354 HAND-BOOK OF PHYSIOLOGY. 



its internal surface. It is constructed of three principal coats (a) an 

 outer, tough, fibrous and elastic coat; (b) a middle, muscular coat, of 

 which the fibres are unstriped, and arranged in three layers the fibres 

 of the central layer being circular, and those of the other two longitudinal 

 in direction; and (c) an internal mucous lining continuous with that of 

 the pelvis of the kidney above, and of the urinary bladder below. The 

 epithelium of all these parts (Fig. 246) is alike stratified and of a some- 

 what peculiar form; the cells on the free surface of the mucous mem- 

 brane being usually spheroidal or polyhedral with one or more spherical or 

 oval nuclei; while beneath these are pear-shaped cells, of which the broad 

 ends are directed toward the free surface, fitting in beneath the cells of 

 the first row, and the apices are prolonged into processes of various 

 lengths, among which, again, the deepest cells of the epithelium are 

 found spheroidal, irregularly oval, spindle-shaped or conical. 



Structure of Urinary Bladder. The urinary bladder, which 

 forms a receptacle for the temporary lodgment of the urine in the intervals 

 of its expulsion from the body, is more or less pyriform, its widest part, 

 which is situate above and behind, being termed the fundus: and the 

 narrow constricted portion in front and below, by which it becomes con- 

 tinuous with the urethra, being called its cervix or neck. It is constructed 

 of four principal coats, serous, muscular, areolar or submucous, and 

 mucous, (a) The serous coat, which covers only the posterior and upper 

 half of the bladder, has the same structure as that of the peritoneum. 



FIG. 246. Epithelium of the bladder; a, one of the cells of the first row; 6, a cell of the second 

 row; c, cells in situ, of first, second, and deepest layers. (Obersteiner.) 



with which it is continuous, (b) The fibres of the muscular coat, which 

 are unstriped, are arranged in three principal layers, of which the external 

 and internal (Ellis) have a general longitudinal, and the middle layer a 

 circular direction. The latter are especially developed around the cervix 

 of the organ, and are described as forming a sphincter vesicce. The mus- 

 cular fibres of the bladder, like those of the stomach, are arranged not in 

 simple circles, but in figure-of-8 loops, (c) The areolar or submucous 

 coat is constructed of connective tissue with a large proportion of elastic 

 fibres, (d) The mucous membrane, which is rugose in the contracted 

 state of the organ, does not differ in essential structure from mucous 



