568 
ON ANIMAL HEAT. 
state. But it often happens, during the exertions of the animal, 
or when the body is placed in an unusually high temperature, or 
wrapped in materials which conduct heat slowly, that the gene- 
ration of heat exceeds the immediate consumption, and the actual 
temperature of the whole body becomes increased. Nature, how- 
ever, has beautifully adapted the animal body to keep up a tem- 
perature no higher than is necessary to its economy. Accordingly, 
when any circumstances occur which tend to an unnecessary mani- 
festation of vital heat, by the very power which propels the blood 
with increased force along the arteries into the capillaries is the 
reduction of caloric obtained. The pores of the skin are opened, 
and, giving out abundant watery exudation, act by the powerful 
means of evaporation to diminish the temperature of the body. 
The local effect of evaporation is well marked in the coldness of 
the dog’s nose: the part being generally moistened is surprisingly 
cold; but should the exudation be stopped by any internal cause, 
the nose acquires a temperature identical with that of the other 
external parts of the body. 
36. It usually happens that the atmospheric air received into 
the lungs during respiration becomes considerably increased in 
temperature before leaving the body. It carries off an important 
quantity of the superabundant caloric generated in the animal 
frame. It serves to graduate that heat, and the heat of the ex- 
pired air is always proportionate to the internal warmth of the 
body. The air, therefore, which enters the lungs, instead of 
being a source of warmth, becomes the means of refrigerating the 
internal parts of the frame. The extensive surface offered by the 
air-cells of the lungs for evaporation materially assists in cooling 
the body. The powerful influence which the admittance of air 
into the lungs has on the regulation of the internal temperature 
may be inferred from the fact of the land tortoise of hot countries 
(7) keeping itself several degrees cooler than the atmosphere. 
It must be recollected that this animal is covered with a horny 
shell, almost wholly precluding the possibility of evaporation 
from the external parts of the body. Still we must not generally 
regard all the heat acquired by expired air as derived solely from 
the blood. The whole respiratory apparatus, including the 
trachea and bronchial tubes, to their minutest ramifications, is 
provided with elastic tissue for the production of warmth. 
Throughout the whole course of these tubes, on every inspiration, 
heat is generated. By these means the air acquires an increase 
of temperature before it comes into juxta-position with the blood, 
and the shock is not so great to the system as i t wou Id otherwise be. 
The air admitted into the lungs affords an abundant source of 
latent heat, which the elastic tissues may absorb during their re- 
laxation. 
