260 DIGESTION. 



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. 

 They cannot, as has been supposed by some, have any considerable influence upon the 

 rapidity of the passage of the alimentary mass along the intestinal canal. 



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 Brnnner. 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 muscu- 

 lar coat from the mucous membrane, 

 they may be seen with the naked eye, 

 in the areolar tissue, in the form of lit- 

 tle, rounded bodies, about one-tenth of 

 an inch in diameter. Examined micro- 

 scopically, these bodies are found to 

 consist of a large number of short, blind 

 tubes branching in every direction and 

 held together by a few fibres of con- 

 nective tissue. The tubes have blood- 

 vessels ramifying on their exterior and 

 are lined with glandular epithelium. 



They collect together to terminate in 

 FIG. W. GlandofBmnner, from the human subject. (Frey.) ., , . , ,, 



an excretory duct which penetrates the 



mucous membrane and opens into the intestinal cavity. When these structures are ex- 

 amined in a perfectly fresh preparation, the excretory duct is frequently found to contain 

 a clear, viscid mucus, of an alkaline reaction. This secretion has never been obtained in 

 quantity sufficient to admit of the determination of its chemical or physiological proper- 

 ties. Its quantity must be infinitely small as compared with the secretion produced 

 by the glandular tubes found in such immense numbers throughout the intestinal tract, 

 and it cannot be regarded as constituting an important part of the fluid known as the 

 intestinal juice. 



The intestinal tubules, or the follicles of Lieberkuhn, the most important glandular 

 structures in the intestinal mucous membrane, are found throughout the whole of the 

 small and large intestine. In examining a thin section of the mucous membrane, these 

 little tubes are seen closely packed together, occupying nearly the whole of its structure. 

 From the great extent of the membrane, it can readily be conceived that their number 

 must be immense. Between the tubules, are blood-vessels, embedded in a dense stroma 

 of fibrous tissues with numerous unstriped muscular fibres. In a vertical section of 

 the mucous membrane, the only situations where the tubules are not seen are in that 

 portion of the duodenum where the space is occupied by the ducts of the glands of Brun- 

 ner 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. A 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 are usually simple, though sometimes bifurcated, are composed externally 

 of a structureless basement-membrane, and are lined with a single layer of columnar epi- 

 thelium like the cells which cover the villi, the only difference being that, in the tubes, 

 the cells are a little shorter. These cells never contain fatty granules, even during the di- 

 gestion of fat. The central cavity which the cells enclose, which is about one-fourth of 

 the diameter of the tube, is filled with a clear, viscid fluid, which is the most important 



