592 FAT SUPPORTS RESPlllATIOJN IN SOMli, 



— the proportion of carbonic acid being very small. But as it is breathed 



out again it consists of about-— 



Nitrogen 79-16 



Oxygen 16-84 to 12 



Carbonic acid 4-00 to 8 



100 

 — the proportion of oxygen being considerably less, that of carbonic acid 

 very piuch greater, than before. On an average the natural proportion 

 of carbonic acid in the air is found to be increased 100 times after it is 

 expelled by breathing from the lungs. 



Now carbonic acid consists, as we have previously seen, of carbon and 

 oxygen. In breathing, therefore, the animal throws off' into the air a 

 quantity of carbon — in the form of carbonic acid — which varies at dif- 

 ferent times, in different species of animals, and in different individuals of 

 the same species. By a heahhy man the quantity of carbon thus 

 thrown off* varies from 5 to 13 ounces, and by a cow or a horse from 3 to 

 5 pounds, in 24 hours. All this carbon must be derived from the food. 

 The animal eats, therefore, not merely to support or to add weight to its 

 body, but to supply the carbon also which is wasted by respiration. 



2°. How the respiration is fed. — What part of the ibod supplies the 

 waste caused by respiration ? How is the respiration fed ? 



In animals which live upon flesh — carnivorous animals — it is the fat 

 of their food from which the carbon given off" by their lungs is derived. 

 It is only when the fat fails in quantity that the lean or muscular part 

 of the flesh they eat is decomposed for the purpose of supplying carbon 

 to their lungs. 



In an animal to which no food is given for a time, the lungs are fed, 

 so to speak, from fat also. But in this case it is the living fat of the 

 animal's own body. When digestion is fully performed and hunger is 

 keenly experienced, the body begins to feed upon itself— the lungs still 

 play, respiration continues for many days after food has ceased to be ad- 

 ministered, but the carbon given otfis derived from the substance of the 

 body itself. The fat first disappears — escapes with the breath — and af- 

 terwards the muscular part is attacked. Hence the emaciation which 

 follows a prolonged abstinence from food. 



In animals which live upon vegetable food again — herbivorous ani- 

 mals — it is the starch, gum, and sugar, of the food which supply th") 

 carbon for respiration. It is only when the food does not contain a suf 

 ficient supply of these compounds that the oil first, and then the gluten, 

 are decomposed, and made to yield their carbon to the lungs. 



In man, who lives on both kinds of food, and in the domestic dog, and 

 the pig, which also eat indifferently both animal and vegetable food, the 

 carbon of respiration may be derived in part from the fat, and in part 

 from the starch and sugar which they eat — according as they are chiefly 

 supported by the one or by the other kind of food. 



It may be asked how we know that such are the parts of the food, to 

 which the duty of supplying the demands of the lungs is especially com- 

 mitted. There are several considerations which lend force to this opin- 

 ion. Of these I willtlraw your attention to one or two. 



a. Why is the fa", rather than the lean part of the food of carnivorous 



