RESPIRATION 



245 



greater in youth than in adult life, and greater in the vigorous 

 adult than in the old man. So that the age of the organism has 

 an influence apart from the extent of surface. The taking of 

 food increases the gaseous exchange, partly from the increased 

 mechanical and chemical work performed by the alimentary 

 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 

 instance, 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 pro- 

 duction 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 amount of carbon dioxide produced and of oxygen con- 

 sumed ; but probably only by increasing muscular movements, 

 including the movements of respiration. The external tempera- 

 ture also has an influence. In poikilothermal animals (such as the 

 frog), the temperature of which varies with that of the surround- 

 ing 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 production 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 : 



