1. It converts the starch into maltose, completing the work begun by the
saliva. This action is due to the _amylopsin_,(61) which is similar to
ptyalin but is more vigorous.

2. It changes proteids into peptones and proteoses, completing the work
begun by the gastric juice. This is accomplished by the _trypsin_, which
is similar to, but more active than, the pepsin.

3. It digests fat. In this work the active agent is the _steapsin_.

The necessity of a milk-curding enzyme, somewhat similar to the rennin of
the gastric juice, is not understood.

*Digestion of Fat.*—Several theories have been proposed at different times
regarding the digestion and absorption of fat. Among these, what is known
as the "solution theory" seems to have the greatest amount of evidence in
its favor. According to this theory, the fat, under the influence of the
steapsin, absorbs water and splits into two substances, recognized as
glycerine and fatty acid. This finishes the process so far as the
glycerine is concerned, as this is soluble in water; but the fatty acid,
which (from certain fats) is insoluble in water,(62) requires further
treatment. The fatty acid is now supposed to be acted on in one, or both,
of the following ways: 1. To be dissolved as fatty acid by the action of
the bile (since bile is capable of dissolving it under certain
conditions). 2. To be converted by the sodium carbonate into a form of
soap which is soluble in water.

The emulsification of fat is known to occur in the small intestine. By
this process the fat is separated into minute particles which are
suspended in water, but not changed chemically, the mixture being known as