ARRANGEMENT OF THE BILIARY DUC'l’S. 
393 
External diameter. 
Internal diameter. 
r -008 
•004 
Pig 
} -006 
•003 
1 "001 
•001 
Human subject . 
r "01 
•0045 
. 
{ "002 
•002 
Human foetus 
. 
•01 
•0045 
Cat 
•015 
•0075 
Monkey . , . 
, -02 
•01 
Seal ..... 
r -001 
•001 
< 
1 -0003 
•0003 
The diameter of the ducts from which my drawings were taken can be readily 
determined by reference to the scales appended to the Plates. 
The narrowing of the excretory duct, just before it becomes connected with the 
secreting tube, is seen in other glands ; thus the straight and ductal portion of the 
renal tube is narrower than the convoluted and secreting portion. It must, however, 
be borne in mind, that although the excretory portion of the tube is so small, its 
cavity, through which the secretion passes, is wider than in that part in which the 
secretion is separated. In the liver, where the secretion is highly elaborated and 
slowly removed in a more concentrated form, we should naturally expect to find the 
contrast between the ductal and secreting portions of the gland still greater than in 
most other secreting organs. 
Epithelium of the small ducts . — As is well known, the larger ducts have a thick 
lining of columnar epithelium, the cells of which become shorter and altogether 
smaller in the finer ducts. In the narrowest tubes the cells of epithelium are some- 
what flattened, and usually of a circular form. Sometimes they are oval or angular, 
depending probably upon being stretched. They have a faintly granular appearance, 
and rarely a nucleus can be seen within. The quantity of this epithelium varies 
very much ; sometimes it completely lines the tube. In some instances it is so 
abundant as apparently to leave no distinct cavity in the duct, while it is not 
uncommon to find some of the finest ducts containing only a very few cells scattered 
at irregular intervals over the basement membrane, of which the coats of these small 
ducts are alone composed. In a perfectly normal condition, when the minute ducts 
are undisturbed by manipulation, and are examined in a proper medium, they are 
generally seen to be lined by epithelium ; but from the extreme minuteness of these 
ducts and tender character of their walls, it is scarcely surprising that one should 
fail in making out distinctly the epithelium in every instance. The epithelium of 
the small ducts presents very similar characters in all the animals in which I have 
been able to demonstrate it. In the human subject it is shown in Plate XV. figs. 20 
& 21, and in the seal in Plate XIV. fig. 16, at h. 
This ductal epithelium does not pass by insensible gradations into the secreting 
