THE BKAIN. 



a nerve of motion be cut, the member which it moves will be 

 immediately paralysed ; and if a nerve of sensation be cut, the 

 part which it connects with the brain will become insensible. 

 Thus, for example, if the nerves of motion proceeding from the 

 brain to the arm be divided at the shoulder, the entire arm and 

 hand will be paralysed, the will losing all power over it. In like 

 manner, if the nerve connecting the optical membrane of either 

 eye be divided at any point between that membrane and the point 

 where the nerve unites with that which proceeds from .the other 

 eye, the former eye will become blind, the sight of the latter 

 remaining unimpaired. But if the optic nerve be divided beyond 



Fig. 2. Section of the human brain, made by a vertical plane passing 

 through the middle of the forehead. 



the point where the nerves from the two eyes are united, both eyes 

 will lose the power of vision. 



4. The brain being the organ of intelligence, it has, as might 

 naturally be expected, a greater development and more perfect 

 structure in man than in the inferior animals. The cerebral 

 hemispheres, as they are called, are more voluminous, and their 

 convolutions are more prominent and numerous, and extended 

 over a much larger region of the skull. They cover, for example, 

 that part of the organ called the cerebellum, while in inferior 

 animals they never extend over it, and in many cases have no 

 existence at all. 



The part of the brain which occupies the front of the skull in 

 man is remarkable for the extent of its volume, and gives that 



53 



