ANIMAL HEAT 579 



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. 398) of the epithelium 

 of the alveoli is 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 



B, copper tube with mouth- 

 piece, 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 capsules. 

 A is placed in another wider 

 copper cylinder. 



FIG. 197. RESPIRATION CALORIMETER. 



the amount often to double or even treble. A diagram of a respira- 

 tion calorimeter (for measuring the heat given off in respiration) is 

 shown in Fig. 197. (See Practical Exercises, p. 613.) 



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. 



Per Cent. Calories. 



r Evaporation of water - - 15 \ 400 



Skin J Radiation* - 25 80 650 



(Conduction (and convection) 40 J i ooo 



Heating the excreta - 2*5 70 



100 2,590 



In the rabbit, according to Nebelthau, the heat lost by evapora- 

 tion 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. 



Sources of the Heat of the Body Heat-production. Some 

 heat enters the body as such from without in the food, and 

 by radiation from the sun and from fires. The ultimate source 



* The relative amounts lost by radiation and conduction cannot be 

 accurately fixed. The proportion is extremely variable. 



