EFFECT OF EXTERNAL TEMPERATURE 507 



The organs concerned in the assimilation of food respond 

 most readily and efficiently to the natural stimulus, food, if the 

 meal has been preceded by a fast. It is, therefore, interesting 

 to note that there is generally among all classes of men an 

 interval of twelve hours between the last meal in the evening 

 and the first meal on the following morning ; the respiratory 

 exchange may thus fall to a very low value, or even reach the 

 minimum observed during fasting, and the digestive organs 

 will be in the most fit condition for the digestion and assimilation 

 of food. 



The very constant value of the minimal respiratory exchange 

 and the rapidity with which it is attained during the first day of 

 a fast indicate that the vital activity of an animal is associated 

 with certain chemical changes, which must be maintained above 

 a definite level. The processes of oxidation may be greatly in- 

 creased beyond the minimum, but cannot fall below it without 

 endangering life. To this there is an apparent exception in 

 hibernating animals ; but it is apparent only, for the great decrease 

 in the respiratory exchange is accompanied by a reduction of vital 

 activity to the lowest ebb. 



Effect of External Temperature upon the Respiratory Exchange. 

 The two groups of animals, the cold-blooded and the warm-blooded, 

 show a marked difference in their response to a change in the 

 temperature of their surroundings ; the temperature and respira- 

 tory exchange of the former vary with and in the same direction 

 as the external temperature, whereas the temperature of the latter 

 is fairly constant for a wide range of heat and cold, and their 

 respiratory exchange is increased by cold and diminished by heat. 

 The warm-blooded animal is able to maintain its constant high 

 temperature by the control which its nervous system exerts over 

 the production and loss of heat. The calorimeter is the only 

 efficient method of measuring the loss of heat, but variations in 

 the production can be studied with the respiration chamber as 

 well as the calorimeter, for the respiratory exchange is a measure 

 of the combustion. Thus it is that the influence of changes of 

 external temperature upon the respiratory exchange cannot be 

 discussed apart from the question of the regulation of temperature. 

 In no case is this connection so clear as in those animals which 

 form a link between the warm-blooded and the cold-blooded 



