HUMAN PHYSIOLOGY. 



INTEODUCTIOlSr. 



THE study of Physiology* embraces all the active phenomena pre- 

 sented by living beings such as growth, reproduction, movement, 

 sensation, the chemical changes manifested in the body during life, as 

 well as its action upon external substances and its dependence upon 

 external conditions. 



Living bodies are distinguished, as regards their structure, from 

 those of the inorganic world mainly by the fact that they are organized ; 

 that is, they are composed of a number of different parts, or organs, 

 connected with each other and mutually dependent. In all the higher 

 orders, both of animals and plants, these various organs belonging to 

 the same body are quite numerous, and are very different from each 

 other both in their structure and properties. 



In an animal, for example, there is an external integument covering 

 the surface of the body, bones which form a framework for the protection 

 and attachment of other parts, muscles by which the limbs are put in 

 motion, an alimentary canal for the digestion of the food, and various 

 glands for the secretion of the animal fluids. In a plant there are roots 

 which absorb the ingredients of the soil, leaves which elaborate the 

 vegetable juices, and the various parts of the blossom which are con- 

 cerned in the production of the fruit. Thus each different organ has a 

 special structure, and plays a distinct part in the living organism. 



The peculiar action or result accomplished in this way l>y a particular 

 organ is called its function. There are, therefore, a variety of functions 

 going on in the living body, each one as distinct as the organ by which 

 it is performed. But no one of them is entirely independent of the rest. 

 The circulation of the blood, which is carried on by the organs of the 

 vascular system, requires that the blood should be incessantly renovated 

 by the process of respiration in order that it may continue undisturbed ; 

 and the circulation is in its turn necessary to the functions of secretion 

 and nutrition, for which it supplies the necessary material to all parts 

 of the body. Thus all the different functions are in a state of mutual 

 dependence, and the life of the whole body is a result of the simultaneous 

 and harmonious action of its different parts. 



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