ACTIVITIES OF LIVING BEINGS 38 



waste matters. The muscle substance must be 

 repaired. This is done by nutritional changes 

 in the muscle. Materials are supplied in food, 

 which, after many chemical and physical 

 changes, are rebuilt into the muscle sub- 

 stance, thus renewing both the matter and 

 the energy that have been expended in 

 the muscle during its contractile activity. 

 The muscle liberates energy as heat, motion, 

 and, to a small extent, under the form of 

 electrical change. 



(3) Secretion. This is a peculiar form of 

 vital activity occurring in glands, of which 

 there are many forms. The product of this 

 activity is termed a secretion. The essential 

 structures in a gland are a thin membrane on 

 one side of which we find living cells, and on 

 the other, and in intimate relation to these 

 cells, a network of minute blood vessels, 

 termed capillaries. From the capillaries a 

 fluid exudes from the blood, and this fluid 

 supplies nutrient material to the living cells. 

 These cells take such material into their sub- 

 stance and complex chemical processes then 



go on. Each cell is a minute laboratory in 



c 



