MUCOUS MEMBRANE OF THE SMALL INTESTINE 



209, 



The position and arrangement of the valvulae conniventes are such 

 that they move freely in both directions and may be applied to the 

 inner surface of the intestine either above or below their lines of attach- 

 ment. It is evident that the food, as. it passes along in obedience to the 

 peristaltic movements., must, by insinuating itself beneath the folds and 

 passing over them, be exposed to a greater extent of mucous membrane 

 than if these valves did not exist. This is about the only definite use 

 that can be assigned to them. 



Thickly set beneath the mucous membrane in the first half of the 

 duodenum, and scattered here and there throughout the rest of its 

 extent, are the duodenal racemose glands, or the glands of Brunner. 

 These are not found in other parts of the intestinal canal. In their 

 structure they closely resemble the racemose glands of the oesophagus. 

 On dissecting the muscular coat from the mucous membrane, they may 

 be seen with the naked eye, in the areolar tissue, in the form of small 

 rounded bodies, about one-tenth of an inch (2.5 millimeters) in diameter. 

 Examined microscopically, these bodies are found to consist of a large 

 number of rounded follicles held together by a few fibres of connective 

 tissue (see Plate IV, Fig. 4). They have bloodvessels ramifying on 

 their exterior and are lined with glandular epithelium. They communi- 

 cate with an excretory duct which penetrates the mucous membrane 

 and opens into the intestinal cavity. When these structures are 

 examined in a fresh preparation, the excretory duct frequently is 

 found to contain a clear viscid mucus of an alkaline reaction. This 

 secretion, however, has not been obtained in quantity sufficient to admit 

 of the determination of its chemical or physiological properties. 



The intestinal tubules, the follicles or crypts of Lieberkiihn, are 

 found throughout the small and the large intestine. In a thin vertical 

 section they are seen closely packed together, extending through nearly 

 the entire thickness of the mucous membrane (see Plate IV, Fig. 4). 

 Between the tubules are bloodvessels embedded in a dense stroma of 

 fibrous tissue, with non-striated muscular fibres. In vertical sections 

 the only situations where the tubules are not seen are in that portion 

 of the duodenum occupied by the ducts of the glands of Brunner and 

 immediately over the centre of the larger solitary glands and some of 

 the closed follicles which are collected to form the patches of Peyer. 

 The tubes are not entirely absent in the patches of Peyer, but are here 

 collected in rings, twenty or thirty tubes deep, which surround each of 

 the closed follicles. Microscopical examination of the surface of the 

 mucous membrane by reflected light shows that the openings of the 

 tubules are between the villi. 



The tubules in the human subject usually are simple, though some- 



