ANIMAL HEAT 



370 



loss of heat. All warm-blooded animals living in the sea are 

 bulky. It takes considerable heat -loss to lower the body 

 temperature of a horse i°, so that small animals in which, 

 as we have seen, the cooling surface is relatively greater than 

 in large animals,, must be able to increase rapidly their heat 

 production. Heat production varies as the cube of the body 

 volume, heat-loss varies as the square of the body surface. 



The skin as a source of loss of heat is largely controlled by 

 the nervous system. Through the vasomotor nerves the 

 vessels of the skin are constricted or dilated ; when the vessels 

 are constricted the skin becomes pale (though this may not 

 be seen, owing to hair and pigment) and the blood is throw r n 

 upon the internal viscera, where it is additionally shielded from 



K 



X 





k: ::M 



1 1 

 1 1 



"ITS 



5 





^ 



x 



T-rfr 



i i 



*fc 



i ' 





-ti-X 



Fig. 107. — Diagram to illustrate the Relation between Volume or 

 Weight and Surface (Waller). 



The volumes are 1 8 27 cubic centimetres. 



The weights are 1 8 27 grammes. 



The surfaces are 6 24 54 square centimetres. 



That is to say, their ratio of increase is 1 4 9. 



loss. In consequence the skin becomes cold and the loss of 

 heat less, not merely as the result of the lessened radiation, but 

 chiefly as the outcome of the diminished evaporation. When 

 the vessels are dilated the skin becomes flushed and hot, the 

 veins stand out with blood, and a large amount of heat is lost. 

 This vasomotor action is an automatic reflex act, as also is the 

 nervous control over the sweat-glands, by which more or less 

 water is poured out on the surface of the body and heat lost by 

 its evaporation. This nervous control is normally set in action 

 by changes in the temperature of the surroundings. 



The loss of heat is influenced by the thickness of the natural 

 covering, its colour, etc. The loss of heat from a rabbit after 

 shaving off the fur is one and a half times greater than before 

 shaving. Sheep before shearing excrete less C0 2 and more 

 H 2 than the same sheep after shearing. White rabbits lose 



