

 In the milHons of years which elapsed during the Tertiary period, 

 there was a marked increase in the size of mammalian brains. This 

 increase affected all parts, and was accompanied by a corresponding 

 increase in the size of the cranium. The growth of the cerebral cortex 



