[Sidenote: Action of pepsin on proteids]
[Sidenote: Peptone and proteoses]
The gastric juice contains three principal enzyms or digestive
principles. These are hydrochloric acid, pepsin, and rennet. The
hydrochloric acid and the pepsin are secreted by different cells, and
could be considered as separate digestive juices, but as the action of
one is dependent upon the other, I will consider these actions as one.
Pepsin, in the presence of hydrochloric acid, acts on proteids, and
changes them into proteoses and peptone. Comparatively little food is
completely peptonized in gastric digestion. Proteoses are intermediate
products between food proteids and peptone, being the principal
product of the action of the gastric juice. Thus it is seen that this
stomach-action is only preparatory for the digestive processes of the
intestines.
[Sidenote: Action of gastric juice on fat]
The gastric juice does not act on fat, but in the case of animal food,
in which the membranes or connective tissues that enclose the fat-cells
are formed of proteid material, the gastric juice sets the fat-globules
free by dissolving these enclosing membranes.
[Sidenote: Purpose of hydrochloric acid]
The chief action of hydrochloric acid in the stomach is to aid the
action of the pepsin. Pepsin alone has no digestive power. There are no
other acids produced by the secretive glands of the stomach. If other
acids are found in the contents of the stomach, it is because they have
been taken in with the food, or produced by abnormal fermentation.
[Sidenote: How hydrochloric acid is formed]
The source of hydrochloric acid is from the sodium chlorid or common
salt of the blood. The secreting cells of the stomach-glands are thought
to have the power to form hydrochloric acid by uniting the chlorin of
the salt with the hydrogen of the water. This is a very unusual chemical
process, and has not yet been successfully produced in a laboratory.
[Sidenote: Hydrochloric acid as an antiseptic]
One of the chief functions of hydrochloric acid in the stomach is that
of an antiseptic. In other words, hydrochloric acid kills bacteria. This
is not true of all bacteria, for some germs can live in an acid medium,
while others may live best in an alkaline solution. The alternation of
the digestive juices from alkali to acid is a provision of Nature which
has a dual purpose:
1 To reduce food to the finest possible solution; that is, to
subdivide or to digest food elements into a form that will admit of
assimilation and use
2 To destroy bacteria and enzyms of plant and animal origin
that are taken into the digestive tract with food
(These two facts constitute additional reasons for the
thorough mastication of food)
[Sidenote: Object of alternating digestive juices]
By such plan Nature provides for the digestion of food only by such
enzyms and ferments as will produce a finished product wholly suited
to the particular requirements of the body. When we attempt by
artificial processes to digest our food with other enzyms than those
of our own digestive organs, or take into the stomach large quantities
of food without proper mastication, which causes fermentation, we may
expect that the nutritive material supplied to our tissues will not be
perfectly adapted to the needs of human cell-growth, and, as a natural
result, consequent derangement of the body-functions will take place.
[Sidenote: Rennet]
The rennet of the gastric juice is primarily for the purpose of
digestion. Other than this it has no particular function that has yet
been discovered.
[Sidenote: Why stomach does not digest itself]
The problem as to why the stomach does not digest itself has puzzled
scientists for many years. Investigations of the twentieth century
have at last solved this fascinating question. The walls of the human
stomach are composed of proteid material, and should be dissolved by
the gastric juice according to all known chemical laws. The explanation
formerly given was that the stomach did not digest itself because it was
alive. This answer did not satisfy scientists.
[Sidenote: Antipepsin in the blood]
There has recently been discovered an enzym, known as antipepsin,
which is secreted by the cells in the stomach-walls. This antipepsin
destroys the action of the pepsin, thus in turn preventing its action
on the stomach-wall itself. Were antipepsin secreted in sufficiently
large quantities to mix with the food in the stomach-cavity, no
digestion could take place. The presence of this antipepsin in the
stomach-walls has been proved in the following manner: The arteries
leading to a portion of the stomach-wall of a dog was severed. This
portion, receiving no blood supply, did not form the usual amount of
antipepsin. The secretion of pepsin went on in the remainder of the
animal's stomach, but digested that portion of the stomach-wall which
was receiving no blood supply; that is, secreting no antipepsin.