LECTURES ON THE CENTRAL NERVOUS SYSTEM. 



depends on two factors, the* extent of the cortical layer which 

 the particular class has acquired during the process of evolu- 

 tion, and the capacity of the skull. 



This does not always keep even pace with the growth of 

 the brain, inasmuch as it depends on other factors. The whole 

 pallium of mammals not only grows backward, but it curves 

 and swells downward as well (not visible in figure). 



That portion of the hemisphere lying farthest forward, the 

 frontal lobe, only comes into special prominence in the higher 

 mammals, particularly man (Meynert). From this extensive 

 cortical layer of the mammalian pallium arise a great mass of 



FIG. 17. 

 Mammalian brain. Diagram of a sagittal section. 



fibres, the corona radiata. These pass out of the hemisphere to 

 terminate in the inter-brain, the hind-brain, the after-brain, and 

 the spinal cord. Other large bundles pass through the hemi- 

 spheres, connecting their different regions with one another. All 

 these taken together form a great deposit of medullary matter 

 under the cortex. The extent of this is relatively largest in 

 human beings. In lower mammals it is only small. In the 

 mouse, for example, it is insignificant. Besides this, there is 

 developed in the cortex of all animals a thick net-work of medul- 

 lary fibres, which serves to connect all parts with each other. 

 In all vertebrates, from the cyclostomata up to human beings, 



