of Longitudinal fibers removed.

The outer coat of the stomach, called the _serous coat_, is a continuation
of the peritoneum, the membrane lining the abdominal cavity.

*Stomach Digestion.*--In the stomach begins the definite work of dissolving
those foods which are insoluble in water. This, as already stated, is a
double process. There is first a chemical action in which the insoluble
are changed into soluble substances, and this is followed immediately by
the dissolving action of water. The chief substances digested in the
stomach are the proteids. These, in dissolving, are changed into two
soluble substances, known as _peptones_ and _proteoses_. The digestion of
the proteids is, of course, due to the

*Gastric Juice.*--The gastric juice is a thin, colorless liquid composed of
about 99 per cent of water and about 1 per cent of other substances. The
latter are dissolved in the water and include, besides several salts,
three active chemical agents--hydrochloric acid, pepsin, and rennin.
_Pepsin_ is the enzyme which acts upon proteids, but it is able to act
only in an acid medium--a condition which is supplied by the _hydrochloric
acid_. Mixed with the hydrochloric acid it converts the proteids into
peptones and proteoses.

*Other Effects of the Gastric Juice.*--In addition to digesting proteids,
the gastric juice brings about several minor effects, as follows:

1. It checks, after a time, the digestion of the starch which was begun in
the mouth by the saliva.(58) This is due to the presence of the
hydrochloric acid, the ptyalin being unable to act in an acid medium.

2. While there is no appreciable action on the fat itself, the proteid
layers that inclose the fat particles are dissolved away (Fig. 79), and
the fat is set free. By this means the fat is broken up and prepared for a
special digestive action in the small intestine.

3. Dissolved albumin, like that in milk, is curded, or coagulated, in the
stomach. This action is due to the _rennin_. The curded mass is then acted
upon by the pepsin and hydrochloric acid in the same manner as the other
proteids.

4. The hydrochloric acid acts on certain of the insoluble mineral salts
found in the foods and reduces them to a soluble condition.

5. It is also the opinion of certain physiologists that cane sugar and
maltose (double sugars) are converted by the hydrochloric acid into
dextrose and levulose (single sugars).

After a variable length of t