Describe the structure and functions of the various parts of the alimentary canal.

Human Digestive system: 

Human digestive system consists of alimentary canal and associated digestive glands. 

Alimentary canal: Alimentary canal is a long tube-like structure of varying diameter starting from mouth and ending with anus. It is about 8-10m long. Alimentary canal consists of mouth, buccal cavity, pharynx, oesophagus, stomach, small intestine, large intestine and anus. 

Mouth: 

• It is also called oral or buccal cavity and is bounded by fleshy lips. 
• Its side walls are formed of cheeks, roof is formed by palate and floor by tongue.
• It is internally lined by a mucous membrane. 
• Salivary glands open into the buccal cavity. 

Function: It helps in ingestion of food. 

Teeth: 

• 32 teeth are present in the buccal cavity of an adult human being.
• Human dentition is described as thecodont, diphyodont and heterodont. 
• It is called thecodont type because each tooth is fixed in a separate socket present in jaw bones by gomphosis type of joint. 
• In our life time, we get only two sets of teeth, milk teeth and permanent teeth. This is called diphyodont dentition. We have four different type of teeth hence we are heterodont.
• Types of teeth are incisors (I) canines (C) premolars (PM) and molar (M). 
• Each half of each jaw has two incisors, one canine, two premolars and three molars. 
• Thus, dental formula of adult human can be represented as: i 2/2 , c 1/1 , pm 2/2 , m 3/3 = = 16 × 2 = 32 

Tongue: 

It is the muscular fleshy organ and is roughly triangular in shape. It lies along the floor of the buccal cavity. 

Functions: The upper surface of the tongue bears numerous projections called papillae. These papillae contain sensory receptors called taste buds. 

ii. Pliary nx: 

• The buccal cavity leads to a short pharynx.
• Pharynx is a common passage for food and air. 
• The pharynx opens into trachea through an opening called glottis. 
• The glottis is guarded by a cartilaginous flap called epiglottis. The epiglottis closes during the swallowing (deglutition) action and pre vents entry of food into the trachea. 
• The lower region of pharynx is called oropharynx. 
• Oropharynx opens into oesophagus through gullet. 

iii. Oesophagus: 

• The oesophagus is a thin, muscular tube. 
• It lies behind the trachea. 
• It is approximately 25cm long tube passes through the neck, central aspect of rib cage, pierces the diaphragm and joins the stomach.
• It is lined by mucus cells. Mucus lubricates the passageway of food.
•  Oesophagus is made up of longitudinal and circular muscles.

Function: The rhythmic wave of contraction and relaxation of these muscles is called peristalsis that helps in passage of food through oesophagus.

iv. Stomach: 

The stomach is located in the upper left portion of the abdominal cavity. It is a muscular sac-like ‘J1 shaped organ, around 25 to 30cm in length. It is divided into upper cardiac region and lower pyloric region.

• Cardia or Cardiac: It is first part in which oesophagus opens. The cardia surrounds the band of circular muscles present at the junction of oesophagus and stomach called cardiac sphincter. The cardiac sphincter prevents back flow or regurgitation of food from stomach to oesophagus. 
• Fundus: It is the dome shaped region above and left of cardia. 
• Body: It forms the large central portion of stomach that stores the food. 
• Pylorus: It is a narrow posterior region of stomach. 
• It opens into duodenum, the initial part of small intestine. 
• This opening is guarded by a set of sphincter muscles called pyloric sphincter. 
• It regulates the flow of food from stomach to small intestine. 

Function: The stomach temporarily stores the food. It chums the food and helps in mixing the food with gastric juice.

v. Small Intestine: 

• It is about 6 meters long and 2.5 cm broad tube coiled within abdominal cavity. 
• The coils are held together by mesenteries, supporting the blood vessels, lymph vessels and nerves. 
• It is divided into three parts: Duodenum, jejunum and ileum. 

vi. Large Intestine: 

• It is 1.5 meters in length. 
• It is wider in diameter and shorter than small intestine. 
• It consists of caecum, colon and rectum. 

vii. Anus: 

• Anus is the terminal opening of alimentary canal.
• It is guarded by sphincter. 

Function: It expels faecal matter by a process called egestion or defecation.