HOW ANIMALS BREATHE. 449 



drying for an indefinite time ; and, under such conditions, the waste 

 of the tissues must be entirely suspended. 



In "warm-blooded" animals birds and mammals a constant 

 body-temperature, independent of the surrounding atmosphere, is 

 maintained by the immediate use of the food as fuel. Consequently, 

 in a warm atmosphere less internal combustion is required than in a 

 cold one, and the respiration of birds and mammals is therefore in- 

 versely proportionate to the external temperature, although directly 

 proportionate to the activity of the animal. 



This constant body-temperature is the reason of the difference in 

 the kind and quantity of food required according to season or latitude. 

 While the people of the tropics subsist chiefly on vegetable food, which 

 supplies little fuel, but on the contrary much fluid to cool the body by 

 evaporation, the inhabitants of frigid regions use carbonaceous foods 

 affording the greatest proportion of fuel. 



Experiments tend to prove that human respiration is, as would 

 theoretically be expected, less rapid in the tropics than in the cold 

 regions. Every traveler knows that a less amount of food is required. 

 But on the contrary, and for the reason above stated, respiration in 

 the cold-blooded animals is more rapid in the tropics, and the quantity 

 of food is greater. 



Physiological Principle. The process of respiration is in prin- 

 ciple an interchange of gases between the fluids of the structure and 

 the external medium, air or water. It should be unnecessary to say 

 that aquatic animals do not breathe water, but the air which is ab- 

 sorbed by the water. This exchange is effected by the physical action 

 known as osmosis. It is a question whether vital influence has any 

 part in the process. The principle is identical in all creatures, air- 





Fig. 2. Holothuboidea {Thy one papilloma). (After Forbes.) 



breathers and water-breathers those which have special fluids, or 

 blood, and those having none. It applies to plants also, as far as they 

 have a true respiration. 



In bringing the internal and the external fluids adjacent, Nature in 

 this function, as in all others, is nicely economical of power. In " wa- 

 ter-breathers " the blood, when existing, is commonly sent to the sur- 

 face of the body, and a slight movement of the water produced suffi- 

 cient to renew that in contact with the respiratory organs. But " air- 

 breathers " always draw the mobile atmosphere inward to the blood. 

 vol. xx. 29 



