VESSELS OF ANIMALS. 121 



aislies 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 subjecl 

 to muscular constriction, we always find a contrivance to 

 prevent regurgitation. In some cases valves are used ; in 

 Dther cases, among which is that now before us, a differenl 

 gxpedient 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 finger's breadth betivee7i 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, namely, in the insertion ot 

 the ureters in the bladder. These enter the bladder near 

 its neck, running for the space of an inch between its coats. f 

 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 

 fl.uid 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. 



Y. Among 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 among 

 che most intelligible pieces of mechanism with which we 

 are acquainted. The saliva, we all know, is used in the 

 mouth ; but much of it i? produced on the outside of the 

 cheek by the parotid gland, which lies between the ear and 

 the angle of the lower jaw. In order to carry the secreted 

 juice to its destination, there is laid from the gland on the 

 outside a pipe about the thickness of a wheat straw, and 

 about three finger's breadth in length, which, after riding 

 over the masseter muscle, bores for itself a hole through the 

 very middle of the cheek, enters by that hole, which is a 

 complete perforation of the buccinator muscle, into the 

 mouth, and there discharges its fluid very copiously. 

 * Keill's Anat., p. 62. t Ches. Anat, p. 2C0. 



Nat. Thpol. 6 



