390 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



cell to muscle-cell, the entire musculature behaving as though it were a single, 

 colossal hollow muscle-fibre. The liberation of the stimulus which inaugurates 

 the normal peristalsis of the ureter seems to be connected with the accumulation 

 of urine in its upper or kidney portion. It maybe supposed that the urine 

 that collects at this point as it flows from the kidney stimulates the muscular 

 ti~-ue to contraction, either by its pressure or in some otherway, and thus leads 

 to an orderly sequence of contraction waves. It is possible, however, that the 

 muscle of the ureter, like that of the heart, is spontaneously contractile under 

 normal conditions, and does not depend upon the stimulation of the urine. 

 Thus, according to Engelmann, section of the ureter near the kidney does not 

 materially affect the nature of the contractions of the stump attached to the 

 kidney, although in this case the pressure of the urine could scarcely act as a 

 stimulus. Moreover, in the case of the rat, in which the ureter is highly con- 

 tractile, the tube may be cut into several pieces and each piece will continue to 

 exhibit periodic peristaltic contractions. It does not seem possible at present 

 to decide between these two views as to the cause of the contractions. The 

 nature of the contractions, their mode of progression, and the way in which 

 they force the urine through the ureter seem, however, to be clearly established. 

 Efforts to show a regulatory action upon these movements through the central 

 nervous system have so far given only negative results. 



Movements of the Bladder. — The bladder contains a muscular coat of plain 

 muscle-tissue, which, according to the usual description, is arranged so as to 

 make an external longitudinal coat and an internal circular or oblique coat. 

 A thin longitudinal layer of muscle-tissue lying to the interior of the circular 

 coat is also described. The separation between the longitudinal and circular 

 layers is not so definite as in the case of the intestine ; they seem, in fact, to form 

 a continuous layer, one passing gradually into the other by a change in the 

 direction of the fibres. At the cervix the circular layer is strengthened, and 

 has been supposed t<> act as a sphincter with regard to the urethral orifice — the 

 so-called sphincter vesicae internus. Round the urethra just outside the blad- 

 der is a circular layer <>f striated muscle that is frequently designated as 

 the external sphincter or sphincter urethra'. The urine brought into the 

 bladder accumulates within its cavity to a certain limit. It is prevented from 

 escaping through the urethra at first by the mere elasticity of the parts at the 

 urethral orifice, aided perhaps by tunic contraction of the internal sphincter, 

 although this function of the circular layer is disputed by some observers. 

 When the accumulation becomes greater the external sphincter is brought into 

 action. If the desire to urinate is strong the external sphincter seems undoubt- 

 edly to be controlled by voluntary effort, but whether or not, in moderate filling 

 of the bladder, it is brought into play by an involuntary reflex is not definitely 

 determined. Back-flow of urine from the bladder into the ureters is effectually 

 prevented by the oblique course of the ureters through the wall of the 

 bladder. Owing to this circumstance pressure within the bladder serves to close 

 the mouths of the ureters, and indeed the more completely the higher the pres- 

 sure. At some point in the filling of the bladder the pressure is sufficient to 



