Most digestion in the small intestine happens in the duodenum. This is the beginning of the small intestine. This is probably where the food gets hit the hardest. The middle and end of the small intestine are meant to keep the food going and absorb the rest of the nutrients out of the food. This means that all the digestion and breaking up of the food has to happen before the rest of the nutrients are pulled out of the food.
However, the most digestion actually happens in the stomach, but that is neither here nor there for this question. For the purposes of this answer, we are going to disregard the rest of the digestive system, and focus on the small intestine, so there you go!
In the small intestine there is a section called the duodenum. It is the first section of the small intestine. It is the shortest part of the small intestine. In humans it is a tube that is jointed and it is hollow. It begins with what is called the duodenal bulb. It ends with the suspensory muscle. There are four parts to it. The first part is superior to the rest of the four segments. It is slightly dilated. It is connected to the liver. The second part is the descending part.
The third part is horizontal. The fourth part ascends. It is at the vertebral level. The duodenum has a function of breaking down food using enzymes. Ulcers are common in the duodenum caused by bacteria and infection.