4 PHYSIOLOGY 



It will be seen that the average difference between the calculated 

 and observed results amounts only to T01 per cent. an amazing 

 agreement considering the extreme difficulties of the experimental 

 methods involved. The important deduction to be drawn from these 

 observations is that the food-stuffs which are oxidised in the body 

 develop in this process exactly the same amount of energy as when 

 they are burnt up outside the body. 



From one aspect, therefore, the animal body may be looked upon 

 as a machine for the transformation of the potential energy of the 

 food -stuffs into kinetic energy, represented by the warmth and move- 

 ments of the body as well as by other physical changes involved in 

 vital processes. In the living organism we cannot, however, distinguish 

 between the source of energy and the machinery, as we can in the case 

 of our engines. When we endeavour to trace the food-stuffs after their 

 entry into the body, we lose sight of them at the point where they are 

 built up to form apparently an integral part of the living framework. 

 During activity there is a discharge of the products of oxidation of the 

 food-stuffs from this living matter, which therefore becomes reduced 

 in mass. This reduction, or disintegration of the living matter, 

 associated with activity, is always followed by a period of increased 

 integration, during which the organism grows by the assimilation of 

 more food. Our conception of life must therefore involve the idea of 

 a constantly recurring cycle of processes, one of building up, repair, or 

 integration, and the other, associated with activity, of destruction or 

 disintegration. If the former process predominates, we obtain a 

 steady increase in the mass of the organism, an increase which we 

 speak of as growth, and in many cases, as in that of plants, it is this 

 power of growth which we take as our criterion of the existence of life. 

 In fact, the possession by the green parts of plants of the power of 

 utilising the energies of the sun's rays for the integration of food-stuffs, 

 such as starch, with a high potential energy, is the necessary condition 

 for the existence of all higher forms of life on this earth. 



Closely associated with the property of growth is the power 

 v possessed by all living organisms of repair, i.e. the replacement by 

 newly formed healthy living material of parts which have been 

 damaged by external events. 



The process of growth does not, in the individual, proceed indefi- 

 nitely. At a certain stage in its life every organism divides, and a 

 part or parts of its substance are thrown off to form new individuals, 

 each of them endowed with the same properties as the parent organism, 

 and destined to grow until they are indistinguishable from the organism 

 whence they were derived. In the lowest forms of life, the unicellular 

 organisms, these processes of growth and division may go on until 

 brought to an end by some change in the environment which will not 



