Digestion, in animal economy, signifies the dissolution of food taken into the stomach, in order to supply the continual loss sustained by perspiration, the different functions, or by exercise.

As soon as the food is taken into the mouth, it is first broken and divided by the teeth, being at the same time moistened with a liquor supplied by the salival glands, and consequently formed into a kind of paste. Thus prepared, it passes into the stomach to ferment; a process which is effected, 1. By the salival and gastric juices, which have an effect on aliment similar to that of leaven, or yeast, on dough; 2. By the vital heat of the stomach and viscera of the abdomen; men ; 3. By the remains of food, which adhere to the folds of the stomach, and there become acid and acrimonious ; 4. By the agitation arising from the pressure of the abdomen, and the continual pulsation of the contiguous blood-yessels ; 5. By the liquor which the repeated compression of those muscles causes to be discharged from the glands of the stomach : and, lastly, by air itself, which being mixed with alimentary matter, dilates by the heat of the stomach, and separates the particles of food which, from the concurrence of these causes, are converted into chyle.

From the stomach, the chyle descends into the intestines, where it incorporates with the blood; which, by its volatile nature, toge ther with the saline and nitrous parts of the air, subtilizes the aliment, and perfects its digestion. These powers, however, are frequently impeded, or weakened, from a variety of causes, too minute to be specified here, but which will be occasionally ment oned in their alphabetical series. - See In-digestion.