OF RESPIRATION. 95 



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 Hood, in contradistinction from the venous 

 blood, which is called Hack Hood. 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 

 experiment. 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 correspond 

 with 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 

 violent exercise demand a more rapid supply of blood to 

 repair the consequent waste (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 being rendered more stimulant by greater oxygena- 

 tion, causes an acceleration of the circulation. The quan- 

 tity of air consumed varies therefore with the proportion of 

 the blood which is sent to the lungs. 



