Page:Popular Science Monthly Volume 20.djvu/465

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449
HOW ANIMALS BREATHE.

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 inversely 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 principle 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 absorbed 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-breathers

PSM V20 D465 Holothuroidea.jpg
Fig. 2. Holothuroidea {Thyone papillosa). (After Forbes.)

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 "water-breathers" the blood, when existing, is commonly sent to the surface of the body, and a slight movement of the water produced—sufficient to renew that in contact with the respiratory organs. But "air-breathers" always draw the mobile atmosphere inward to the blood.