48 STEUCTTJEAL UNITS OF HUMAN TISSUES 



In our study of the nervous system we shall investigate 

 further this conductive power and the action of nerve 

 tissue.' 



General Properties of Tissues. 



There are two general facts to be learned from the stud-y 

 of the structure of tissue. First, that all parts of our 

 bodies are made up of tiny structural units or cells. 

 Second, that the functions of a given form of tissue are 

 due to the predominance of at least one of the properties 

 of the protoplasm of its cells over the other properties. 

 Thus, the protoplasm of the bone cell, like that of the 

 muscle cell, has the power of absorbing food, building up 

 new protoplasm, giving off wastes, and forming new cells. 

 In these respects bone cells are like all other cells. The}' 

 differ in that the protoplasm of the bone cell has developed 

 its power of giving off waste matter over all the other prop- 

 erties, while the muscle cell has made a specialty of the 

 power of contraction and expansion. The result is that 

 a combination of bone cells becomes important in the 

 body for the character of material it throws off, namely, 

 bone, while the combination of muscle cells is equally 

 important from the power of motion which it makes 

 possible. A comparison may make this point clearer. 

 Take two men, one a musician, one a carpenter. Each 

 eats, moves, thinks, and performs all the ordinary actions 

 of the other. Both have all the properties of a man. 

 One, however, has trained his hands to one kind of action, 

 the other, to a different kind, and to this specialization 

 are due their respective callings. So tissues are but col- 

 lections of cells whose essential substance is protoplasm. 



