590 PHYSIOLOGY OP THE DOMESTIC ANIMALS. 



As already mentioned, the quantity of oxj^gen taken in at each 

 inspiration does not all appear as carbonic acid ; a certain quantity of 

 the oxygen disappears, varying with the nature of the food 



If a dog be fed upon animal food, only 75 per cent, of the oxygen 

 of inspiration returns in the form of carbon dioxide. If it be fed on 

 vegetable food, 90 to 95 per cent, returns in the carbonic acid. 



There is no absolute relation between the amount of oxygen inspired 

 and the carbon dioxide expired, but a certain amount of oxygen always 

 remains behind, the surplus quantity going to oxidize certain materials, 

 such as sulphur, phosphorus, and, perhaps, certain food-constituents. 



The expired air also contains a notable quantity of carbon dioxide, 

 amounting to about 4 per cent. 



If we take thirty cubic inches as the volume of each expiration, the 

 average of carbon dioxide in each will be 1.29, or about twenty-three 

 cubic inches per minute, about one thousand three hundred and ninety- 

 three per hour, or twenty-seven thousand eight hundred and sixty-four 

 cubic inches per day, or what is equivalent to about seven and one-half 

 ounces of solid carbon. 



The quantity of carbon dioxide thrown off by the lungs is notably 

 diminished by the presence of a certain amount of carbon dioxide in the 

 atmosphere we breathe. 



To throw off the full amount, the surrounding atmosphere must be 

 devoid of this gas. If we breathe the same air over and over again, so 

 as to permit the accumulation of carbon dioxide, the difference in tension 

 between the amount of carbon dioxide in the inspired air and the air in 

 the lungs will be decreased, and diffusion interfered with. 



It has been found that if 5 per cent, of carbon dioxide be allowed to 

 remain in the air we breathe a fatal result ensues in consequence of the 

 difficulty of elimination of the carbon dioxide from the blood. Even when 

 carbon dioxide accumulates in the air below 2^ per cent, it soon begins 

 to produce depressing influences, 'and this, together with the accumula- 

 tion of other deleterious matters which are normally thrown off from the 

 lungs, is the common cause of the languor which comes on in crowded 

 rooms. When gradually subjected to these influences we do not appre- 

 ciate them so much as when suddenly subjected to them, for when grad- 

 ually brought under their influence the vital powers become depressed 

 to such a degree that they do not require the same amount of oxygen as 

 under ordinary circumstances. A person passing from the open air to a 

 room which has been occupied for some time by a large number of per- 

 sons at once experiences a depression, but after remaining there a little 

 while the vital powers become similarly depressed and the inconvenience 

 resulting from the presence of carbon dioxide in the atmosphere is no 

 longer appreciated. 



