CHAP. HI.] ELIMINATION OF WASTE PRODUCTS. G79 



425. Structure of the bladder. The epithelium of the 

 bladder resembles in its characters that of the ureter, but the 

 appearances presented by the cells in sections of prepared bladders 

 will naturally vary a good deal according as the bladder was 

 hardened in a contracted or in a distended state. This epithelium 

 with the underlying fine connective tissue forms a mucous mem- 

 brane, separated by submucous connective tissue from a well- 

 developed muscular coat, which in turn is invested with an outer 

 coat of connective tissue covered, over the greater part of the 

 organ, with peritoneum. 



The well-developed, plain muscular fibre-cells which constitute 

 this muscular coat are gathered into rounded bundles, or flattened 

 bands, which in turn are arranged in a plexiform manner, being bound 

 together by connective tissue carrying blood vessels arid nerves. 

 The direction of these bundles is not very regular, but they may 

 be regarded as forming on the inner side below the mucous 

 membrane a circularly disposed coat, better developed at the 

 lower part of the bladder round the opening of the urethra than 

 elsewhere, and outside this a longitudinally disposed coat, the 

 longitudinal direction of the bundles being better seen at the 

 front and back than at the sides. Many of the bundles and net- 

 works of bundles, however, in both coats run a course which is 

 neither exactly longitudinal nor circular. The inner longitudinal 

 coat of the ureter appears to be represented by a very thin and 

 inconspicuous layer. The thicker and better developed portion 

 of the circularly disposed coat is sometimes spoken of as the 

 sphincter vesicce, and the longitudinally disposed coat is similarly 

 sometimes called the detrusor urinai ; but, as we shall see, these 

 names are undesirable. In the dog the longitudinal bundles are 

 much better developed than the circular; but the relative pro- 

 portion of the two sets of bundles seems to vary in different 

 animals. 



The bladder is supplied with nerves from the hypogastric 

 plexus, the fibres being both medullated and non-medullated 

 They appear, as in the case of the rectum ( 276), to have a double 

 origin, coming on the one hand from the lower dorsal and upper 

 lumbar spinal cord through the sympathetic system, and on the 

 other hand, in a more direct manner, from the sacral spinal 

 nerves. More abundant round the neck of the bladder than 

 higher up they run at first in the outer connective tissue coat, be- 

 neath the peritonaeum "and, forming plexuses, ultimately end partly 

 in the blood vessels and partly in the muscular fibres, though 

 some fibres are said to have been traced to the epithelium. 

 Groups of nerve-cells occur on the plexuses, especially near the 

 neck. 



426. The urine, like the bile, is secreted continuously ; the 

 flow may rise and fall, but, in health, never absolutely ceases for 

 any length of time. The cessation of renal activity, the so-called 



