NATURAL THEOLOGY. 187 



smaller lubes are inserted into larger tubes, or tubes 

 into vessels and cavities, such receiving tubes, ves- 

 sels, or cavities, being subject to muscular con- 

 striction, we always find a contrivance to prevent 

 resuro-itation. 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.* The same 

 contrivance is used in another part, where there is 

 exactly the same occasion for it, viz., in the inser- 

 tion of the ureters in the bladder. These enter 

 the bladder near its neck, running obliquely for the 

 space of an inch between its coats. f It is, in both 

 cases, suflficiently evident that this structure has a 

 necessary mechanical tendency to resist regurgita- 

 tion ; 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 wdiich is included between them. 



V. 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. The 

 saliva, we all know, is used in the mouth ; but much 



* Keill's Anat. p. 62. f Ches. Anat. p. 260. 



