OF RESPIRATION. 121 



252. An immediately obvious effect of respiration in the 

 red-blooded animals is a change of color; the blood, in 

 passing through the respiratory organs, being changed from 

 a very dark purple to a bright scarlet. In the great circula- 

 tion (241) the scarlet blood occupies the arteries, and is usu- 

 ally called red blood, in contradistinction from the venous 

 blood, which is called black blood. In the lesser circulation, 

 on the contrary, the arteries carry the dark, and the veins 

 the red blood. 



253. The quantity of oxygen consumed by various ani- 

 mals in a given time has been accurately ascertained by ex- 

 periment. It has been found, for instance, that a common- 

 sized man consumes, on an average, about 150 cubic feet in 

 twenty-four hours; and as the oxygen constitutes but 21 per 

 cent, of the atmosphere, it follows that -he inhales, during a 

 day, about 700 cubic feet of atmospheric air. In birds, the 

 respiration is still more active, while in reptiles and fishes it 

 is much more sluggish. 



254. The energy and activity of an animal is, therefore, 

 somewhat dependent on the activity of its respiration. Thus 

 the toad, whose movements are very sluggish, respires much 

 more slowly than the mammals, birds, and even insects ; and 

 it has been ascertained that a butterfly, notwithstanding its 

 comparatively diminutive size, consumes more oxygen than 

 a toad. 



255. The circulation and respiration have a reciprocal in- 

 fluence upon each other. If the heart be powerful, or if 

 on violent exercise a more rapid supply of blood to I'epair 

 the consequent waste is demanded, (201,) respiration must be 

 proportionally accelerated to supply air to the greater amount 

 of blood sent to the lungs. Hence the panting occasioned by 

 running or other unusual efforts of the muscles. On the 

 other hand, if respiration be hurried, the blood is rendered 

 more stimulating by greater oxyggnation, and causes an ac- 



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