small intestine is finished and absorbed
here. The important peristaltic waves occurring in this region are
antiperistaltic in character and have the property of churning the
food thoroughly and bringing a larger portion of it in contact with
the absorbing walls. The water which is left in the food mass together
with the products of the digestion of the foodstuffs is absorbed in
the first part of the large intestine, leaving the remainder more
solid. This residue is known as feces or fecal matter.

The diagram on page 179 shows the various processes through which the
foodstuffs pass after absorption.[56]

~Bacterial Action in the Alimentary Canal.~--The changes in the foods
so far mentioned have been chiefly the result of the activity of the
enzymes existing in the various digestive processes throughout the
body. But there are other changes which occur in the foods during
their sojourn in the digestive tract which are not accountable to
enzymic action, but which, in fact, modify to a certain degree the
changes wrought by the enzymes. These are the result of the activity
of certain specific bacteria which inhabit the entire digestive tract
of the individual from a few hours after birth until death. Some of
these have so adapted themselves to the existing conditions that,
unless present in overwhelming numbers, they are not only harmless,
but they actually assist in protecting the organism from the inroads
of more harmful species. Many experiments have been made to find
whether or not bacteria are essential to human nutrition, and the
results of these experiments prove that they are not. However, since
they are so firmly established in the body it is well to study the
various types and learn as much as possible of the products of their
activity and the influence which they exert in human nutrition.

Cells and Tissues
/ | \
Anabolism / | \ Catabolism
Assimilation / | \ Dissimilation
/ | \
Foodstuff < | > End Products
\ | /
\ | /
\ | /
\ | /
Combustion

~Types of Bacteria.~--It would be impossible and unnecessary to
consider the action of all of the bacteria in the body in this text,
but it is necessary to consider those which are prominent in bringing
about decomposition of