STRUCTURE OF THE GALL-BLADDER. 23 



their appearance suddenly, succeeding to the ultimate epithe- 

 lial cells of the ducts, and it is only occasionally that these 

 appear somewhat enlarged at the point of transition. The 

 interior of a biliary duct is frequently bounded on the one side 

 by small epithelial cells, whilst on the other the large hepatic 

 cells are visible. In fig. 123 two fragments are depicted from 

 the periphery of an hepatic lobule, taken from a child three 

 months old, exhibiting the transition of the epithelial into the 

 hepatic cells. Very similar appearances may be obtained from 

 the injected livers of animals, where the fine threads of injec- 

 tion proceeding from the ducts, lined with epithelium, can be 

 seen to penetrate between the hepatic cells. 



It is to the absence of intermediate or transitional forms of 

 cells between the small epithelium and the large hepatic cells, 

 that the obscurity which has so long rested on the passage of 

 the biliary ducts into the biliary canals is mainly to be at- 

 tributed. This transition, in fact, only consists in the sudden 

 alteration in the form of the biliary ducts, whilst the calibre of 

 the tube undergoes but little change. 



The microscopic appearances of the biliary ducts and their 

 epithelial lining are easily observed in thin sections. In 

 order to render the mode of branching of the biliary ducts and 

 the plexuses visible, they should be injected with coloured 

 material, which however, of course, destroys the epithelium. 

 The glands of the biliary ducts are best examined microscopic- 

 ally after they have been injected with slightly tinted gum. 



THE GALL-BLADDER is lined by a mucous membrane which 

 presents numerous decussating rugae and a few less elevated 

 conical processes. Like the mucous membrane of the intestine 

 and the villi, it is highly vascular, and is covered by a layer 

 of very tall columnar cells, which, like those of the small in- 

 testine, present a thickened striated hem or border at their 

 free extremity.* A layer, composed of connective tissue, tra- 

 versed by a wide-meshed plexus of capillaries, and containing 

 numerous decussating bundles of smooth muscular fibres, is 

 regarded by Henle as belonging to the mucous membrane. 



* Yirchow, Virchow's Archivj 1857, Band xi., p. 574. 



