ourishment to the system until this change takes place. _Respiration_,
then, is, in reality, _the completion of digestion_. This interesting
and vital part of the process of digestion will be considered more fully
in the following chapter.

Before passing from this part of the subject, a few remarks of a more
general nature seem called for. The _nerves of the stomach_ have a
direct relation to _undigested_ but _digestible_ substances. When any
body that can not be digested is introduced into the stomach, distinct
uneasiness is speedily excited, and an effort is soon made to expel it,
either upward by the mouth or downward by the bowels. It is in this way,
says Dr. Combe, that bile in the stomach excites nausea, and that tartar
emetic produces vomiting. The _nerves of the bowels_, on the other hand,
are constituted in relation to _digested_ food; and, consequently, when
any thing escapes into them from the stomach in an _undigested_ state,
it becomes a source of irritative excitement. This accounts for the
cholic pains and bowel-complaints which so commonly attend the passage
through the intestinal canal of such indigestible substances as fat,
husks of fruits, berries, and cherry-stones.

The process of digestion, which commences in the stomach, is completed
in the intestines. Physiologists have hence sometimes called the former
part of the process, or chymification, by the more simple term _stomach
digestion_; and the latter, or chylification, has been termed
_intestinal digestion_. The bowels have distinct coats corresponding
with those of the stomach. By the alternate contraction and relaxation
of the muscular coat, their contents are propelled in a downward
direction, somewhat as motion is propagated from one end of a worm to
the other. It has hence been called vermicular, or _wormlike motion_.
Some medicines have the power of _inverting_ the order of the muscular
contractions. Emetics operate in this manner to produce vomiting. Other
medicines, again, excite the _natural_ action to a higher degree, and
induce a cathartic action of the bowels. When medicines become necessary
to obviate that kind of costiveness which arises from imperfect
intestinal contraction, physicians usually administer rhubarb, aloes,
and similar laxatives, combined with tonics. But when the muscular coat
of the bowels is kept in a healthy condition by a natural mode of life,
and is aided by the action of the abdominal muscles, it rarely becomes