ing off branches. It shows the columnar
epithelium of the surface dipping down into the duct D of the gland, from
which two tubes branch off. Each tube is lined with columnar epithelial
cells, and there is a minute central passage with the "neck" at N. Here
and there are seen other special cells called parietal cells, P, which are
supposed to produce the acid of the gastric juice. The principal cells are
represented at C.]

Pepsin the important constituent of the gastric juice, has the
power, in the presence of an acid, of dissolving the proteid food-stuffs.
Some of which is converted into what are called _peptones_, both soluble
and capable of filtering through membranes. The gastric juice has no
action on starchy foods, neither does it act on fats, except to dissolve
the albuminous walls of the fat cells. The fat itself is thus set free in
the form of minute globules. The whole contents of the stomach now assume
the appearance and the consistency of a thick soup, usually of a grayish
color, known as chyme.

It is well known that "rennet" prepared from the calf's stomach has a
remarkable effect in rapidly curdling milk, and this property is utilized
in the manufacture of cheese. Now, a similar ferment is abundant in the
gastric juice, and may be called _rennin_. It causes milk to clot, and
does this by so acting on the casein as to make the milk set into a jelly.
Mothers are sometimes frightened when their children, seemingly in perfect
health, vomit masses of curdled milk. This curdling of the milk is,
however, a normal process, and the only noteworthy thing is its rejection,
usually due to overfeeding.

Experiment 58. _To show that pepsin and acid are necessary for
gastric digestion._ Take three beakers, or large test tubes; label them
_A_, _B_, _C_. Put into _A_ water and a few grains of powdered pepsin.
Fill _B_ two-thirds full of dilute hydrochloric acid (one teaspoonful to
a pint), and fill _C_ two-thirds full of hydrochloric acid and a few
grains of pepsin. Put into each a small quantity of well-washed fibrin,
and place them all in a water bath at 104 degrees Fahrenheit for half an
hour.

Examine them. In _A_, the fibrin is unchanged; in _B_, the fibrin is
clear and swollen up; in _C_, it has disappeared, having first become
swollen and clear, and completely dissolved, being finally converted
into peptones. Therefore, both acid and ferment are required for gastric
digestion.

Ex