CHAP. XXXIV.] PELVIS OF KIDNEY. 505 



urinary constituents in a state of solution involves the necessity of a bladder, 

 or receptacle, in which it can collect, and which would still further add to the 

 weight. We find in the minute anatomy of the bird's lung a beautiful 

 arrangement, by means of which not the smallest space where blood can be 

 exposed to the action of air is lost. (Vide p. 395.) 



Pelvis of Kidney and Ureters. The mucous membrane lining the 

 pelvis of the kidney is continuous with that of the renal tubes at 

 the point where they open upon the papillae, in which situation it is 

 exceedingly thin, and it is difficult to distinguish its epithelium. 

 The epithelium of the pelvis of the kidney generally is polygonal in 

 form, arid constitutes a tolerably thick layer. The deeper cells are 

 small and rounded. Many cells approaching to the columnar 

 form may also be observed ; and these increase in number towards 

 the ureter, which tube is lined with this variety of epithelium. 



The ureters have muscular coats composed of two layers, 

 an internal layer of circular, and an external one of longitu- 

 dinal fibres. These are prolonged upwards into the pelvis of the 

 kidney, and cease at the calyces. The muscular coat is composed 

 entirely of unstriped muscular fibre cells, the nature of which will 

 be particularly described when we come to speak of the uterus, 

 and it is invested with an external coat composed of fibrous tissue. 



The ureters reach the base of the bladder, run obliquely through 

 its coats for the distance of nearly an inch, and open into this 

 viscus by two narrow slit-like openings about an inch and a half 

 behind the prostate on its inferior surface, and separated from 

 each other by the distance of nearly two inches. The openings 

 readily permit the urine to pass into the bladder ; but, by their 

 arrangement, completely prevent its reflux into the ureter; the 

 reflexion of mucous membrane at their mouth serves the office of a 

 valve. 



We have already referred to the contraction of the ureters in 

 p. 457 of the present volume. 



Bladder. The urinary bladder is the large receptacle into which 

 the urine is poured and in which it accumulates as it escapes from 

 the ureters. Its size varies very greatly : it may be distended 

 to such a degree as to contain nine, or even twelve pints of 

 urine, in which case its walls of course become exceedingly thin, 

 or it may be contracted so much as to leave scarcely any visible 

 cavity in its interior. Its contracted muscular walls may be found 

 half an inch or more in thickness, a condition very often met with 

 in cases of cholera. 



The internal surface of the bladder has a reticulated appearance, 



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