450 



THE URINARY SYSTEM 



into deep folds or rugae, whose surface also presents secondary folds 

 of irregular direction. The epithelial layer is relatively thick, and is 

 thicker on the sides of the folds than upon either their apices or bases 

 where the folding of the epithelium increases the tension of its cells. 

 When the organ is distended, the folds are more or less completely 

 obliterated, the epithelial layer is much thinned, often until it appears 



a b 



FIG. 414. THE MUCOSA OF A CHILD'S BLADDER IN THE CONTRACTED STATE OF THE 



ORGAN. 



a-'a, and a'-a', transitional epithelium; b, fibroin uscular tissue of the mucosa. 

 Hematein and eosin. Photo. X 216. 



to consist of no more than a double layer of cells, and its cells be- 

 come much broader and relatively much thinner than in the collapsed 

 or empty condition. Hence the transitional variety of epithelium, by 

 which the organ is lined, may be said to be remarkable for the extreme 

 elasticity of its cells. 



The mucous membrane, except near the urethral orifice, contains 

 no glands. In this location, however, the bladder of adult man contains 

 a few small mucus-secreting glands, lined by columnar cells (Kol- 



