222 



BIOLOGY AND HUMAN LIFE 



4. Neurons in the brain. Many of these are not directly re- 

 lated to outward reactions but are related to knowing, feeling, 

 and the voluntary control of the muscles. 



Suppose a certain part of the sciatic nerve (the main nerve 

 trunk runnino; down the leg) were cut, destroying the afferent 



Fig. 113. Localization of functions in the cerebrum 



By studying human beings and other animals in which the brain had been injured, 

 and by making experiments, it has been ascertained that certain regions of the brain 

 cortex are related to receiving sensations from specific regions of the body, while 

 other regions initiate movements of specific muscles. Most of the sensory and motor 

 nerves pass through the spinal cord, S.C. The thinking is carried on by the so-called 

 association areas, A-i and A~2. The frontal association area has to do with ab- 

 stract thinking, self-control, concentration, and making decisions. The hind associa- 

 tion area has to do with knowing and understanding concrete facts and relations 



fibers (a^, Fig. iii)." One might then walk on carpet tacks or 

 hot iron and not know it, unless he happened to be watching his 

 steps. Under these circumstances a person would be able to 

 move his legs or to jump if he wanted to, but the reflex, or 

 automatic, jumping would be impossible because the arc would 

 be broken. On the other hand, suppose another portion of the 



