DIGESTION. 115 



CHAPTEK VI. 



DIGESTION. 



DIGESTION is that 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. 

 For vegetables are dependent for their nutrition, mostly, if not 

 entirely, upon a supply of inorganic substances, as water, saline 

 matters, carbonic acid, and ammonia. These materials constitute 

 the food upon which plants subsist, and are converted in their inte- 

 rior into other substances, by the nutritive process. These mate- 

 rials, furthermore, are constantly supplied to the vegetable under 

 such a form as to be readily absorbed. Carbonic acid and ammonia 

 exist in a gaseous form in the atmosphere, and are also to be found 

 in solution, together with the requisite saline matters, in the water 

 with which the soil is penetrated. All these substances, therefore, 

 are at once ready for absorption, and do not require any preliminary 

 modification. But with animals and man the case is different. 

 They cannot subsist upon these inorganic substances alone, 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 

 at the time when it is taken, and insoluble in water. Meat, bread, 

 fruits, vegetables, &c., are all taken into the stomach in a solid and 

 insoluble condition ; and even those substances which are naturally 

 fluid, such as milk, albumen, white of egg. are almost always, in 

 the human species, coagulated and solidified by the process of cook- 

 ing, 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 contact with certain digestive fluids, which act 



