Alimentary canal is a tube present in all higher animals starting from mouth and reaching up to anus.  Various glands located on its wall produce digestive juices that help in the process of digestion.  Two glands namely liver and pancreas are also associated with it.  They also produce the digestive juices.  The digested food is also absorbed into the alimentary canal and undigested and indigestible food is passed out of the body through anus.

Mammalian Alimentary System:

 

             In man the total length of alimentary canal is about 21 feet and consists of the following parts:

 Mouth leads into a buccal cavity.  The opening of the mouth is provided with lips.  At the floor of the buccal cavity a muscular tongue is present.  It helps in the ingestion, mastication and swallowing of food.  It has got taste buds on its surface.  Most of the mammals possess teeth on both the jaws.  They are present in the cavity or socket of gums (thecodont dentition).  The number and types of teeth vary in mammals.  In man, there are 32 teeth of four different types namely incisors, canines, premolars and molars.  This type of dentition is known as heterodont dentition.  Their number can be represented by the dental formula: In each half of jaw;

Upper jaw    I(2);  C(1);  PM(2);  M(3)

-----------------------------------------------       = 32

Lower jaw I (2); C (1); PM (2); M (3)            

                                                            

The incisor teeth are chisel-shaped and have sharp cutting edges.  Canines are dagger-shaped and pierce the food.  They are very large and well developed in predatory animals.  Premolars and molars are broad and strong crushing teeth.  Thus the incisors are used for biting; the canines for tearing the food; and premolars and molars for grinding the food.  With the help of the teeth, tongue and jaw movements, food is chewed and mixed with saliva in the mouth. 

Salivary glands:

 

There are three pairs of salivary glands namely parotids, submaxillary (submandibular) and sublingual glands.  Their secretion is collectively known as saliva that is poured into the buccal cavity.  Saliva usually contains enzymes and mucin.  The enzyme present in saliva is known as ptyalin that helps in the digestion of carbohydrates; while mucin helps to lubricate the food for swallowing.    

                                                                                                                                                                                                                                                                                                                                                                                               

The mouth leads to a funnel-shaped pharynx, which communicates with a long muscular tube called oesophagus.  The oesophagus opens into a muscular sac like structure called as stomach.  In man, it is somewhat J-shaped and occupies the left side of the abdomen. The stomach opens into the small intestine.  The stomach has many glands on its wall. Stomach wall produces gastric juice, which chiefly contains HCl, mucin and two protein digesting enzymes – rennin and pepsin.  The muscles of the stomach wall churn and mix the food with gastric juice.  Stomach through its pyloric region opens into small intestine. It is differentiated into three regions viz., duodenum, jejunum and ileum.  Duodenum is U-shaped and gets the common bile duct and pancreatic duct from the gall bladder and pancreas. Jejunum is longer and more coiled.  Ileum is the last part of small intestine and opens into the large intestine.  Its wall has numerous long, finger-like projections called villi, which enhance absorption.  Small intestine is the main region where digestion and absorption of food occurs.  It has large number of tubular glands that produce the intestinal juice containing a number of enzymes, which digest various types of food.  Digestion of different nutrients is completed in the small intestine by the action of pancreatic juice, intestinal juice and bile juice.  The end products of digestion are then absorbed from the small intestine.  

The small intestine opens into the large intestine.  It is comparatively much shorter and wider than the small intestine.  It does not have villi.  It is also differentiated into three regions:  caecum, colon and rectum.  Caecum is a small pouch-like structure and its main part is vermiform appendix.  However, caecum is very well developed in herbivorous animals like horse and ass.  The colon is longest and has four parts; ascending colon, transverse colon, descending colon and pelvic colon.  The pelvic colon opens into the rectum.  Rectum is the last part of large intestine.  Both in colon and rectum most of the water is reabsorbed back while the undigested food is removed from the body as faecal matter through anus.  This is known as Egestion.

 

Glands associated with alimentary canal:

 

Pancreas:  It is located in between the loops of duodenum.  It is the second largest gland of the body.  It secretes pancreatic juice that contains large number of digestive enzymes for digesting starch, lipids, proteins and nucleic acids.  The pancreatic juice is released into the pancreatic duct, which joins with the common bile duct.  

 

Liver:  It is the largest gland of the body lying immediately below the diaphragm in the right upper part of abdomen.  The cells of the liver (hepatic cells) produce bile juice that contains bile pigments and bile salts.  These bile salts help in the digestion and absorption of fats.  Bile juice does not contain any enzyme.  Bile juice flows out of the liver through hepatic ducts forming the common bile duct that opens into the duodenum (when the food is present in the duodenum).  When there is no food in the duodenum, then bile juice is stored in the gall bladder.  The gall bladder is a small elongated, muscular sac below the liver.  When the food comes into duodenum, it contracts to release the bile juice.