Human Digestive System ( Structure of Alimentary Canal )

Alimentary canal is complete as it starts from mouth and ends with anus.  Alimentary canal has following parts-

   

Mouth:  It is a main opening of alimentary canal and guarded with lips. Mouth opens in a cavity i.e. buccal cavity. 

Buccal cavity: 

This is also known as  oral cavity or mouth cavity and it is a space between our cheeks. The roof of buccal cavity is made by palate while tongue is present at floor. Buccal cavity is also bounded by jaws in front. Upper jaw is fixed while lower jaw is movable. Jaws contain teeth for mastication of food .  

Teeth:

 Teeth are used for grinding and mastication of food inside the buccal cavity. Teeth of human beings are “Diphyodont (appears two times in life),Thecodont(fixed in gums)and Heterodont (different types of teeth according to function)” .

We have four types teeth in our buccal cavity i.e.Incisors,Canine Premolar and Molar.-

• Incisors -These teeth are present at front-site of jaws and these help to cut large food pieces into small pieces.(In rodents, these teeth tend to grow throughout the life continuously.) 
• Canines - These teeth are present at side-wise of incisor and help in tearing flesh.( These teeth are well developed in carnivores. ) 
  
• Premolars- These teeth  help in mastication and chewing the food .
• Molars – These teeth also help in mastication and chewing of food.In children ,these teeth are absent and appeared later. 

Our teeth are diphyodont. We have two sets of teeth, milk-teeth(Childhood teeth)  and permanent teeth. 

Arrangement of teeth on one side of each jaw is shown by dental formula. 

Dental formula of milk teeth - i.c.pm.m/i.c.pm.m =2.1.2.0/2.1.2.0 x2 =20

Dental formula of permanent teeth-i.c.pm.m/i.c.pm.m =2.1.2.3/2.1.2.3 x2 =32

Structure of tooth: 

A tooth has three regions Crown ,neck and root. 

1.Crown –This  part is projected above gum.

2.Neck -  This part is surrounded by gum.

 Structure of single and double rooted teeth 

3.Root - This part is  embedded in bone.The incisor and canine and lower premolar have one root, upper premolar and lower molar have 2 roots and upper molar have 3 roots. Tooth consists of enamel which is the hardest part of human body. It covers the dentine of crown. Dentine has many canaliculi that pass radially from the pulp cavity. Cement covers root of tooth. Periodontal membrane covers cement and fixes tooth in socket .

Tooth has a pulp cavity ,filled with mass of cells, blood vessels and nerve which  constitute pulp. It is for growth of tooth. Dentine is formed by odontoblast and enamel forming ameloblast cells are also present.

Tongue 

It is flatten muscular organ attached at the floor of buccal cavity by a fold called frenulum. Tongue has  numerous papillae on upper surface with  taste buds. Papillae are filiform,fungiform,vallate and foliate. 

St. of tongue 

• Filiform- These are smallest, most numerous, conical, mostly found at center of tongue, white in color.
• Fungiform - These are less in no. red and rounded, found at tip and margin of tongue.
• Vallate papillae-These are large in size, about 5 to 12 in no. arranged in inverted v shape at the base of tongue.
• Foliate – These are leaf like, and not developed in man, found at sides of tongue.

Functions of tongue:

Main function of tongue to detect taste of food . Different parts of tongue identify different types of taste as “tip of tongue distinguish  sweet taste ,sides of tongue identifie sour and posterior end of tongue detect bitter.” Besides it ,tongue helps in saliva mixing ,chewing and swallowing the food. It also assists the meaning-full talking.

Palate: 

The roof of buccal cavity is called palate. Anterior part is called hard palate. It bears transverse ridges called rugae. The posterior part is smooth and called as soft palate. The hinder part freely hangs down as a small flap called uvula. Buccal cavity receives saliva from salivary gland.

Pharynx: 

Buccal cavity opens in pharynx at posterior side . It is the common part of digestive system and respiratory system. Pharynx has three parts i.e. “Nasopharynx (above the soft palate),Oropharynx(just behind the buccal cavity) and Laryngopharynx (posterior most part of pharynx and common pathway for food and air)” . It has two openings : gullet (opening of food-pipe)  and glottis (Opening of windpipe i.e. trachea). There is a muscular flap called epiglottis which closes glottis when food is swallowed.  

Oesophagus:

It is a long narrow muscular tube which connect mouth to stomach. It is about 25 cm long. It pierces diaphragm to open into stomach. It undergoes peristalsis to carry down food and water or fluid.

Stomach:

It is a large muscular elastic bag situated below diaphragm on left side. It has four parts

1.Cardiac: it is so called because it lies near heart. In between oesophagus and cardiac part of stomach there is cardiac sphincter. 

2. Fundus : it extends Superior from the cardiac part. It is usually filled with air.

3.Body: it is main part of stomach. 

4.Pyloric part: it is distal part of stomach. it opens into duodenum. It opens and closes several times. At the time of opening, a small amount of partially digested food(chyme) is passed into duodenum.

Gastric gland secretes gastric juice.

Small intestine:

Stomach opens into small intestine where process of digestion is completed and process of absorption takes place.Small intestine has three parts i.e. duodenum,jejunum and ileum. 

Duodenum – it is c shaped and about 25 cm long. It receives bile juice and pancreatic juice through common bile duct.

Jejunum - it is about 2.5 meter in length. It is coiled part.

Ileum – it is about 3.5 meter long. It is highly coiled part. Both jejunum and ileum are suspended by mesenteries. The inner wall of ileum has number of folds called villi. Villi are vascularise with blood capillaries and lacteals . Villi  increase the surface area for the process of absorption. 

Large intestine- It is the last part of gastrointestinal tract in human .It is also known as large bowl.It is about 1.5 meter long and divisible into caeum,colon and rectum.

• Caecum: It is pouch like structure about 6 cm long. There is ileocaecal valve preventing backflow. A slender vermiform appendix of about 10 cm long is attached to caecum. It is vestigial in man but functional in herbivores. The inflammation of appendix is called appendicitis at the time of infection.
• Colon: It is inverted U shaped and meant for water absorption . 
• It is  divisible into following parts - 
  
   

1. Ascending colon: It is the first part on right side. It moves upward from caecum.
2. Transverse colon: It is horizontal part.
3. Descending colon: It moves down at left side. It descends down
4. Pelvic colon: It is s shaped or sigmoid. Undigested material can remain in colon for about 36 hours.
5. Rectum:It is last part of large intestine. It is small muscular region about 15 cm long. It opens to the exterior through anus. Undigested material remains here for a short time. The anus is guarded by 2 sphincters.