THE NERVOUS SYSTEM. 149 



self into active relations with it. By means of the vege- 

 tative processes, his life and species are maintained; while, 

 by means of certain animal functions, he feels, acts; and 

 thinks. These functions, among which are sensation, 

 motion, and volition, not only distinguish the animal from 

 the plant, but, in proportion to their development, elevate 

 one creature above another ; and it is by virtue of his pre- 

 eminent endowment, in these respects, that man holds his 

 position at the head of the animal creation. 



3. Among animals whose structure is very simple, the 

 hydra, or fresh- water polyp, being an example, no special 

 organs are empowered to perform separate functions ; but 

 every part is endowed alike, so that if the animal be cut 

 into pieces, each portion has all the properties of the entire 

 original; and, if the circumstances be favorable, each of 

 the pieces will soon become a complete hydra. As we ap- 

 proach man, in the scale of beings, we find that the orgaifs 

 multiply, and the functions become more complete. The 

 function of motion, the instruments of which the muscles 

 and bones have been considered in former chapters, and 

 all the other animal functions of man, depend upon the set 

 of organs known as the nervous system. 



4. The Nervous System. The intimate structure 

 of this system differs from any tissue which we have be- 

 fore examined. It is composed of a soft, pulpy substance, 

 which, early in life, is almost fluid, hut which gradually 

 hardens with the growth of the body. When examined 

 under the microscope, it is found to be composed of two 

 distinct elements: (1) the white substance, composing 

 the larger proportion of the nervous organs of the body, 

 which is formed of delicate cylindrical filaments, about 

 -g-jJL- of an inch in diameter, termed the nerve-fibres ; and 

 (2) the gray substance, composed of grayish-red, or ashen- 



3. Animals whose structure is simple? As we approach man? Dependence 

 of the animal functions of man ? 



4. The nervous tissues, of what composed ? When examined by the aid of the 

 microscope ? The white substance ? The gray substance ? 



