578 A MANUAL OF PHYSIOLOGY . 



the mechanism by which the loss of heat is adjusted to its pro- 

 duction, so that upon the whole the one balances the other. 



Heat-loss. Heat is lost (i) from the surfaces of the body 

 by radiation, conduction, and convection ; (2) as latent -heat in 

 the watery vapour given off by the skin and lungs ; and (3) in 

 the excreta. Even in the bulky excrement of herbivora a com- 

 paratively trifling part of the total heat is lost. The second 

 channel of elimination is much more important ; the first is in 

 general the most important of all. 



The loss of heat by direct radiation from a portion of the skin 

 or clothes, or from hair, fur, or feathers covering the skin, may 

 be measured by means of a thermopile or a resistance radiometer 

 (bolometer) . The latter instrument is similar in principle and allied 

 in construction to the resistance thermometer used in measuring 

 superficial temperatures, and already described (Fig. 194, p. 574). 

 It may consist of a grating of lead-paper or tinfoil fixed vertically 

 in a small box which protects it from draughts. The box has a 

 sliding lid, which is kept closed till the moment of the observation, 

 when it is withdrawn and the portion of skin applied to the opening 

 at a fixed distance (5 to 10 cm.) from the grating. The intensity 

 of radiation depends on the excess of temperature of the radiating 

 surface over that of the surroundings, as well as on the nature of the 

 surface. The uncovered parts of the skin (face and hands in man) 

 radiate more per unit of area than the clothes or hair ; and the warm 

 forehead more than the comparatively cool lobe of the ear or tip of 

 the nose. When a man is sitting at rest in a still atmosphere, pure 

 radiation plays a greater, and conduction and convection play a 

 smaller, part in the total loss of heat from the skin than when he is 

 walking about or sitting in a draught. The more rapidly the air in 

 contact with the skin and clothes is renewed, the lower, other things 

 being equal, the temperature of the radiating surfaces is kept, the 

 greater is the loss of heat by conduction to the adjacent portions of 

 air, and the smaller the loss by radiation to the walls of the room, 

 the furniture, and other surrounding objects. It is probable that, 

 under the most favourable conditions, the amount of heat lost from 

 the surface by true radiation does not exceed the amount lost by 

 conduction and convection. 



The loss of heat by evaporation of water from the skin can be 

 calculated if we know the quantity of water so given off. For a 

 gramme of water at the ordinary temperature (say 15 C.) needs 

 '555 calories to convert it into aqueous vapour at the average tempera- 

 ture of the skin. If we take the average quantity of water excreted 

 as sweat in twenty-four hours as 750 c.c., this will be equivalent 

 to a heat-loss of 41 6' 25 say, in round numbers, 400 calories. 



The quantity of heat given off by the lungs may be also deduced 

 from calculation, the data being (i) the weight, temperature, and 

 specific heat of the expired air, and (2) the excess of water it contains 

 in the form of aqueous vapour over that contained in the inspired 

 air. Helmholtz calculated the quantity of heat needed to warm the 

 air 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 



