Plicae circulares or circular folds are large, 1-cm-high transverse folds of the small
intestine. The surface of these folds has fingerlike projections called villi. They are
approximately 1-mm-tall extensions of the mucosa that contain blood vessels and lacteals. The
epithelial layer covering these villi consists of columnar absorptive cells each having mircovilli
extensions on their apical surface.

The intrinsic muscles are confined within the tongue and are not attached to bone. They
are responsible for changing the shape of the tongue but not its position in the mouth. This latter
function belongs to the extrinsic muscles that are attached to bone. The genioglossus is
responsible for protracting, retracting, and lateral movements of the tongue. The intrinsic
muscles are arranged in transverse, longitudinal, and vertical layers, allowing the tongue to
change shape while chewing and speaking.

A mesentery consists of two serous membranes that sandwich blood vessels
and nerves in between. The mesentery extends from the abdominal wall to intraperitoneal
organs, supporting the organs and fixing them to the wall. The two ventral mesenteries are the
falciform ligament, which attaches the liver to the anterior wall and diaphragm, and the lesser
omentum, which attaches the lesser curvature of the stomach to the liver. The dorsal mesenteries
include the greater omentum, mesentery proper, transverse mesocolon, and sigmoid mesocolon.
The greater omentum attaches the greater curvature of the stomach to the posterior abdominal
wall. The mesentery proper connects the jejunum and ileum to the posterior abdominal wall. The
two mesocolons attach to the region of the colon for which they are named.

The enteric nerve plexus consists of neurons located entirely within the walls of the GI
tract. These neurons form reflex arcs that can function independently from the central nervous
system. The enteric plexus includes neurons from both the submucosal and myenteric plexuses.
The submucosal nerve plexus, located in the submucosa, innervates the secretory glands of the
mucosa. It also causes some movements of the mucosa. The myenteric nerve plexus lies in the
muscularis layer of the tract. It controls peristalsis and segmentation.