Distribution of Secretory Cells.~--Secretory cells are distributed in
each of the three regions of the stomach, but are more numerous in the
middle region than at either end. The third region includes the
pyloric vestibule through which all foods must pass before they can
enter the small intestine, and terminates in the pylorus, the valve
which shuts off the stomach from the duodenum and the rest of the
intestinal canal.

~Chemical Digestion.~--The chemical changes in the food materials,
after they are eaten, are brought about through the action of certain
substances known as soluble ferments or "enzymes." These enzymes exist
in every tissue of the body, and their province is first to break down
the food materials themselves into simpler compounds, and then to
reconstruct the simpler substances into those of a more complex
character, which are more available for the various uses of the
organism.

~Action of the Enzymes.~--As Sherman has stated, "all fermentation is
brought about either directly or indirectly by the activity of animal
or vegetable organisms or cells. When the organisms or cells act
directly and the chemical changes occur only in their presence, the
fermentation is said to be due to an organized ferment. When the
action is not brought about directly by the cell itself, but by means
of a substance secreted by the cell but acting apart from it, this
substance is called a soluble or unorganized ferment or 'enzyme.'" The
enzymes concerned in digestion and metabolism, their source and their
action, may be found in the following table:

TABLE OF ENZYMES[54]

SOURCE AND ACTION OF ENZYMES

==============+===================+==================+===================
| | _Where chiefly_ |
| _Enzymes_ | _found_ | _Action_
--------------+-------------------+------------------+-------------------
{|Ptyalin (salivary |Salivary |Converts starch
{| amylase) | secretions | to maltose
{|Amylopsin |Pancreatic juice |Converts starch
{| (pancreatic | | to maltose
{| amylase) | |
{|Liver diastase |Liver |Converts glycogen
{| | | to glucose
{|Muscle diastase |Muscles |Converts glycogen