SOME OF THE LIFE PROCESSES 121 



nerves. We know that the building up of new protoplasm 

 takes place in growth, when new cells are formed, and that it 

 is also made necessary on account of the slow and impercep- 

 tible destruction of the protoplasm in oxidation (see below). 



It will be helpful at this point to know that of all the food 

 taken into the circulatory system of an animal but very 

 little, except during growth, is actually made into proto- 

 plasm. The carbohydrates and the fats that are absorbed, 

 and those that are- made from proteins by the protoplasm, 

 together with some unassimilated proteins, are destroyed 

 after they have been stored temporarily in the various tis- 

 sues of the body, especially in the muscle cells or in the liver. 



Respiration. So far we have traced the food through those 

 changes leading to its assimilation by the living animal. 

 Protoplasm and the food materials stored within it are 

 chemical substances which unite rather readily with oxygen. 

 In spite of the fact that oxygen destroys the substance of 

 which protoplasm is composed, there are few animals which 

 are able to exist without oxygen. Oxygen is necessary to 

 life because the union of oxygen with protoplasm and stored 

 food material sets free the energy which is used by the 

 animal in its movements and other life activities. The 

 stored food material and the living protoplasm are broken 

 down, not as wanton destruction but as the only means 

 whereby the animal obtains its energy. 



When oxygen combines with the material of a living 

 organism, the process is called respiration. Special organs 

 of respiration are found in most of the higher animals. 

 Thus in man the lungs, in fish the gills, and in insects the 

 tracheae are organs adapted to supplying the body with 

 oxygen and to giving off* certain types of wastes. The es- 

 sential characteristic of a breathing organ is a thin, moist 

 membrane, with thin-walled capillary blood vessels on one 

 side and air on the other. In man these conditions permit 



