rous tubes
upon its inner surface is discharged a colorless fluid, called the
_gastric juice_, which mingles with the food and dissolves it. When the
food is reduced to a liquid condition, it accumulates in the pyloric
portion of the stomach. Some distinguished physiologists believe that
the food is kept in a gentle, unceasing, but peculiar motion, called
_peristaltic_, since the stomach contracts in successive circles. In the
stomach the food is arranged in a methodical manner. The undigested
portion is detained in the upper, or cardiac extremity, near the
entrance of the esophagus, by contraction of the circular fibers of the
muscular coat. Here it is gradually dissolved, and then carried into the
pyloric portion of the stomach. From this, then, it appears, that the
dissolved and undissolved portions of food occupy different parts of the
stomach. After the food has been dissolved by the gastric fluid, it is
converted into a homogeneous, semi-fluid mass, called _chyme_. This
substance passes from the stomach through the pyloric orifice into the
duodenum, in which, by mixing with the bile and pancreatic fluid, its
chemical properties are again modified, and it is then termed _chyle_,
which has been found to be composed of three distinct parts, a
reddish-brown sediment at the bottom, a whey-colored fluid in the
middle, and a creamy film at the top. Chyle is different from chyme in
two respects: First, the alkali of the digestive fluids, poured into the
duodenum, or upper part of the small intestine, neutralizes the acid of
the chyme; secondly, both the bile and the pancreatic fluid seem to
exert an influence over the fatty substances contained in the chyme,
which assists the subdivision of these fats into minute particles. While
the chyle is propelled along the small intestine by the peristaltic
action, the matter which it contains in solution is absorbed in the
usual manner into the vessels of the villi by the process called
_osmosis_. The fatty matters being subdivided into very minute
particles, but not dissolved, and consequently incapable of being thus
absorbed by osmosis, pass bodily through the epithelial lining of the
intestine into the commencement of the lacteal tubes in the villi. The
digested substances, as they are thrust along the small intestines,
gradually lose their albuminoid, fatty, and soluble starchy and
saccharine matters, and pass through the ileo-caecal valve into the
caecum and large intestine. An