104 
OF THE VESSELS 
is, to force out from the bag, and send into the duodenum, 
an extraordinary quantity of bile, to meet the extraordinary 
demand which the repletion of the stomach by food is about 
to occasion.* Cheselden describes! the gall-bladder as 
seated against the duodenum, and thereby liable to have its 
fluid pressed out, by the passage of the aliment through that 
cavity; which likewise will nave the effect of causing it to 
be received into the intestine, at a right time, and in a due 
proportion. 
There may be other purposes answered by this contri¬ 
vance; and it is probable that there are. The contents of 
the gall-bladder are not exactly of ihe same kind as what 
passes from the liver through the direct passage.J It is 
possible that the gall may be changed, and for some pur¬ 
poses meliorated, by keeping. 
The entrance of the gall-duct into the duodenum, furnish¬ 
es another observation. Whenever either smaller tubes 
are inserted into larger tubes, or tubes into vessels and 
cavities, such receiving tubes, vessels, or cavities, being 
subject to muscular constriction, we always find a con¬ 
trivance to prevent regurgitation. In some cases, valves 
are used; in other cases, amongst which is that now be¬ 
fore us, a different expedient is resorted to; which may 
be thus described: The gall-duct enters the duodenum 
obliquely: after it has pierced the first coat, it runs near 
two fingers’ breadth between the coats, before it opens into 
the cavity of the intestine.§ The same contrivance is used 
in another part, where there is exactly the same occasion 
for it, viz. in the insertion of the ureters in the bladder. 
These enter the bladder near its neck, running obliquely for 
the space of an inch between its coats.|| It is, in both 
cases, sufficiently evident, that this structure has a ne¬ 
cessary mechanical tendency to resist regurgitation; for, 
whatever force acts in such a direction as to urge the fluid 
back into the orifices of the tubes, must, at the same time, 
stretch the coats of the vessels, and thereby compress that 
part of the tube, which is included between them. 
IV. Amongst the vessels of the human body, the pipe 
which conveys the saliva from the place where it is made, 
to the place where it is wanted, deserves to be reckoned 
amongst the most intelligible pieces of mechanism with 
which we are acquainted. [PI. XX. fig. 1, 2.] The saliva, 
we all know, is used in the mouth; but much of it is 
* Keill’s Anat. p. 64. f Anat. p. 164. 
t Keill’s from Malpighius, p. 62. § Keill’s Anat. p. 62. 
II Ches. Anat. p. 260. 
