CHAPTEE VIII. 



DIGESTION. 



DIGESTION is the process by which the food is reduced to a form in 

 which it can be absorbed from the intestinal canal, and taken up by the 

 bloodvessels. This process does not occur in vegetables, which are 

 dependent for their nutrition upon materials which are supplied to them 

 in a form already fitted for absorption. 



Carbonic acid, ammonium carbonate, and ammonium nitrate exist in 

 a gaseous form in the atmosphere, or are brought down in solution by 

 the rain, and penetrate the soil to the roots of the growing plants ; while 

 many of the mineral salts, as sulphates, nitrates, and carbonates, are 

 also present in the soil in a soluble condition. Thus they require no 

 alteration before being taken up by the tissues of the plant. The only 

 known exception to this is in the case of materials composed of the 

 earthy carbonates and phosphates, which are insoluble or nearly so in 

 water, but which are known to be corroded and rendered soluble by the 

 acid juices of the plant-roots in contact with them. As a general rule, 

 the substances requisite for vegetation are directly absorbed from the 

 exterior in their original condition. But with animals and man the 

 case is different. They cannot subsist upon inorganic substances only, 

 but require for their support materials which have already been organ- 

 ized, and which have previously constituted a part of animal or vegetable 

 bodies. Their food is almost invariably solid or semi-solid when taken, 

 and insoluble in water. Meat, bread, fruits, vegetables, and the like, are 

 all taken into the stomach in a solid and insoluble condition ; and even 

 substances naturally fluid, such as milk, albumen, white of egg, are nearly 

 always, in the human species, more or less solidified by the process of 

 cooking, before being taken into the stomach. 



In animals, accordingly, the food requires to undergo a process of 

 digestion or liquefaction, before it can be absorbed. In all cases, the 

 general characters of this process are the same. It consists essentially 

 in the food being received into a canal, running through the body from 

 mouth to anus, called the " alimentary canal," in which it comes in con- 

 tact with certain digestive fluids, which act upon it in such a way as to 

 liquefy and dissolve it. These fluids are secreted by the mucous mem- 

 brane of the alimentary canal, and by certain glandular organs situated 

 in its neighborhood. The food consists, as we have seen, of a mixture 

 of various substances, having different physical and chemical properties ; 

 and the several digestive fluids are also different from each other, each 



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