218 THE NERVOUS SYSTEM 



2. The animal, in addition to these vegetative functions, has 

 another set of powers, by the use of which he becomes con- 

 scious of a world external to himself, and brings himself into 

 active relations with it. By means of the vegetative 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 dis- 

 tinguish 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 approach man, in the scale 

 of beings, we find that the organs multiply, and the functions 

 become more complete. The function of motion, the instru- 

 ments of which the muscles and bones have been con- 

 sidered 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 before examined. 

 It is composed of a soft, pulpy substance, which early in life is 

 almost fluid, but which gradually solidifies with the growth of 

 the body. When examined under the microscope, it is found 

 to be composed of two distinct elements : (1) the white sub- 

 stance, composing the larger proportion of the nervous organs 

 of the body, which is formed of delicate cylindrical filaments, 



2. What secor d set of powers has the animal ? What functions are mentioned ? The 

 advantage they give ? 



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 micro- 

 scope ? The white substance ? The gray substance ? 



