Question

The longest part of the alimentary canal is
A) Oesophagus
B) Large intestine
C) Small intestine
D) Stomach

Answer 1

Hint: The alimentary canal along with some associated glands constitutes the complete digestive system of our body. The structure and dimension of different organs of the alimentary canal depends on the function of that organ.

Complete Answer:
The important organs of the alimentary canal are – Mouth, Oesophagus, Stomach, Small intestine and Large intestine.
The alimentary canal has two openings. The opening at anterior end is the mouth and at posterior end it opens through anus. The different organs of alimentary canal can be explained as –
1) Mouth is the first part of the alimentary canal where partial digestion of food takes place with the help of saliva, teeth and tongue. From mouth food enters into the food pipe or Oesophagus.
2) Oesophagus is a long and thin muscular tube which passes through the neck, thorax and the diaphragm and opens into a muscular bag-like structure called stomach.
3) Stomach consists of three parts – cardiac region (where oesophagus opens), fundic region and pyloric region. The pyloric region of the stomach opens into the ‘C’ shaped duodenum of the small intestine.
4) The longest part of the alimentary canal is the small intestine which is divided into three parts- the Duodenum, long and coiled Jejunum, and highly coiled ileum that opens into the large intestine.
5) The large intestine consists of three parts- Caecum, Colon, and Rectum. Caecum is a small pouch-like structure that contains symbiotic microorganisms. The vermiform appendix that is a vestigial organ remains attached to this part. Caecum opens into colon which is divided into four regions-ascending colon, transverse colon, descending colon and sigmoid colon. Through the sigmoid colon the descending part of the colon opens into Rectum. The rectum is the distal part of the alimentary canal that opens into the anus.

Fig: Digestive system

Now we will discuss the dimensions of these organs. Oesophagus is a tube like structure with an average length of about 25cm, an empty human stomach is a small muscular bag which is about 12 inches long and 6 inches wide, the large intestine is about 1.5m in length, and, the small intestine is about 6 to 7m in length.

This shows that the correct answer is C, i.e. small intestine.

Note: The primary function of the small intestine is to absorb nutrients, and, for this function it needs a maximum amount of surface area. Also it bears numerous finger-like projections called villi and microvilli on its inner surface to increase its surface area. That is why our small intestine is so long. The nutrients are absorbed very slowly as compared to the act of eating, so, the length of the small intestine allows the process to take its time (in hours).