PERSPIRATION AND DIGESTION. 31 



that of the sheep or pig, it is somewhat less than is given off 

 from the lungs of a man. 



But the source of this carbon the kind of food from which 

 it is usually derived is necessary to be understood. Starch, 

 sugar, and gum consist of carbon and water (or its elements) 

 only one hundred pounds of each of these substances, by 

 weight, on an average representing or containing forty-four of 

 carbon. These substances, and especially the starch, form a 

 very large proportion of all vegetable productions which are 

 used for food either by man or by other animals ; and the pur- 

 pose served by them in the system, is to supply the carbon 

 thrown off by the lungs during respiration. If this be their 

 natural use, then their presence in sufficient quantity in the 

 food becomes almost a necessity of nature. 



The fatty substances and oils which occur in plants also con- 

 tain much carbon united to hydrogen, and less oxygen than 

 exists in starch. One hundred pounds of these fatty substances 

 contain upwards of eighty pounds of carbon weight for weight, 

 therefore, they are capable of supplying the wants of the respi- 

 ration for a much longer period than starch. It is believed 

 that they really do, to a certain extent, supply carbon for res- 

 piration, and that this is one of the incidental purposes served 

 by the accumulation of fat in the animal body. But that this 

 is not intended to be the ordinary source of carbon, at least to 

 herbivorous animals, is shown by the comparative scarcity of fatty 

 matter in plants, and the abundance of starch, gum, and sugar. 



2. Perspiration. Animals also perspire ; and besides car- 

 bonic acid and a little nitrogen, saline, fatty, and other sub- 

 stances exude from the pores of the skin. In reference to this 

 function, it is necessary to be borne in mind, that the means of 

 a free perspiration are necessary to a healthy and thriving con- 

 dition of the animal, and that the substances given off from the 

 skin must be supplied by the food. 



3. Digestion. Properly speaking, this function consists sim- 

 ply in the conversion of the nutritive parts of the food into a 

 fluid form, and the absorption of them by the proper vessels 

 which are to convey them to the blood. That the presence of 

 free muriatic acid in the stomach is necessary to this resolution 



