244 



RESPIRATION 



increased mechanical and chemical work performed by the ali- 

 mentary canal and the digestive glands. But that this is not the 

 sole cause of the increase is shown by the fact that it varies with 

 different kinds of food to a greater extent than can be explained 

 by differences in the ease with which they are digested. For in- 

 stance, maize produces a much greater increase than oats when 

 given in equal amount, and a protein diet a greater increase than a 

 diet of carbo-hydrate or fat. Sleep diminishes the production of 

 carbon dioxide partly because the muscles are at rest, but also to 

 some extent because the external stimuli that in waking life excite 

 the nerves of special sense are absent or ineffective. Even a bright 

 light is said to cause an increase in the atmount of carbon dioxide 

 produced and of oxygen consumed ; but probably only by increasing 

 muscular movements, including the movements of respiration. 

 The external temperature also has an influence. In poikilothermal 

 animals (such as the frog), the temperature of which varies with 

 that of the surrounding medium, the production of carbon dioxide, 

 on the whole, diminishes as the external temperature falls, and 

 increases as it rises. In homoiothermal animals, that is, animals 

 with constant blood temperature, external cold increases the pro- 

 duction of carbon dioxide and the consumption of oxygen. But if 

 the connection of the nervous system with the striated muscles has 

 been cut out by curara, the warm-blooded animal behaves like the 

 cold-blooded (Pfliiger and his pupils in guinea-pig and rabbit). 

 These interesting facts will be returned to under ' Animal Heat.' 



Cold-blooded animals produce far less carbon dioxide, and con- 

 sume far less oxygen, per kilo of body-weight than warm-blooded. 



The following table shows the relation between the body-weight 

 and the excretion of carbon dioxide in man : 



The next table illustrates the difference in the intensity of metab- 

 olism in different kinds of animals, a difference, however, largely 

 dependent upon relative size: 



