158 MAN, THE ANIMAL 



which he has defined as light and sound, and by a 

 gaseous medium which gives him what he desig- 

 nates as heat, cold and odors. As already indi- 

 cated, the central nervous system is entirely encom- 

 passed by skin, muscles and bone which prevents 

 these environmental agencies from coming in di- 

 rect contact with either the brain or spinal cord. 

 But it is self-evident that no animal could adjust 

 itself to its surroundings unless it somehow became 

 aware of such changes. This it does through 

 special cells grouped into special sense organs as 

 the eye or ear or through scattered cells such as 

 those of taste or smell. 



In the eye is found a cell that is easily stimulated 

 by light vibrations but does not react to sound vi- 

 brations; while in the ear are cells that readily 

 react to sound vibrations but do not appreciate 

 light waves. Any cell that is thus specialized to 

 appreciate changes of a specific kind in its im- 

 mediate environment is known as a receptor. Such 

 a cell may be located entirely within the body, as 

 in the muscles. Thus there are receptors for light, 

 sound, heat, pain, taste, etc. Some idea of the 

 great difference existing between the several re- 

 ceptors is gained by reviewing the several forms of 

 stimuli that affect them. 



Touch and pressure are due to mechanical con- 

 tact with the number of vibrations varying from 

 one to 1552 per second. There are no organs 



