658 



ANIMAL HEAT 



expired by a man in twenty-four hours from an initial temperature of 

 20 to body-temperature, at 70 calories, and that required to evaporate 

 the water given off by the lungs at 397, making the total heat-loss by 

 the lungs in these processes from 400 to 500 calories. A certain amount 

 of heat is also absorbed in connection with the escape of the carbon 

 dioxide. The reason why a great deal more water and therefore more 

 heat is not given off by the lungs with their enormous surface, and the 

 high degree of imbibition (p. 420) of the epithelium of the alveoli, is 



Fig. 213. Calorimeter for 

 Measuring Heat given off in 

 Respiration. B, copper tube 

 with mouthpiece, connected with 

 the thin brass capsule 4; 4 is 

 connected with a similar capsule 

 3 by a short tube, which passes 

 out from it at the side opposite 

 to that at which B enters; 2 and 

 i are similar capsules. From i 

 an outlet tube C passes off. 

 The whole is set in a copper 

 cylinder A filled with water. 

 A piece is supposed to be cut out 

 of A in order to show the cap- 

 sules. A is placed in another 

 wider copper cylinder. 



that the air is already saturated with aqueous vapour, or nearly so, 

 before it reaches the alveoli. By direct calorimetric observations it 

 was found that a man of 70 kilos weight gave off in normal breathing, 

 with an air-temperature of 12 to 15 C., from 350 to 450 calories. 

 Forced respiration, as might be expected, increased the amount often 

 to double or even treble. A diagram of a calorimeter for measuring 

 the heat given off in respiration is shown in Fig. 213. (See Practical 

 Exercises, p. 694.) 



The following table gives an analysis of the heat-loss of an average 

 man. It must be understood that the figures are only approximate. 

 In round numbers we may say that two-thirds of the heat-loss is 

 due to radiation, conduction, and convection, and one-third to the 

 evaporation of water. 



["Evaporation of water 

 Skin j Radiation* - 



[Conduction (and convection) 

 T ] Evaporation of water - 

 n & s (Heating the expired air- 

 Heating the excreta 



Per Cent. 



15 

 25 

 40 



15 



80 



17-5 



Calories. 

 400 

 650 



I,OOO 



| 4 oo 



I 70 

 70 



100 2,590 



In the rabbit, according to Nebelthau, the heat lost by evaporation 

 of water is about 16 per cent, of the whole, or about half the proportion 

 in man, according to the above calculation. This is not surprising 

 when we reflect that the rabbit does not sweat, and drinks comparatively 

 little water. 



* The relative amounts lost by radiation and conduction cannot be accur- 

 ately fixed. The proportion is extremely variable. 



