496 A MANUAL OF PHYSIOLOGY 



impaired by such violent fluctuations. But it is necessary, 

 not only for health, but even for life, that the internal 

 temperature (the temperature of the blood) of a man should 

 vary only within relatively narrow limits around the mean 

 of 37 to 38 C. 



Why it is that a comparatively high temperature should 

 be needed for the full physiological activity of the tissues 

 of a mammal, while the in many respects similar tissues 

 of a fish work perfectly, although perhaps more sluggishly, 

 at a much lower temperature, is not quite clear ; nor do 

 we know the precise significance of that constancy of 

 temperature in the warm-blooded animal, which is as im- 

 portant and peculiar as its absolute height. The higher 

 animals must possess a superior delicacy of organization, 

 hardly revealed by structure, which makes it necessary that 

 they should be shielded from the shocks and jars of varying 

 temperature that less highly-endowed organisms endure with 

 impunity. Leaving the discussion of the local differences 

 and periodic variations of the temperature of warm-blooded 

 animals to a future page, let us consider now the mechanism 

 by which the loss of heat is adjusted to its production, 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 comparatively 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. 138, p. 493). 

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 when it is withdrawn and the portion of skin applied to the opening 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



