ORGANS OF THE BODY. 507 



More internally still these tubules are continuous with an exquisitely 

 delicate mesh-work of the finest tubes, known as the biliary capillaries 

 (d). These are passages of extremely small calibre, measuring in the 

 rabbit only '0025-0 '00 18 mm. Arranged in a dense network (3 a), 

 they pass between the hepatic cells (b) in such a way that the sur- 

 face of each of the latter conies in contact with them at various 

 points. 



The meshes are cubical, so that the network presents the same appear- 

 ance from almost every point of view. The breadth of each mesh is, on 

 an average, G'0144-0'0201 mm. in the rabbit, and corresponds as a rule 

 with that of the gland cells. The whole is characterised by the wonder- 

 ful delicacy of arrangement, and the regular way in which this third and 

 finest network is interwoven with the two others formed by the blood 

 capillaries and bands of gland cells. 



These biliary capillaries have been known for many years past to 

 exist in many mammals, among which the rabbit appears to be best 

 suited for their demonstration. They have recently, however, been dis- 

 covered in the other three classes of vertebrata also (Eberth, Hering). 



The questions now arise Do the biliary capillaries possess independent 

 walls, or are they simply lacunar passages ; and what relation do they 

 maintain to the hepatic cells *? 



For our own part, we would with MacGillavry, as formerly, so still 

 answer the first question in the affirmative, having always held the 

 opinion that the biliary ducts do possess independent walls. Isolation of 

 the latter has, however, up to the present been impossible, but the signi- 

 ficance of this fact seems of minor importance when we consider the great 

 delicacy of all the component tissues of the part. Again (2), the 

 interlacement of the blood capillaries (a) is seen to take place in such a 

 peculiar manner through the network of the biliary capillaries (b), and in 

 many localities the latter present such regularity when injection has been 

 successfully accomplished, that the existence of a system of lacunae of this 

 kind between cells endowed with vital contractility seems highly impro- 

 bable. Further, we may at times encounter points, at the junction 

 of injected and uninjected portions of tissue, at which the amount of 

 granules of colouring matter of the former diminish in the latter in a way 

 that permits of our following on the network of biliary capillaries a little 

 farther by the thin lines of coloured fluid, until they appear eventually in 

 the tissue around the several hepatic cells quite destitute of coloured con- 

 tents. Under very high magnifying power, also, the empty network may 

 be seen clearly, presenting great regularity, the canals of the same calibre 

 throughout, with no enlargements at the nodal points, and sharply con- 

 toured. Sometimes we are even so fortunate as to obtain a section so 

 thin that it is almost entirely formed of a network of bands of hepatic 

 cells only one tier thick ; and here, along the middle of each band, some 

 of these biliary passages may take their course, maintaining the axis, and 

 lying quite free and uncovered by other rows of cells. An appearance of 

 this kind is easily explained, if we accept the presence of a special wall 

 to each canal, but is, on the other hand, difficult to account for if the pas- 

 sages be regarded as lacunar. The existence of these walls has been since 

 recognised by both Eberth and Koelliker. 



The next question is : How are these biliary ducts related to the hepatic 

 cells? 



On this point the opinions of histologists have until recently been very 



