BESPIEATIOX. 237 



acid gas, will have taken its place. The essential office of 

 respiration is to supply oxygen to the blood, at the same 

 time that carbon is removed from it. 



396. An immediately obvious effect of respiration in the 

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

 passing through the respiratory organs, being changed from a 

 very dark purple to a bright scarlet. In the great circulation 

 the scarlet blood occupies the arteries, and is usually called 

 red blood, in contradistinction to the venous blood, which is 

 called black blood. In the lesser or pulmonary circulation, on 

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

 red blood. 



396*. The quantity of oxygen consumed by various ani- 

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

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

 sized man consumes, on an average, about one hundred and 

 fifty cubic feet in twenty-four hours ; and as the oxygen con- 

 stitutes but twenty-one per cent, of the atmosphere, it follows 

 that he inhales, during a day, about seven hundred cubic feet 

 of atmospheric air. In birds, the respiration is still more 

 active, while in reptiles and fishes it is much more sluggish. 



397. The energy and activity of an animal is somewhat 

 dependent on the activity of its respiration. Thus the toad, 

 whose movements are very sluggish, respires much more slowly 

 than mammals, birds, and even insects ; and it has been ascer- 

 tained that a butterfly, notwithstanding its comparatively 

 diminutive size, consumes more oxygen than a toad. 



398. The circulation and respiration have a reciprocal 

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

 violent exercise demand a more rapid supply of blood to 

 repair the consequent waste, respiration must be propor- 

 tionally 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 ren- 

 dered more stimulant by greater oxygenation, causes an ac- 

 celeration of the circulation. The quantity of air consumed 

 varies therefore with the proportion of the blood which is 

 sent to the lungs. 



339. The proper temperature of an animal, or what is 

 termed ANIMAL HEAT, depends on the combined activity of 



