The upper portion of alimentary canal is mouth. It is bounded by lips and contains cheeks, gums, teeth, tongue and muscles. The teeth in the mouth chew and masticate the food. It increases the surface area of the food so that various enzymes can act on it more effectively.
Tongue it is a muscular sensory organ, with taste buds on it. It helps in tasting the food and also helps in the movement in the cavity. In adult man 32 teeth are found which perform different functions. Formation of teeth takes place twice in man. The first set, called the milk teeth, starts appearing at the age of 6 months. This continues up to 2-3 years. Cutting, biting and chewing of food are the main functions of teeth.
The oral cavity passes into a pharynx. The masticated and partially digested food then goes down into the stomach through a tube called oesophagus.
It is a muscular, tubular part of the alimentary canal. It is about ten inches long. It corresponds to the neck region of man. It represents the narrowest portion of the alimentary canal. By peristalsis (a type of muscle movement) the food passes down the oesophagus.
It is located below the diaphragm and appears as a sac-like structure. It serves as the storehouse of food where partial digestion takes place. The wall of the stomach is muscular and contains many small gastric ducts. About 40 million gastric glands are found in the wall of the stomach.
The small intestine is about 6 metres in length and 2.5 centimetres in thickness. There are three divisions of the small intestine: duodemum, jejunum and tleum. Duodenum is the first part which begins from the pyloric stomach, and is C-shaped. In the middle of the duodenum two different ducts open through a common aperture. One of the ducts is the common bile duct and the other is the pancreatic duct. Bile juice is poured into the duodenum through the common bile duct.
The ileum passes behind into the egressive (outgoing) zone of the digestive tract commonly called the large intestine, which in man is about 1.5 m in length. The large intestine can be divided into two parts: anterior, colon and posterior, rectum. At the junction of iloeum and colon, there is a blind (one end closed) out pushing called caecum. The free end of this caecum forms a process which is termed the vermiform appendix (vermes=worm, vermiform = worm-shaped). In man, the vermiform appendix has outlived its usefulness i.e., it is a vestigial organ. It is 8 cm long blind tube which may become a source of trouble. Swelling or infection in the vermiform appendix causes severe pain. Rupture of vermiform appendix may be fatal.
Under the circumstances, vermiform appendix has to be removed by surgery (appendectomy). The colon is arranged in the abdominal cavity in ascending, transverse, and descending manner. The last part, the descending part, opens into the rectum which is 21-23 centimetres long. Its terminal part is called anal canal which opens through the anus, guarded by circular muscles, the sphincter muscles. The large intestine allows the passage of residue of food mass (faecal matter) for its expulsion through the anus which is called egestion. As the residue of the food mass passes along the large intestine, a considerable amount of water contained in the residue is absorbed and brought into the blood. The passage of the faecal matter along the colon is regulated by the specialized longitudinal muscles present in its wall.