ESSAY ON SECRETION. 
11 
posterior vena-cava; which vessels running along the base of 
the lobules, are called sub-lobular veins. Besides these, we 
have a few branches ramifying upon the sheaths in which the 
vessels are contained, and called the vaginal plexuses. 
Each of these lobules contains a number of secreting cells, 
which are placed in the interspaces of the capillary network, 
and are enveloped by a very thin membrane, analogous to the 
basement membrane of ordinary mucous or serous membranes, 
and continuous with the basement membrane of the biliary 
ducts. 
These ducts, commencing at the surfaces of the lobules, 
anastomose freely among themselves, and, becoming larger, 
pass along the same canals which give passage to the branches 
of the portal vein, until at last they form a single tube, the 
biliary duct. 
In the horse this passes directly to the duodenum, but in 
most other animals we have connected therewith, a pear- 
shaped bag, known as the gall-bladder. This may be viewed 
as a pouch-like dilatation of one portion of the duct, which 
has become narrow at its point of junction with the duct 
whence it originated, and being situated on the inferior sur¬ 
face of the duct, it is conveniently placed for the reception of 
a quantity of the secretion as it flows along that tube ; and 
is enabled again to return it into the duct by the muscular 
tissue which forms a part of its walls. The hepatic artery 
supplying the liver pours its blood ultimately into the inter¬ 
lobular plexus of veins, which has been described among the 
branches of the portal vein ; and it is on record that the portal 
vein has been found to terminate directly in the posterior 
vena cava, without supplying the liver, so that the secretion 
of bile can be formed from arterial blood. 
As this gland and its secretion are of very great importance 
in the animal economy, you will not, I am sure, think that I 
am unprofitably occupying your time, if I now describe the 
course which the bile would take from its first secretion to its 
destination in the intestine. 
We say, then, that it is first formed within the lobules of 
the liver, by the cells which they contain, from the blood 
existing in a capillary plexus, derived from the interlobular 
branches of the portal vein ; thence, either by rupture of the 
cells, or by exudation through their walls, it passes into the 
commencing small portions of the biliary duct, and thence 
into the larger ducts which are found in company with the 
large branches of the veins in certain canals within the liver, 
and thus finally into that part of the duct situated upon the 
outer surface of the liver, within the transverse fissure; whence 
