2 PHYSIOLOGY 



their simplest form in the most highly differentiated organisms. In 

 the unicellular animal all the essential functions which we associate 

 with living beings are carried out, often simultaneously, in one little 

 speck of protoplasm. An analysis of these functions, the determina- 

 tion of their conditions and mechanism, is obviously impossible under 

 such circumstances. It is only when, as in the higher animals, one 

 part of the living body is differentiated into an organ which has one 

 function and one function only as the outlet for its activities, that it 

 becomes possible to peer into the details of the function with some 

 chance of discovering its ultimate mechanism. 



Our especial preoccupation with the physiology of man will not 

 prevent our employment of examples from any part of the animal or 

 vegetable kingdom, when light can be thrown by their study on 

 fundamental physiological phenomena common to the whole of 

 the living world. In many cases such a study will enable us to 

 separate the essential features in a process from those which have 

 been added as auxiliary, with increasing complexity of the structures 

 concerned. 



What are the fundamental phenomena which are Wrapt up in our 

 conception of living beings ? When dealing with the higher animals, 

 we are inclined to lay greater stress on the phenomena involving a 

 discharge of energy. Thus we should say that a man was alive if his 

 body were warm and if he were presenting spontaneous movements, 

 such as those of respiration or of the heart. The life of a man in the 

 ordinary sense of the term is made up of those movements which place 

 him in relationship with his environment. For the production of 

 these movements, as for the maintenance of a constant body-tempera- 

 ture, a continual expenditure of energy is necessary. Experience 

 teaches us that these movements come to an end in the absence of 

 food or of oxygen, and that an increased call on the energies of the 

 body must always be met by a corresponding increase in the air and 

 in the food supplied. Two further processes must therefore be 

 included among those making up our conception of life, viz. the 

 function of assimilation (the taking in and digestion of food), and the 

 function of respiration, in which oxygen is absorbed and carbon 

 dioxide is excreted into the surrounding atmosphere. 



The substances which make up our food-stuffs are all capable of 

 oxidation. Composed chiefly of carbon and hydrogen, with some 

 oxygen, nitrogen, and sulphur, they yield on complete combustion 

 carbon dioxide, water and small amounts of ammonia or allied bodies, 

 and sulphates. In this process of oxidation there is liberation of heat. 

 In the body a similar oxidation occurs, the products of oxidation 

 being discharged into the surrounding medium. An amount of energy 

 is thus set free which is available for the activities of the living organism. 



