When the food reaches the stomach, gastric juice is secreted. This juice
contains a ferment called pepsin and hydrochloric acid. Pepsin is only
active in an acid media. Starch digestion proceeds in the stomach to such
a time--stated as from 15 to 30 minutes--when the acid gastric juice has
been poured out in sufficient quantity to neutralise the alkalinity of
the saliva. The gastric juice acts upon the proteids only. After a time
the liquefied contents of the stomach are passed into the first portion
of the small intestine, called the duodenum. Here it meets with the
pancreatic juice, which like the gastric juice attacks proteids, but even
more energetically, and only in an alkaline media. The proteolitic ferment
is called trypsin. The pancreatic, the most important of the digestive
fluids, contains other ferments; one called amylopsin, takes up the
digestion of any remaining or imperfectly converted starch left from the
salivary digestion. Amylopsin is much more powerful and rapid than the
ptyalin of the saliva, especially on uncooked starch. Its absence from the
pancreatic juice of infants is an indication that starch should not be
given them. Another ferment, stearopsin, emulsifies fats. The bile is
alkaline and assists the pancreatic juice in neutralising the acid mixture
that leaves the stomach; it also assists the absorption of fats. The
digestion of proteids is not completed in the stomach. There are some who
look upon the stomach as chiefly of use as a receptacle for the large mass
of food, which is too quickly eaten to be passed at once into the
intestines; the food being gradually expelled from the stomach, in such
quantities as the duodenal digestion can adequately treat. A frequently
used table, showing the time required for the digestion of various foods
in the stomach, is of little practical value. There is ample provision for
the digestion of food, there is a duplication of ferments for the proteids
and starch. In health, the ferments are not only very active, but are
secreted in ample quantities. The digestive or unorganised ferments must
not be confused with the organised ferments such as yeast. The latter are
living vegetable cells, capable of indefinite multiplication. The former
are soluble bodies, and though capable of transforming or digesting some
thousands of times their mass of food, their power in this direction is
restricted within definite limits. Another and preferable name for them is
enzymes.
The action of saliva on starch is powerfully retarded by tea, this is due
to the tannin. Coffee and cocoa are without effect. Tea infused for two
minutes only, was not found to have sensibly less restraining effect than
when infused for thirty minutes. On peptic digestion both tea and coffee
had a powerful retarding effect. When of equal strength cocoa was nearly
as bad, but as it is usually taken much weaker, its inhibitory effect is
of little consequence.
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