2 PHYSIOLOGY 



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 physio- 

 logical 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 con- 

 ception 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 respira- 

 tion 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 environ- 

 ment. For the production of these movements, as for the maintenance of a 

 constant body-temperature, 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. Before we can make any accurate in- 

 vestigations of the conditions which determine these activities, we must 

 know whether the two great laws of chemistry and physics, viz. the conser- 

 vation of mass and the conservation of energy, hold good for the processes 

 within the living body. The many experiments which have been made on 

 this point have decided the question in the affirmative. Thousands of 

 experiments have been made, both on man and on animals, in which the 

 total income of the body, viz. food and oxygen, has been weighed, and 

 compared with the total output, viz. carbon dioxide, water, and bodies 

 allied to ammonia (urea, &c.). In every case complete equality has been 



