Illustration also
shows arrangement of mucous membrane in the two organs. _D._ Bile duct.
*The Small Intestine.*--This division of the alimentary canal consists of a
coiled tube, about twenty-two feet in length, which occupies the central,
lower portion of the abdominal cavity (Fig. 71). At its upper extremity it
connects with the pyloric end of the stomach (Fig. 70), and at its lower
end it joins the large intestine. It averages a little over an inch in
diameter, and gradually diminishes in size from the stomach to the large
intestine. The first eight or ten inches form a short curve, known as the
_duodenum_. The upper two fifths of the remainder is called the _jejunum_,
and the lower three fifths is known as the _ileum_. The ileum joins that
part of the large intestine known as the caecum, and at their place of
union is a marked constriction which prevents material from passing from
the large into the small intestine (Fig. 73). This is known as the
_ileo-caecal valve_.
_The mucous membrane_ of the small intestine is richly supplied with blood
vessels and contains glands that secrete a digestive fluid known as the
_intestinal juice_. The membrane is thrown into many transverse, or
circular, folds which increase its surface and also prevent materials from
passing too rapidly through the intestine.
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