The document describes the four main layers that make up the walls of the alimentary canal (also known as the gastrointestinal tract): the mucosa, submucosa, muscularis externa, and serosa. The mucosa is the innermost layer that lines the canal and has functions like secreting mucus and enzymes, absorbing nutrients, and protecting against pathogens. It contains epithelial tissue, connective tissue, and smooth muscle. The submucosa lies just outside the mucosa and contains blood vessels, lymphatics, nerves and connective tissue. The muscularis externa is responsible for movements like segmentation and peristalsis through its inner circular and outer longitudinal smooth muscle layers. The outer

1. Alimentary Canal
 alimentary canal, which is also known as the (GI) gastrointestinal
tract.
 The walls of the alimentary canal have the same four basic layers,
(also known as tunics) — the mucosa, submucosa, musclaris externa,
and serosa. Each layer contains a certain tissue type that plays a
crucial role in the breakdown of food.
 The Mucosa
 The mucosa (also known as mucus membrane) is the innermost
layer of tissue. It’s a moist epithilial membrane that lines the
alimentary canal lumen from mouth to the anus.
 The mucosa has three major functions:
 Secrete mucus, digestive enzymes, and hormones
 Absorb the end products of digestion into the blood
 Protect against infectious disease

2.  In certain regions of the alimentary canal, the mucosa may
perform one or all three of these functions. Digestive
mucosa is made up of three sublayers: (1) a lining
epithelium, (2) a lamina propria, and (3) a musclularis
mucosae. Except in the mouth, esophagus, and anus
where it’s stratified squamous, the epithelium of mucosa
is a simple columnar epithelium rich in mucus-secreting
cells. The mucus it produces protects certain digestive
organs from being digested by enzymes working within
the same cavity, it also eases food passage along the GI
tract.
 The lamina propria, which underlies the epithelium, is
loose areolar connective tissue. Its capillaries nourish the
epithelium and absorb

3.  digested nutrients. mucosa associated lymphoid
tissue) help defend against bacteria and other
pathogens, which have free access to our digestive tract.
Large collections of lymphoid follicles occur in the
pharynx (tonsils) and appendix.
 External to the lamina propria is the musularis
mucosae, a layer of smooth muscle cells that produces
local movements of mucosa. In the small intestine, this
muscle layer’s tone throws the mucosa into a series of
small folds that immensely increases its surface area.

4. The Submucosa
 The submucosa, just external to the mucosa, is areolar
connective tissue containing a rich supply of blood and
lymphatic vessels, lymphoid follicles, and nerve fibers
which supply the surrounding tissues of the GI tract wall.
Its elastic fibers enable the stomach to regain its normal
shape after temporarily storing a large meal.
The Muscularis Externa
 Th muscularis externa, (also called the muscularis)
surrounds the submucosa. The muscularis is responsible
for segmentation and peristalsis. It typically has an inner
circular layer and an outer longitudal layer of smooth
muscle cells. In several places along the tract, the circular
layer thickens and forms sphincters that act as valves that
control food passage from one organ to the next, they also
prevent backflow.

5. The Serosa
 The serosa is the outermost layer of the intraperitoneal
organs (it’s also considered the visceral peritoneum). In most
alimentary canal organs, its made up of areolar connective
tissue covered with mesothelium, a single layer of squamous
epithelial cells.
 In the esophagus, which is located in the thoracic instead of
the abdominopelvic cavity, the serosa is replaced by
an adventitia, ordinary fibrous connective tissue that binds
the esophagus to surrounding structures. Retroperitoneal
organs have both a serosa (facing the peritoneal cavity) and
an adventia (on the side abutting the dorsal body wall).