1X.J 



ACTION OF THE LUNGS. 



81 



This shows a very small quantity of carbonic acid in 

 the atmospheric air, and how greatly the carbonic 

 acid is increased by a single respiration of the animal. 

 We have therefore, by the healthy action of the lungs 

 upon blood properly supplied with non-nitrogenous 

 matter, an internal source of heat which maintains the 

 warmth of the body, up to a healthy standard. 



167. Two conditions are essentially necessary to 

 secure this result 



The lungs must be sufficiently powerful, and, 

 The blood must contain sufficient heat-producing 

 matter. 



The demands made for the maintenance of the 

 warmth of the body, are entirely regulated by the loss 

 of heat from the body. If the loss of heat be great, 

 the demand for heat is great also ; and if the loss of 

 heat be small, there is only a small supply needed. 

 If the air should be very cold, there will be a great 

 loss of warmth, and the demand for internal heat will 

 be great; but if, by sheltering the animal from the 

 cold air, the loss of heat is reduced, then the demand 

 for internal heat will be decreased also. It is there- 

 fore quite within our power to reduce the loss of heat 

 from the body, and thereby to economize the 

 fuel or heat-producing matter of the food. 



1 68. The demands made upon the lungs and the 

 food, are therefore entirely regulated by the waste of 

 warmth from the body. If there be plenty of heat- 



