THERMOTAXIS 



697 



tion per square metre of body-surface, which is low at birth, increases 

 rapidly during the first year, reaches a maximum between the ages 

 of one and six, then falls steeply until the age of twenty, and thereafter 

 very slowly. The basal metabolism of boys of twelve or thirteen years 

 old per unit of surface is 25 per cent, higher than that of the adult. 



The next table, calculated by Rubner from the quantity of tissue- 

 protein and fat consumed, gives the relative intensity of heat-produc- 

 tion in fasting dogs of different sizes: and along with it is given tor 

 comparison some of the results on the output of the heart in dogs of 

 different weight, obtained by the method of injecting salt solutu 

 into the ventricle described on p. 139. 



It is obvious that the two quantities, heat-production (or loss) and 

 heart-output, vary in the same general way. The reason is clear. 

 Oxygen must be absorbed by the lungs in proportion to the heat 

 produced, and where more oxygen is to be absorbed, more blood passe? 

 through the lungs to take it up.* It is interesting to inquire whether 



* For simplicity, the possibility that the coefficient of utilization of the oxy- 

 gen-carrying capacity of the blood (i.e., the quantity of oxygen absorbed by 

 a litre of blood during its passage through the lungs divided by the total 

 quantity of oxygen which it can take up) may vary, is disregarded. This 

 possibility would imply that the average oxygen content of the venous blood 

 coming to the right side of the heart varied in animals of different size, more 

 oxygen being abstracted, for example, from the blood in passing through the 

 tissues of small animals than of large. The greater the utilization of the 

 oxygen the smaller would the quantity of blood passing through the lungs 

 require to be. There is no evidence, however, that such differences exist be- 

 tween animals of the same species of different size, although it has been sug- 

 gested that a higher coefficient of utilization coupled with a proportionately 

 smaller heart output may be one way in which training diminishes the diffi- 

 culty and discomfort of hard muscular effort. 



