STARCH AND SUGAR IN OTHEil RACES. 693 



animals devoted to the service of the lungs, and why do starving ani- 

 mals lose their fat first ? Because the chemical decomposition by which 

 carbon can be derived from the tat is simpler and more easily etfected 

 than thai by which it can be ob a-ned from muscular fibre. By combi- 

 nation with oxygen, fat can be converted into carbonic acid and water 

 only, of which the former will pass oIFby the lungs and the latter in the 

 urine. The muscular fibre, on the other hand, contains much nitrogen 

 (p. 444), and, if deprived of its carbon for the uses of respiration, must 

 undergo very complicated decompositions, and form a series of com- 

 pounds, the use of which, in the animal economy, it is not easy to perceive. 



Besides, in producing the carbonic acid of the luairs from the fat of the 

 animal food or of the living body, there is less waste of material. Fat 

 consists wholly of the three elements, carbon, hydrogen, and oxygen. 

 These all disappear entirely in the form of carbonic acid and water — both 

 of which are used up. Muscle, on the other hand, besides nitrogen, con- 

 tains a constant proportion of sulphur and phosphorus. If the muscle, 

 then, be decomposed for tlie purpose of supplying carbon to the lungs, 

 not only the large quantity of nitrogen, but the sulphate and phosphorus 

 also, would go to waste, and would pass off in the urine. In nature, 

 however, such waste is rarely seen to take place ; and, therefore, as a 

 general rule, the resj^iration will be supported by the muscular fibre only 

 when other kinds of food are deficient. 



b. But in the stomachs of^ herbivorous animals, why are the starch and 

 sugar especially appropriated to the use of the lungs ? The food of ani- 

 mals which live upon vegetable substances contains fat as well as starch 

 — why then is the starch in this case dissipated by the process of respira- 

 tion, while the fat is applied as it is supposed to another use ? The 

 answer to this question is both beautiful and satisfactory. 



Starch, gum, and sugar, consist of carbon and water only, and we can 

 conceive them in their passage through the body to be actually separated 

 into these two substances — in which case the carbon has only to combine 

 with oxygen and form carbonic acid, to be ready to pass off by the lungs. 

 Here, therefore, only one chemical combination is required — the union 

 of carbon with oxygen. It is the simplest way in which we can con- 

 ceive carbon to be supplied for the use, or for the purposes of the lungs.* 



But it is otherwise with fat. Though nearly all kinds of fat consist en- 

 tirely of carbon, hydrogen, and oxygen—yet they cannot be supposed to 

 consist only of carbon and water. They contain much more hydrogen than 

 is necessary to form water with the oxygen which is present in them. If, 

 'hen, the carbon of these fats be separated, this excess of hydrogen will 

 ilso be set free, and if the f()rmer be made to combine with oxygen to 

 form carbonic acid, the latter must also combine with hydrogen to form 

 water. Thus two chemical changes must go on simultaneously, for 

 which more oxygen will be required, and which involve more labour in 

 the system than when the carbon alone is to be combined with oxygen. 

 It is natural, therefore, that where both starch and oil are present to- 

 gether, the former should be first converted to the uses of the lungs, the 

 latter only when the supply of starch or sugar has been exhausted. 



'The chemical reader will understand that I am here only giving a popular view of the 

 final result of the several changes through which the carbon no doubt passes before it 

 escapes in the form of carbonic acid. 



