1076 EXCRETION 



this matter. The pelvic segment of the ureter is undoubtedly well 

 equipped with nerve fibers and it appears that this portion acts as the 

 pace-maker for the lower segments. Experimentally, however, it is 

 possible to evoke peristalsis in any part of this organ. Since smooth 

 muscle tissue is seldom richly supplied with nerve tissue, the relatively 

 "nerve-free" central portion might normally be dependent upon in- 

 fluences conveyed to it from the pace-maker through the agency of 

 the aforesaid ganglion cells. 1 



The Urinary Bladder. The bladder is composed of a mucous, 

 submucous, muscular and serous layer. Its muscular coat contains 

 an outer longitudinal, a middle circular, and an inner anastomosing 

 or oblique layer of fibers. The fibers of the first pass in an almost 

 direct line from the fundus to the urethra, where some of them become 

 attached to the pelvis as the pubovesical muscle. Posteriorly, the 

 strands end, in the male, in the prostate and, in the female, in the 

 urethral-vaginal septum. The median coat is much thicker and con- 

 sists of fibers arranged transversely to the long axis of the organ. At 

 the cervix, this layer is materially strengthened, forming here the inter- 

 nal sphincter vesicse. Farther outward, and enveloping the root of 

 the urethra, is a second sphincter which is composed of striated muscle 

 tissue, and is usually designated as the external sphincter or sphincter 

 urethrae. The inner coat of muscle tissue consists of obliquely ar- 

 ranged fibers which are distributed in an irregular manner and per- 

 meate the different layers. 



As each ureter continues to empty small quantities of urine into 

 the fundus of the bladder, its walls are forced outward more and more 

 until they have attained a physiological degree of distention. A 

 contraction of the musculature then ensues which drives the urine 

 through the relaxed sphincters to the outside. Under ordinary con- 

 ditions, therefore, the peristaltic waves of the ureters need not over- 

 come a considerable resistance and their power is more than ample to 

 force the urine into the fundus. But a regurgitation of the urine into 

 the ureters is quite impossible even during the interims, because the 

 orifices of the ureters are firmly closed. This end is not accomplished 

 by special sphincters, but in an indirect way by the distention of the 

 walls of the bladder. Inasmuch as the ureters perforate the latter in 

 an oblique direction and open by means of slit-like orifices, the gradual 

 filling of the bladder must cause the lip-like margins of these openings 

 and neighboring segments of the ureters to become firmly approxi- 

 mated. Consequently, the greater the internal pressure, the more 

 firmly will these orifices be closed. In general, therefore, it may be 

 said that three factors are at work to prevent the regurgitation of 

 the urine, namely, gravity, the peristaltic action of the ureters, and the 

 mechanical closure of their orifices by the distention of the walls of 

 the bladder. 



The foregoing discussion also shows that the high pressures which 

 1 Lucas, Am. Jour, of Physiol., xvii, 1906, 392. 



