Villi in Man and certain of' the Mammalia. 171 



are continually exerting their absorbing function, and draw 

 into their cavities that portion of the chyme in the gut ne- 

 cessary to supply materials for the chyle. When the vesi- 

 cles respectively attain in succession their specific size, they 

 burst or dissolve, their contents being cast into the tissue of 

 the villus, as in the case of any other species of interstitial cell. 



Tlie looped network of lacteals, like the other lymphatics, 

 continually exerting their peculiar function, take up the re- 

 mains, and the contents of the dissolved chyle cells, as well as 

 the other matters which have already subserved the nutri- 

 tion of the villus. As long as the cavity of the gut contains 

 chyme, the vesicles of the terminal extremity of the villi con- 

 tinue to develope, to absorb chyle, and to burst, and their re- 

 mains and contents to be removed by the interstitial absorb- 

 ent action of the lacteals. 



AVhen the gut contains no more chyme, the flow of blood to 

 the mucous membrane diminishes, the development of new 

 vesicles ceases, the lacteals empty themselves, and the villi 

 become flaccid. 



The function of the villi now ceases till they are again 

 roused into action by another flow of chyme along the gut. 



During the intervals of absorption, it becomes necessary to 

 protect the delicate villi from the matters contained in the 

 bowel. They had thrown oft* their protective epithelium 

 when required to perform their functions, just as the stomach 

 had done to affbrd gastric juice, and the intestinal follicles to 

 supply their peculiar secretions. In the intervals of digestion, 

 the epithelium is rapidly reproduced. Repeated examina- 

 tions have induced me to believe that this reproduction is ac- 

 complished in the following manner. 



That peculiar diaphanous membrane which I have denomi- 

 nated (Trans. Roy. Soc Ed., 1842) the primary membrane, 

 and which, as I have stated, not only forms the outer mem- 

 brane of the follicles, under the epithelia, but also the under- 

 lying membrane of the villi, contains in its substances nuclei 

 of an oval form, situated at pretty regular distances. These 

 nuclei have a dark spot in the centre, and are always visible 

 when the epithelium is removed. The membrane consists of 

 flattened cells, the nuclei of which continue active. Blood 

 vessels, therefore, do not exist in this membrane, but ramifv 



