40 THE HUMAN BODY 



include oxidative processes, processes involving waste and repair, 

 and doubtlessly many others of which we know nothing. The 

 study of these cell activities is comprehended under the head of 

 Metabolism. 



The chemical activities which go on in the cells of the Body give 

 rise to much heat. Some cells generate more heat than others. 

 One of the functions of the circulating medium is to distribute this 

 heat uniformly over the Body. There is constant loss of heat from 

 the surface of the Body. In warm-blooded animals, which have 

 a nearly constant body temperature, the maintenance of balance 

 between heat production and heat loss in the face of constantly 

 varying outside temperatures is a function of great importance. 

 It is studied under the head of Heat Production and Heat Regula- 

 tion. 



Not immediately concerned with the well-being of the Body it- 

 self, but devoted to the well-being of the race as a whole through 

 perpetuating the species is the Reproductive System. 



Before we turn from this discussion of the various systems into 

 which, for convenience, we have grouped the various Bodily struc- 

 tures, we may well emphasize again the unity of operation of the 

 Body, so that we shall not fall into the habit of thinking of the 

 different systems as separate mechanisms, operating independ- 

 ently of one another. This unity of operation is well illustrated 

 in one of our commonest every-day experiences, namely, vigorous 

 muscular exercise. Whenever we use our muscles briskly definite 

 activities of the various systems we have classed as maintenance 

 systems occur. Thus the heart is thrown into rapid beating; the 

 skin is flushed; the breathing is quickened; the sweat glands are 

 active; if the exercise is prolonged and not too fatiguing, there is 

 likely to be a sharpening of appetite, leading to a greater consump- 

 tion of food and so to increased digestive activity. All these mani- 

 festations accompany muscular exercise, as a matter of course. 

 We shall see later how they are all part of the provision whereby 

 the Body is able to use its muscles effectively. This is but one of 

 many illustrations that might be cited to show the interdependence 

 of the various systems. True insight into Human Physiology re- 

 quires that this interdependence be thoroughly realized. 



Animals Compared with Plants. We divide the world of living 

 things into two kingdoms; the plant kingdom and the animal 



