The small intestine is a long tube-like structure that plays a vital role in the digestion and absorption of nutrients. It is divided into three parts: the duodenum, jejunum, and ileum.

1. Caecum: This is incorrect, as the caecum is not a part of the small intestine. The caecum is actually a pouch-like structure that is connected to the large intestine, specifically the beginning of the colon. It is involved in the absorption of water and electrolytes, as well as the fermentation of certain indigestible materials.

2. Duodenum: This is a correct part of the small intestine. It is the first and shortest part, located just after the stomach. It receives the partially digested food from the stomach and receives secretions from the liver, gallbladder, and pancreas to aid in digestion.

3. Jejunum: This is also a correct part of the small intestine. It is the middle part and is responsible for the majority of nutrient absorption.

4. Ileum: This is another correct part of the small intestine. It is the final and longest part, and it continues the absorption of nutrients as well as the absorption of bile salts and vitamin B12.

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