498 A MANUAL OF PHYSIOLOGY 



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 pro- 

 portion 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 of all the heat produced in the body is the chemical 

 energy of the food substances. Whatever intermediate 

 forms this energy may assume whether the mechanical 

 energy of muscular contraction ; the energy of electrical 

 separation by which the currents of the tissues are pro- 

 duced ; the energy of the nerve impulse ; or the energy, be 



it what it may, which enables 

 the living cells to perform their 

 chemical labours it all ultimately, 

 except so far as external mechani- 

 cal work may be done, appears in 

 the form of heat. We do not 

 know at what precise stage of 

 metabolism the chief outburst of 

 heat takes place, but we may be 

 sure that the food, whether it is 

 burned' in a calorimeter to simple 

 FIG. 140. RESPIRATION end-products like carbon dioxide 

 and water, or more slowly oxidized 



B, copper tube with mouthpiece, . 



connected with the thin brass cap- in the body, yields the same 



sule 4 ; 4 is connected with a similar r , .... 



capsule 3 by a short tube, which amount ot heat, provided always 



passes out from it at the side fUof j n K n fh rac^c if ic onfirolw 



opposite to that at which B enters ; tnat l CaS6S lt 1S entirely 



2 and i are similar capsules. From consumed, and that no WOrk is 

 i an outlet tube C passes off. The 



whole is set in a copper cylinder A transferred to the outside. In the 



filled with water. A piece is sup- i_ j ji i , r i 



posed to be cut out of A in order to body the combustion of carbo- 



1 hydrates and fats is complete ; but 

 the nitrogenous residues of the 

 proteids urea, uric acid, etc. can be further oxidized, and 

 the remnant of energy which they yield must be taken into 

 account in any calculation of the total heat -production 

 founded on the heat of combustion of the food substances. 



