Alimentary Canal

4. Human Digestive System

The digestive system consists of two main parts: Alimentary Canal and Digestive Glands.
(A)Alimentary Canal

1. Oral Cavity.The roof of oral cavity is called Palate.Anterior part of the palate is Hard Palate and with transverse ridges called rugea.Posterior part of the palate is Soft Palate.The hinder free part of the Soft Palate hangs down as a small flap,the Uvula.
Floor of oral cavity has tongue that has four types of papillae (little projections)
i)Vallate Papillae-Largest
ii)Filiform Papillae-Smallest and most numerous
iii)Fungifirm Papillae-Less numerous than Filiform
iv) Foliate Papillae-Not developed in human tonue.

Teeth: Most of mammals have diphyodont teeth (i.e. two sets of teeth-milk or deciduous and permanent).
-Thecodont teeth (i.e. teeth are embedded in the sockets of the jaw bone)
-Heterodont teeth (i.e. different types of teeth)
-Four kinds of teeth (Incisor, Canine, Premolar and Molar)
-Last molar teeth in human being are called wisdom teeth.
-Milk teeth of man (Incisors-8, Canines-4, Molars-8)
-Permanent teeth of man (Incisors-8, Canines-4, Premolars-8, Molars-12)
-The Incisors and Canines have one root.
-The upper Premolars have two roots.
-The lower Premolars usually have only one root.
-The upper Molars have three roots and the lower Molars have two roots.
-Enamel is the hardest part of the human body.
-Odontoblast(Dentine forming cells)
-Ameloblast(Enamel forming cells)
-First permanent teeth to appear is first Molar.
-Last permanent teeth to appear are third Molars.
The teeth of the lower jaw usually appear before those of the upper jaw.

-Teeth in females appear earlier than in males.


Pharynx (Throat):
-Divided into three parts for easily descriptive purpose


i)Nasopharynx
-Lies behind the Nasal Chamber
-The Eustachian Tube (Auditory Tube) connects Nasopharynx with Middle Ear.
ii)Oropharynx
-Lies behind the Oral Cavity (Buccal Cavity)
iii)Laryngopharynx
-Lies behind the opening into the Larynx and posterior surface of Larynx.


Oesophagus(Food Pipe):
-Passes through the diaphragm and opens into the Stomach.
-Human Oesophagus is about 25cm long.
-Have three parts
i) Cervical part (in the neck)
ii) Thoracic part (in the Thorax)

iii) Abdominal part (in the Abdomen)


Stomach (Gaster):
-Widest part of the Alimentary Canal
-Divided into four parts
i) Cardiac part
-Upper part of the stomach
-Cardiac sphincter (not true valve, functional sphincter) lies in the opening between Oesophagus and Stomach
ii)Fundus part
-Middle part of Stomach
-It extends superiorly from the Cardiac part
-It is commonly filled with air or gas
iii)Body part
-Main part of the Stomach
iv)Pyloric part
-Distal part of the Stomach
Function of Stomach:
-It stores food for some time
-It churns and breaks up the food into smaller particles and mixes with gastric juice secreted by Stomach
-It secretes gastrin (hormone) and Castle’s intrinsic factor (Glycoprotein)
-Castle’s intrinsic factor is important for the absorption of Vit. B12 absorbed in small intestine
-partial digestion of food(protein and fat) takes place here 


-Alcohol, Aspirin, some lipid-soluble drug,moderate amount of sugar and water are absorbed by the Stomach wall.

Small Intestine:
-It has small diameter
-Length is correlated with the height of the individual and not the weight.
-Longest part of the Alimentary Canal (about 6.25m long)
Divided into three parts
i)Duodenum(about 25cm long)
-Shortest and widest part of the small intestine
-The Hepatopancreatic Ampulla(Ampulla of Vater) opens into it.
-This Ampulla receives bile duct from Liver and Pancreatic Duct from Pancreas.
ii)Jejunum(about 2.5m long)
-Middle part of the small intestine
iii)Ileum(about 3.5m long)
-Longest part of the small intestine.
-Small nodules of Lymphatic tissue can be seen along the entire length of small intestine. In some places, particularly along the Ileum , these nodules are clustered together in groups called Peyer’s Patches or Lymph nodules. Peyer’s patches are the distinguishing feature of Ileum, which produce Lymphocytes(a type of WBC).Though less conspicuous , lymph nodes are found throughout the small intestine. Finger-like projection of mucosa, the Villi are present in the small intestine.Villi are absent over the Peyer’s patches. The villi increase the surface area of the small intestine.Villi are absent over the Peyer’s patches. The villi increase the surface area of the small intestine.Each villus is covered with epithelium and contains a lymph capillary (lacteal) and blood capillaries.The entire small intestine has circular folds of the submucosa, the plicae circularis (‘Valves’ of Kerkring). These folds are more prominent in the Jejunum. They further increase the absorptive surface considerably.

Function of Small Intestine:

-Small Intestine completes the digestion of Proteins, Carbohydrates, Nucleic Acid and Fats.

-It absorbs nutrient materials into the blood and lymph.

-It secretes some hormones such as Cholecystokinin, Secretin, Enterogasterone, Duocrinin, Enterocrinin and Villikinin.


Large Intestine:

-Its diameter is larger than small intestine.

-Divisible into three parts

i)Caecum and Vermiform Appendix:

-Vermiform Appendix(Appendix) is an outgrowth of Caecum.

Appendix is thought to be vestigial but according to some scientists Appendix takes part in Immunity System.

-Caecum is well developed in Herbivores like Rabbit, Horse etc.

ii)Colon

-It is divided into four regions

a)Ascending Colon(shortest)

b)Transverse Colon

c)Descending Colon

d)Sigmoid Colon(Pelvic Colon)

-The colon has tree longitudinal bands called taniae coli.

-Taniae coli contract and draw the remainder of the wall into small pouches called haustra.

iii)Rectum

-Two centimeter anal canal comprises rectum.

Opening of anal canal is called anus.

-Anus has an internal anal sphincter composed of smooth muscle fibre and external anal sphincter comprised of striped(Voluntary) muscle fibres.

Function of Large Intestine:

-Absorption of water and elimination of solid wastes is the main function.

-Moderate amount of Vit. K and Vit. B-complex are manufactured by bacteria which are found in the large intestine.