THE NERVOUS SYSTEM 



36: 



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

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

 consequently the greater the possible number of interrelationships between 

 cellular elements. A correlated increase appears in the number of asso- 

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



NEOCORTEX 



MEDIAL AREA 



EPISTRIATUM 



A LEPIDOSIREN 



A UPIOOSIREN 



.CORTEX OF 



g \ HIPPOCAMPUS 



EPISTRIATUM 



B. RANA. VENTRAL C. LACERTA. 



Fig. 324. — 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.) 



In the millions 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 

 was, however, out of all proportion to the enlargement of the rest of the 

 brain. This increase was made possible by the complex folding of the 

 outer layers of the brain and resulted in the formation of the gyri and 



