OF ANIMAL BODIES. 105 



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 the same kind as what 

 passes from the liver through the direct passage.* It is 

 possible that the gall may be changed, and, for some purpo- 

 ses, 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 cavi- 

 ties, 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 

 before 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. t The same contrivance is used 

 in another part, where there is exactly the same occasion 

 for it, viz. in the insertion of the ureters into 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 

 necessary 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 

 manufactured on the outside of the cheek, by the parotid 

 gland, which lies between the ear and the angle of the low- 

 er jaw. In order to carry the secreted juice to its destina- 

 tion, there is laid from the gland on the outside, a pipe, 

 about the thickness of a wheat straw, and about three fin- 

 gers breadth in length ; which, after riding over the masse- 

 ter iiiuscle, bores for itself a hole through the very middle 



* Keill's from Malpighius, p. 62. IKeill's Anat. p. 62 



t Ches, Anat. p. 260. 



K 2 



