PHYSIOLOGY FOR BEGINNERS 



The coal can only burn where it is in contact with the air, 

 for only the particles on the surface of the piece can get oxygen. 

 There are no means by which oxygen can get freely into the 

 piece of coal so that the particles inside the solid mass can be 

 oxidised at the same time as those at the free surface. All the 

 parts of a living body, on the other hand, are slowly taking up 

 oxygen and giving out energy at the same time. Every one of 

 the living particles, the fundamental units, or cells of the body, 

 has oxygen brought to it. The two movements, those of the chest 

 and of the heart, by the presence of which we recognise that 

 the animal is living, have for one of their chief purposes the 

 bringing of oxygen to all the cells of which the animal's body 

 is formed. By breathing, air is brought almost into contact 

 with the blood in the lungs, and by the blood oxygen is taken 

 from the air, and then the blood thus charged with oxygen is 

 driven by the heart to the cells. 



The burning coal gives out all its energy in the form of 

 heat. An animal, on the other hand, even when not moving 

 its body or limbs, uses some of the energy resulting from the 

 oxidation of its living cells to do work, namely, the work con- 

 cerned in the movements of respiration and circulation. When 

 the animal has for a time to do heavy work the energy given 

 out by the living body is for the time greatly increased, both 

 the part which is expended in doing work, and the part which 

 is given out as heat. The burning coal, on the other hand, 

 cannot thus change the amount of energy it is to give out at 

 any time. 



"Waste and Renewal. The complex substances forming 

 each cell are then constantly breaking down into simpler com- 

 pounds, of which the simplest and final ones are water and 

 carbonic acid, and are setting free a certain amount of energy. 

 Each cell must therefore gradually waste away if it does not get 

 more of the complex substances of which it is formed. In the 

 same way the total of the cells, that is, the animal, does, as we 

 know, waste away if it cannot get food. From the food which 

 the animal eats certain substances are carried to each cell, by 

 the blood, from the stomach and intestines. From these sub- 

 stances, brought to them by the blood, the cells can build up 

 the complex substances of which they are themselves composed. 

 This power which cells have, on the one hand, slowly to break 



