Human digestive system:

Body parts including mouth from where food enters, digestive organs & elimination organs in digestive system are collectively called alimentary system or alimentary canal.

It has 4 layers:-

1) Visceral peritoneum (viscera): Outermost layer with loose connective tissues.

2) Muscular coat (muscularis): It has 2 Layers:-  

Outer longitudinal & inner circular muscle fibers.

Between these 2 layers, there is a layer of nerve fibers called Plexus of auerbach that controls muscular contraction during digestion of food.      

3) Sub-mucosa:- Contain dense connective tissue.

Between muscularis & sub-mucosa, layer of nerve fibers lies called Plexus of meissner.  It Controls the secretion of intestinal juice.

4) Mucosa:- Innermost layer.

Parts of alimentary canal:

1) Mouth: It has 2 movable lips upper & lower.

2) Vestibule: Space between   lips and gums.

Superior labial	Inferior labial
It connects middle of upper lip to jaw.	It connects middle of lower lips to jaw.

 

3) Buccal cavity:- It consists tongue, teeth, throat and  two jaws. Throat supports tongue.

• Palate: Posterior part of palate is fleshy soft & called soft palate which helps in easy swallowing of food & front part is hard called hard palate & contains bones to help the food to be in place for chewing.
•  Tongue:  Large, movable organ. Base is fixed but apex is free & movable its base is attached to the floor in oral cavity by a fold called Frenulum of tongue.

Upper surface of tongue bears small papillae that has taste buds.  Papillae are of 3 types:

Vallate  papillae	Fungiform  papillae	Filiform  papillae
Large	Smaller in size	Smallest
8 to 10 in no.	More in no.	Most of papillae are this type.
Have taste buds	Round, red	Smallest, whitish

 

Functions of tongue: Helps in swallowing of food. It has taste buds – salty, sour, bitter & sweet. Also play role in speech.

• Teeth: Teeth are present in both the jaws. Teeth are embedded in pits called sockets or alveoli. These type of teeth are called The Dicodont. There are  2 sets of teeth:-

Temporary set (milk teeth) and permanent set:

Temporary set appear after 2 years of age and shed between 6 to 12 years of age & replaced by permanent set.

There are 4 types of teeth:

1) Incisors: For cutting as they are sharp. Incise means to cut.

2) Canines: Used for holding, grasping, and tearing food.

3) Premolars: They help both canines and molars in their respective functions(holding and crushing).

 4) Molars: For grinding the pieces into smaller ones. Last molars are called wisdom teeth.

Dental formula:

For temporary set →  

Incisor   (2/2) canines (1/1) molars (2/2).

8+ 4 + 8 = 20 teeth

For permanent set →

Incisors (2/2) canines (1/1) premolars (2/2) molars (3/3).

8  +  4 + 8 + 12 =32 teeth

Structure of tooth:

Enamel à Hardest substance in our body rich in calcium. It forms the outer layer of crown which is whitish.

 Dentine à Not as hard as enamel, forms  the second innermost layer of crown of the tooth and can be sensitive if the protection of the enamel is lost.

Pulp à Soft, vascular connective tissue, forms the innermost layer of tooth and provide blood supply to tooth by having blood vessels.

Cementum à The layer of bone-like tissue covering the root. It is not as hard as enamel.

Tooth has 3 parts:

(1) Crown → The visible part of tooth.

(2)  Neck → This part is covered by a soft skin called gum.

(3)  Root → This part is embedded in a pit of saw bone called socket.

(4) Rabbits and squirrel lack canines between premolars and incisors. There is a space called Diastema.

4) Pharynx:- 12 cm long, vertical canal. & has 3 parts.

Naso-pharynx	Oro-pharynx	Laryngo-pharynx
Upper part of pharynx. Nose leads to naso-pharynx.	Middle part of pharynx,  oral cavity opens into it.	Lower part of pharynx lead to food pipe (oesophagus) and also opens into voice box( larynx).
Function:  It passes air from nose to oro-pharynx.	Function: It accepts air from naso-pharynx, food from mouth and passes it to laryngo-pharynx.	Function: Its function is to pass food and air to their appropriate destinations that is, oesophagus (food pipe)and larynx(wind pipe), respectively.

 

5) Oesphagus:  25 cm long, narrow tube.

Function:- It serves to convey the food from pharynx to stomach.

6) Stomach:- It is j shaped muscular body organ. It has 3 smooth muscle layers, which helps to churn the food & to mix it with gastric juice.

It has 4 parts:-

Cardiac part	Fundus	Body	Pyloric
Left broad upper, part where oesophagus opens. Here cardiac sphincter is present, helps so that no food goes back into oesophagus.	Small, upper part contain gas or air.	It is middle region of stomach. Here, main digestion of food occurs in stomach.	It is lower narrow part of stomach. It opens into small intestine and regulated by a pyloric sphincter.

   

Functions of stomach:

• storage of food
•  churning of food
• partial digestion  
• flow of food into small intestine. 

7) Small intestine: It is narrow tube, 6 meters long.  It has 3 parts:

• Duodenum:- Follow stomach, C shaped. It receives hepato- pancreatic ducts formed by union of bile duct & pancreatic duct.
•  Jejunum:- Middle part of small intestine long follows the duodenum.  
• Ilium:- Lower part small intestine (Longest part)

Functions of small intestine:

• Completion of digestion & absorption of digested food.
•  Also secrets some hormones secretion, enterocrinin & enterogastrone, duocrinin.  
• Mucosa layer in small intestine has some finger tike projections called villi & small projections called microvilli, that increases surface are of small intestine for the efficient absorption of food.
• Also some white patches of lymphoid tissue occur on mucosa layer of small intestine that fight against infection & are called payer’s patches.

8) Large intestine:

It has no villi or micro villi. It is shorter than small intestine but wider than small intestine. It is 1.5 meter long. & has 3 regions:

Caecum	Colon	Rectum
Small sac like structure. There is a junction between ilium and coecum called ilio-coacal junction. Which regulates the passage of digested food from small to large intestine. Coecume has a worm like projection called vermiform appendix. Which is vestigial organ & can be removed by appendictomy.	Wider than small intestine & is tube like. It has 4 region:- (1) Ascending colon upward (2) Transverse colon  (3) Descending downward (4) Sigmoid colon	Follows the colon. It leads to a small and canal like structure called anus, for removal of waste.

     Functions of large intestine:

• Absorption of water from undigested food.
• Formation, storage & elimination of foeces.
•  Production of mucus for lubrication of mucosa.

9) Anus: Lies at the base of abdomen.

Function: It is for the removal of waste outside of body.