Complete information on the alimentary canal in man

The alimentary canal of man consists of the following parts from mouth to anus.

1. Mouth
2. Buccal cavity
3. Pharynx
4. Oesophagus
5. Stomach
6. Small intestine(Duodenum, Jejunum and Ileum)
7. Large intestine(Caecum,Colon and Rectum)
8. Anus

(1) Mouth:

It is transverse wide slit bounded by upper and lower lips. A space between lips and jaws is present called vestibule.

(2) Buccal Cavity:

Behind the vestibule, a spacious buccal cavity is present. It is bounded from above by palate, below by throat and from sides by cheeks and jaws.

Palate- It is the root of the buccal cavity and differentiated into anterior hard and posterior soft palate. Horizontal processes of premaxilla, maxilla, and palative bones from the skeleton of the hard palate and the rugae. A pair of nasopalatine ducts from Jacobsons organs opens into buccal cavity anteriorly on the hard palate. The soft palate is fleshy and hangs like a flap posteriorly into the pharynx.

Tongue- An elongated, wide muscular toucge is attached posteriorly to the floor of buccal cavity. Frenum is a membranous median fold that joins the ventral side of tounge with the floor of buccal cavity.

On the tougne, various taste buds are distributed on papillae like fungiform cicumvallate, filiform, foliate and conical. Different tastes like salt, sour, sweet, bitter can be tasted by different taste buds.

Tounge helps in mastication, tasting, deglutition, secretion, clearing, drinking and speech.

Teeth- Structurally and functionally four types of teeth are present in man. They are incisors, canines, premolars and molars. These are thecodont and of diphydont types . the dental formula of man is

I2/2, C1/1,PM2/2,M3/3=16/16

Glands of the buccal cavity- Mucous glands like palatine gland and Ebner’ s gland help in swallowing and cleaning. Three pairs of salivary glands i.e. parotid, sublingual and submaxillary open into buccal cavity.

(3) Pharynx:

Behind the soft palate a short but wide pharynx is present; here the food passage and air passage cross each other. The pharynx has three parts: (i) nasopharynx,(ii) oropharynx, (iii) laryngopharynx.Nasopharynx is the upper part and has a pair of openings of Eustachian tube on lymphoid tissue called tonsil, posterior extension of the buccal cavity beyond uvula of soft palate is the oropharynx. Laryngopharynx has gullet posteriorly and glottis in front of it a cartilaginous flap called epiglottis closes the glottis at the time of swallowing.

(4) Oesophagous:

Oesophagous is a long narrow muscular tube strating from gullet passes through neck, mediastinalspace of stomach and pierces the diaphagram to enter into the stomach.It is of about 25 compounds and passes food by peristalsis to stomach.

Histologically it has all the four concentric layers of tissues like (a) serosa,(b)muscular coat,(c) sub-mucosa and (d) mucosa.

The innermost layer of the mucosa has muscularis mucosa, lamina propia and stratified epithelium that surrounds the lumen of gut.

(5) Stomach:

It is a J-shaped bilobed sac and kept transeversely below the diaphagram. A curve called incisura angularis present on the anterior lesser curvature imparts the shape to the stomach. There are three zones in the stomach i.e. the beginning wide part is called cardiac or fundus, the middle zone the body and narrow posterior part of the pylorous. Its normal capacity is about 1500 ml.

(a) Fundus: The oesophagus opens to it through cardiac sphincter. It is the large left lobe.

(b) Body: It lies between in the fundic and pyloric part.

(c) Pyloric Part: It is the right posterior narrow some what conical lobe. It is divided into antrum and pyloric canal. At its end its opening is guarded by a pyloric sphincter.

Stomach wall is thrown into large number of longitudinal ridges or rugae internally.

Histologically stomach wall shows all the four concentric coats but they are much more evolved than those in other parts.muscular layer has longitudinal, circular muscles as well as a third layer called oblique muscle. Submucosa is much thick and his aerolar connective tissue with blood and lymph vessels and islands of adipose tissue. Mucosa has thick muscularis mucosa, lamina propia and columnar epithelial cells. The gastric epithelium penetrates into lamina propia and forms several tubular gastric glands.

(i) Gastric glands in the cardiac region secret mucous only.

(ii) The gastric glands in the fundic regions have three kinds of cells. Mucous secreting cell secret mucous, Zygomen or peptic cells secret pepsin and oxyntic or parietal cells produce hydrochloric acid. Some Kulchitsky or Argentaffian cells are found scattered among chief cells. G-cells of gastric mucosa secrete a hormone called gastrin.

(iii) Pyloric gastric glands secrete mucous only.

(6) Small Intestine:

It is about 6-7 meters long narrow tube and consists of duodenum, jejunum and ileum.

Duodenum- Starting from pylorous it moves to right side and makes a U shaped loop to come to left around pancreas. A heapatopancreatic duct opens into it at its beginning. It is about 25 compounds long.

Its wall has usual four coats. A large number of Brunner’s glands are located in the submucosa. The mucous layer forms enumerable folds or villi into the lumen.Numerous simple tubular glands open among the bases of the villi called crypts of Lieberkuhn, columnar epithelial cells have globet cells.

Jejunum: It is amuch coiled and mobile tube consisting of 2/5^th of total small intestine. Its inner wall is thrown into numerous folds called folds of Kerckring or valvulae conniventes.

Ileum: It is a narrow, tubular and coiled structure (2.2 to 2.5 meters) connecting to the large intestine at caecum. On its whole length lymphoid Peyer’s patches are present.

Its mucous layer has formed numerous villi. Simple tubular glands and Brunner’s glands present in the submucosa open into the crypts of Lieberkuhn. These secretion form the intestinal juice of succus entericus. Goblet cells are also present on the mucosa layer.

(6) Large Intestine:

Ileum and large intestine are connected at ileocaecal junction. The large intestine is divisible into caecum, colon and rectum.It is about 1.5 meters long.

Caecum: It is a thin-walled sac.At one end it continues into the colon and at its blind end it has a blind tube called vermiform appendix. Its proximal opening is guarded by ileocaecal valve.

Colon: It is a sacculated wide tube forming major portion of the large intestine. It has ascending, tyransverse, descending and sigmoid porions. A longitudinal band of muscle present on the midline of colon is called taeniae. A series of constrictions forming pocket-like pouches called haustra on both sides of taeniae are present.

Rectum: It is about 12cm long a narrow tube. It stores faecal matters, absorbs water and helps in excretion. The terminal part of rectum (3.8 cm) is called anal canal.

Histologically the large intestine though has all the four coats; all of them are thin and simple. A few glands of Lieberkuhn and goblet cells are present.