

CHAP. XXV.] THE INTESTINAL MUCOUS MEMBRANE. . 225 



than in the latter intestine. The mucous membrane is thickest 

 where the tubes are most developed, namely, in the jejunum. 



Lieberkiihn's follicles are doubtless secreting agents, resembling 

 in that respect the tubes of the Fig. 153. 



stomach. Probably their office, 

 in reference to the intestinal 

 contents, is the same through- 

 out the whole intestinal tube, A 

 as they present everywhere so 

 much uniformity of arrange- 

 ment and structure, and as each 

 portion of the intestine pos- 

 sesses other and peculiar glands. B 

 As yet, however, nothing is 

 known respecting the nature 

 of the mucus which is secreted 



bv them A - Transverse section of Lieberkiihn's tubes or folli- 



* cles, showing the basement membrane and sub- 



By the infinite multitude Of columnar epithelium of their walls, with the areolar 

 J tissue which connects the tubes, a. Basement mem- 



mi nute and micrOSCOpic involu- brane and epithelium, constituting the wall of the 



tube. 6. Cavity or lumen of the tube. Magnified 200 



tions which form Lieberkuhn's diameters. 



B. A single Lieberkiihn's tube, highly magnified. A 

 follicles, the extent of Surface ha PPy accidental section in the oblique direction has 



served to display very distinctly the form and mode of 

 Of the mUCOUS membrane is packing of the epithelial particles, the cavityof the tube, 



and the mosaic pavement of its exterior, a. Basement 

 CnormOUslv increased. It is membrane, c. Internal surface of the wall of the 



tube. Magnified 200 diameters 



still further enlarged by the 



existence of various folds and processes which project into the 



cavity of the intestine ; these we shall now proceed to describe. 



Of the Valvulce Conniventes, Folds, and Villi. The mucous 

 membrane of the small intestine exhibits numerous folds, which, 

 small, irregular, and resembling the rugae of the stomach in the 

 superior third of the duodenum, assume a much more definite form, 

 and are much more highly developed in the remaining portions of 

 the small intestine, but especially in the jejunum. 



The irregular foldings of the upper portion of the duodenum 

 very soon exhibit the tendency to assume a transverse direction 

 with reference to the axis of the intestine. In the middle and 

 inferior portions of the duodenum we find numerous transverse 

 plaitings or folds from one-eighth to a quarter of an inch in depth. 

 These are simple folds of the compound mucous membrane, including 

 a process of the submucous areolar tissue they are called valvulce 

 conniventes, from their valvular form, and from their movements 

 under water resembling the flapping of valves, or the winking 

 motion of the eyelids. Each fold passes round the intestine for 



