ORGANS OF THE BODY. 509 



these (although liable to be simulated by imperfectly injected canals), can- 

 not be denied in our opinion. 



It is only when we ascend to birds, that we meet with a higher deve- 

 lopment of this system of lateral branches. 



Among those mammals, on the other hand, which have been hitherto 

 made the subject of research, it is in many cases found in an extremely 

 high state of development in the form of that exceedingly complex network 

 of biliary capillaries, represented in fig. 503. Here the surface of each 

 hepatic cell comes in contact with one or more biliary ducts. But even 

 still, and though presenting complex and various modifications, the funda- 

 mental plan, as seen in fig. 504 (3), remains distinct. The biliary (c) and 

 blood-capillaries (a) never come into actual contact ; they are always sepa- 

 rated from one another by a whole or fraction of an hepatic cell (b). In 

 the lower vertebrates several hepatic cells combine to enclose the former, 

 while higher up the scale the contact of fewer, and at last of two, is suffi- 

 cient for their formation. 



Finally, we are met by the inquiry, What is the nature of the delicate 

 wall of the biliary duct ? 



The cuticular border of the epithelial cells in the terminal ramifications 

 of the bile ducts, is pointed out by Ebertlt, as its probable source. Just as 

 the cell secretion or cuticular formation becomes thickened and perforated 

 by pores towards the larger branches, as already mentioned ( 92), so 

 does it, as we advance upon the biliary capillaries, acquire greater delicacy, 

 forming eventually the walls of the biliary capillaries at the points of con- 

 tact of the hepatic cells. 



266. 



There are still left for our consideration the larger biliary ducts, the 

 lymphatics, and nerves of the organ. 



Eesembling to a considerable extent the ramifications of the portal 

 system, in their course and mode of confluence, the bile ducts present for 

 our consideration a homogeneous membrane with a clothing of small low 

 cells from the ductus interlobularis, which has been already mentioned 

 in the preceding chapter. In the larger trunks, instead of homogeneous 

 walls, fibrous coats and long cylindrical epithelial cells make their appear- 

 ance, upon whose surface a porous cuticular border may be recognised 

 with increasing distinctness, as we advance from within outwards. In 

 those passages of large size, which have already left the parenchyma of 

 the liver, a mucous membrane and external fibrous layer are to be seen 

 composing their walls. It was formerly supposed that, besides these, a 

 series of longitudinal contractile fibre cells entered into the structure of 

 the tube : this has not, however, been since confirmed. 



The coats of the gall bladder are formed, according to Henle, of layers 

 of connective-tissue alternating with muscular Iamina3, consisting of 

 unstriped fibres which cross each other in all directions. The mucous 

 membrane is marked by beautifully regular folds, and is covered by the 

 same coating of nucleated columnar cells met with in the small intestine. 

 These latter are also endowed with the same power of absorbing fats as 

 those of the intestine. 



The bile ducts possess also numerous follicles and racemose glands. 

 The first are to be found in the larger canals, as in the ductus chole.docliu& 

 cysticns and hepatic duct with its larger branches : they are arranged 

 sometimes irregularly, sometimes in rows. The racemose mucous glands 



