MUCOUS MEMBEANE OF THE SMALL INTESTINE. 313 



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 which can be assigned to them. They cannot, 

 as has been supposed by some, have any considerable influ- 

 ence on 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 through- 

 out the rest of its extent, are the duodenal racemose glands, 

 or the glands of Brunn. These are not found in other parts 

 of the intestinal canal. In their structure, they closely re- 

 semble the racemose glands of the oesophagus. On dissect- 

 ing the muscular coat from the mucous membrane, they may 

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

 of little rounded bodies, about one-tenth of an inch in diam- 

 eter. Examined microscopically, these bodies are found to 

 consist of a large number of short blind tubes ramifying in 

 every direction and held together by a small quantity of 

 fibrous tissue. The tubes have blood-vessels ramifying on their 

 exterior and are lined with glandular epithelium. They col- 

 lect together to terminate in an excretory duct which pene- 

 trates the mucous membrane and opens into the intestinal 

 cavity. When these structures are examined 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 

 properties. Its quantity must be infinitely small compared 

 with the quantity of secretion produced by the glandular 

 tubes found in such immense numbers throughout the intes- 

 tinal tract ; and it cannot be regarded as constituting an im- 

 portant part of the fluid known as the intestinal juice. 



The intestinal tubules, or the follicles of Lieberkuhn, the 



