628 LECTURE XXVII. 



advisable to depend upon this respiratory quotient alone, but we should 

 also take into consideration the other eliminations, especially that of 

 nitrogen. 



Now that we have roughly sketched the outlines of the methods employed 

 for studying metabolism, we will briefly discuss, before taking up the 

 important facts that have been ascertained concerning metabolism under 

 definite conditions, the influence of the conditions created by the subject 

 experimented upon itself. First of all, there is the question of size. It 

 is perfectly clear that the total metabolism will be more extensive in pro- 

 portion to the size of the organ in function. Eventually the consumption 

 of material is to be traced to the work of the individual cells, and the more 

 cells there are the greater will be their total requirement. Thus a small 

 animal will require absolutely less nourishment than a larger one. Of 

 course individual peculiarities play a part which must not be left out of 

 consideration. If, on the other hand, instead of paying attention to the 

 absolute amount of material consumed, we consider the energy trans- 

 formed per kilogram of body- weight, provided we are working under 

 otherwise parallel conditions, we shall find that the metabolism of the 

 smaller animal is greater than that of the larger one. In order to obtain 

 values which shall be actually comparable, the separate experiments upon 

 metabolism must be carried out with the animal at rest, and also fasting. 

 In this way the so-called fasting value is obtained. 1 The reason that 

 a small animal decomposes more substance in proportion to its own 

 weight lies in the following: The smaller an animal is, the larger the 

 surface of its body in comparison to the volume and weight of the body. 

 It may be assumed that about four-fifths of the total heat given off by 

 the body is through the skin. The amount of heat lost by the skin is very 

 nearly proportional to the amount of surface covered by it, so that the 

 smaller animal with its relatively larger surface loses more heat than the 

 larger animal. Consequently the smaller animal requires a greater supply 

 of heat energy than the larger one, as otherwise its body temperature, 

 which is regulated by two factors, the amount of heat generated and that 

 given off, will not be maintained at the proper height. 



The tables on the following page show how the amount of oxygen 

 consumed depends upon the size of the body, 2 and give also a comparison 

 of the metabolism of energy in animals of various sizes with their relative 

 surface development. 3 



The influence of the greater surface becomes apparent when we compare 

 the metabolism of younger and older individuals of the same species. 



1 Cf. Max Rubner: Z. Biol. 19, 535 (1883). Slowtzoff : Pfliiger's Arch. 96, 158 (1903). 

 Karl Oppenheimer: Z. Biol. 42, (1901). 

 1 Max Rubner: Z. Biol. 19, 536 (1883). 

 3 Max Rubner: ibid. p. 549. 



