THE NERVOUS SYSTEM 



487 



animal. This line of research is especially associated with the 

 name of Flechsig. The study of the structural features of the 

 brains of imbeciles and other pathological conditions, have also 

 largely contributed to a more exact knowledge of the use of the 

 various parts of the cortex. 



The structure of the cortex points to it being a reflex arc, the 

 complexity of which differs according to the animal scale. The 

 cortex of man and the frog possesses the same anatomical 

 elements, but the complexity of the neurones of the former are 



Fig. 145. — Pyramidal Cells of Cerebral Cortex in Different Animals 

 (Donaldson, after Ramon y Cajal). 



A — D shows the different degree of complexity in the fully-developed pyramidal 

 cells in different vertebrates : A, frog ; B, lizard ; C, rat ; D, man. a — e Shows 

 the development of the pyramidal nerve-cells of the cerebral cortex in a 

 typical mammal : a, neuroblast with commencing axon ; b, dendrites 

 appearing ; d, commencing collaterals. 



in marked contrast to the simplicity which distinguishes those 

 of the frog (see Fig. 145). A rich system of dendrites means 

 more numerous and complex connections. Apart from this 

 evidence has recently been brought forward to show that there 

 are structural peculiarities accompanying specialisation in 

 function, to which attention will be drawn later. 



In the white substance of the cerebrum are nerve fibres con- 

 veying impulses to the cortex, either from its different parts or 

 from the basal ganglia, bulb, and cord. There are also efferent 

 fibres which have their origin in the cells of the cortex, which 



