The duodenum is a strong muscular tube, about twelve inches in length (whence the name), which curves round the head of the pancreas or sweetbread. At about the centre of the duodenum will be found the orifices of the tube or duct by which the pancreatic juice is poured into the duodenum; it here becomes mixed with the chyme or half-digested food contained in the duodenum.

The pancreas is a glandular organ resembling the salivary glands in structure. It is concerned in secreting a fluid, which has the very important parts to play in the digestive process of changing starch into sugar and of emulsifying the fats. It has been already stated that it is necessary for insoluble starch to be converted into soluble sugar before it can pass through the walls of the blood vessels. The first step of this process commences in the mouth by the action of the saliva, but it is here incomplete, and it is stopped altogether as long as the food remains in the stomach owing to the acidity of the gastric juice. The substance called pancreatine, which forms ten per cent. of the pancreatic juice, has the power of almost instantaneously changing starch into sugar. There are various forms of sugar, and the kind of sugar into which starch is changed by the action of the pancreatic juice in the duodenum, is that known as glucose.