The duodenum, being thus in the immediate vicinity of the spine, is fixed firmly down in its position by the connecting membrane, and is not left to float loosely in the cavity of the _abdomen_ or belly. Had it not been tied down in this way, it would not only have acted by its weight as a continual drag upon the stomach and disturbed its functions, but likewise have been constantly altering its own relation to the pancreatic and hepatic (or liver) ducts, and thereby affecting the flow of their respective fluids into its cavity, by which the chylification would have often been interrupted. [Illustration] The duodenum is much smaller in diameter than the stomach, but larger than the _jejunum_ or _ileum_; and its muscular coat is also thicker. From its size and the importance of the changes effected in it, it has been considered by some as a secondary stomach or _ventriculus succenturiatus_. Its mucous coat, which has a soft velvety feel, presents a greater multitude of the folds or plaits already described, and which have for their object to extend its surface and delay the passage of the chyme. Some notion of their appearance may be formed from the subjoined wood-cut. These folds or _rugæ_ are called _valvulæ conniventes_ or _folding valves_, and are inherent in the nature of the mucous coat, and not produced by mere folds of the whole thickness of the intestine, consequently they exist even when the latter is distended. They are comparatively few in number in the part of the duodenum near the stomach, and gradually multiply in the course downwards till they arrive at the maximum of development at its lower end and in the jejunum: they again diminish in the ileum, and disappear altogether in the large gut. The bloodvessels and nerves of the duodenum are extremely numerous, and indicate the importance of its functions. The duodenum serves to receive the chyme as it issues from the stomach, and to prepare it for the farther changes which it is about to undergo. But as other organs, namely the _liver_ and _pancreas_, are directly concerned along with the duodenum in effecting chylification, it will be proper to take a cursory view of them also before describing the rest of the intestine.