262 HUMAN BIOLOGY 



date back not merely to the prehistoric but to the entirely 

 prehuman. 



The portion of the human brain which on account of 

 much and well-established evidence must be regarded as the 

 material seat of man's mind is that great surface structure 

 rooted in the forebrain and a relatively new excrescence 

 from it known as the cortex and its fibers; and, probably 

 subsidiarily to that, the thalamus deep underlying the 

 cortex, and of far older evolutionary history. The thalamus 

 forms a relay station for practically all of the nerve paths 

 ascending to the cortex. The cortex itself is for the most 

 part of comparatively late evolutionary history. The pre- 

 mammalian vertebrates possessed merely a trace of it or 

 none at all. The higher mammals, especially the monkey, 

 possess it in large proportions. In man it is so greatly devel- 

 oped that even in brute bulk it dwarfs the whole of the rest 

 of the nervous system. From the biological point of view it 

 represents the very culmination of integration of the 

 animal organism. It consists of two broadly symmetrical 

 hemispheres, one right and one left. The relation with the 

 voluntary muscles is for each hemisphere a crossed one, so 

 that it is the left hemisphere which is concerned with the 

 skilled acts of the right hand. A curious fact is that in right- 

 handed persons the left hemisphere is functionally the more 

 important mentally. Medical experience shows that small 

 lesions of certain parts of the left hemisphere destroy speech 

 and even memory, while similar lesions of the right hemis- 

 phere may pass for years unnoticed because productive of 

 no obvious defect or symptoms. In fact the recent experience 

 of brain-surgeons dealing with malignant growths indicates 

 that the entire right cerebral hemisphere of a right-handed 

 human being may be removed without there ensuing mental 

 defects that are recognizable (Dandy), though of course 

 the patient becomes in result of such an operation paralyzed 

 on the left side. 



This supreme development of the brain, the cerebral 

 cortex, has the property of relatively quickly adjusting 

 its reactions to meet various conditions. It can establish 

 new functional habits requiring new functional connections. 

 A dog secretes saliva when food is placed in its mouth; 



