The small intestine, histologically is covered by four coats of tissues.
1. An outer serous coat (peritoneum)
2. A muscular coat
3. A sub mucous coat and
4. Mucous membrane
1. Outer serous coat:
The serous coat or serous membrane is also called the peritoneum. Like all serous membranes it has a smooth shiny surface composed of a layer of simple squamous epithelium. This is supported by a thin layer of areolar tissue. The membrane forms the outer wall with the epithelial surface facing the cavity which is referred to as peritoneal cavity.
2. Muscular coat:
The muscle coat is made up of two layers of visceral muscle fibres. These are -a) longitudinal muscle fibres and b) circular muscle fibres. The longitudinal muscle fibres are superficial while the circular muscle fibres form the inner layer (there are no oblique fibres in the small intestine as seen in stomach). The muscle fibres are arranged along the length of the organ.
The muscle coat has two important functions. 1. The tone of the muscle coat maintains the intestine in a functional state and 2.The rhythmic contraction of the muscle fibres allows (even without nervous stimulation) the movement of the food in the intestine.
3. Sub mucous coat:
The sub mucous coat binds the mucous membrane to the muscle coat and constitutes the main area for the circular folds of the mucous membrane. In the duodenum, this coat contains duodenal glands (Brunner’s glands) which secrete an alkaline mucous to protect the mucous membrane from the acidity of the food chyme coming from the stomach.
The sub mucous coat is made up of loose connective tissue and contains many elastic fibres. It contains blood vessels to supply the mucous membrane.
4. Mucous membrane:
This is the inner most membrane of the intestine. The surface layer of the mucous membrane is made up of epithelial cells and it comes in direct contact with the food materials. The cells are of the stratified squamous type where food is rough but columnar where digestion and absorption takes place. –
The mucous membrane is thrown up into permanent circular folds called Plicae circulares. These greatly increase the surface area for secretion and absorption. At the sites of absorption the folds of the mucous membrane are filled with minute finger like processes called villi. These villi give the surface a soft velvety appearance.
The collection of these villi gives a brash border appearance. A microscopic examination of the villi (singular, villus) shows that these are tiny folds of the membrane projecting out from the surface. The surface of each villus is covered by a layer of columnar epithelial cells. These epithelial cells have a brush bordered appearance forming thousands of microvilli. The villi and microvilli greatly increase the total absorptive area of the small intestine.
Scattered among the columnar epithelial cells of the surface of the villi are found a number of goblet cells each of which contains a large drop of colourless mucous. The mucous secreted by the goblet cells acts as a lubricant. Each villus has a core area of loose connective tissue and some smooth muscle fibres.In the centre of this is found a lymph capillary with a blind end (lacteal). Into this lacteal some of the absorbed fatty substances get in.
Each villus is provided by a small arteriole which feeds the capillary plexus lying close to the epithelium. Food materials absorbed from the lumen by the columnar cells are passed on to the blood through these capillaries.