URINARY PASSAGES. 221 



B. URINARY PASSAGES. 

 (a) Kidney Calyces, and Pelvis ; Ureter and Urinary Bladder. 



In all of these parts of the apparatus which conducts 

 urine from the kidney we find the walls made up of the fol- 

 lowing layers: 1, mucosa; 2, submucosa; 3, muscularis; 4, 

 fibrosa. 



The mucosa consists of an epithelium and a tunica propria. 

 The former is the so-called transitional epithelium, and is quite 

 similar in all parts of the canal, so that in pathological condi- 

 tions of the tract where the cells appear in the urine it is diffi- 

 cult to say from what part these cells are derived. The cells 

 differ, however, in the various layers. The uppermost layer 

 consists of large cubical or somewhat flattened cells ; the middle 

 layers of cylindrical, pyriform, spindle-shaped, or polygonal 

 cells ; and the deepest layer of relatively small cubical or irreg- 

 ularly oval cells. The cells of the first two layers often possess 

 processes which extend between neighboring cells. Those of 

 the outer row may contain more than one nucleus (Fig. 173). 



FIG. 173. 



End of epithelial 

 ceil 



$:.,. BH 



- . n />4| * > A_ *"- *>-:* "*- "*?. ^^** *- propria 

 From a section through the mucous membrane of an ape's urinary bladder. X 300. 



The epithelial layer is considerably thinned at the place where 

 the calyces of the kidney pass over to the kidney papillae, and 

 usually consists of only a single layer of cubical cells. The 

 mucosa of the ureter is thrown into longitudinal folds (Fig. 

 172). In the bladder also the surface is much folded in many 

 directions on account of the contraction of the muscularis and 

 submucosa. In the distended bladder most of these folds dis- 



