

PASSAGE OF THE URINE TO THE BLADDER. 411 



millimetres of Hg, at which pressure the secretion stops and 

 becomes somewhat changed in chemical composition (kreatin 

 appearing in greater quantity). 



Normally, however, the passage of the urine along the ureters 

 is accomplished by the peristaltic motion of the ducts, which goes 

 on alternately in the two ureters, so that the urine flows into the 

 bladder at different periods from the right and left kidney. 



The ureters have a strong middle coat of smooth muscle along 

 which a wave of contraction, lasting about one-third of a second, 

 passes rhythmically in about six to ten seconds from the pelvis 

 of the kidney to the bladder. 



Having reached the bladder, the urine cannot return into the 

 ureters on account of the oblique way in which these ducts pass 

 through the walls of the bladder. When the pressure in the 

 bladder increases, the opening of the ducts becomes closed and 

 acts as a kind of valve. 



The urine, which is continuously secreted and rhythmically 

 conveyed to the bladder, is only voided at convenient seasons ; 

 therefore special arrangements exist for its retention and expul- 

 sion. 



The retention of urine in the bladder up to a certain point 

 depends on the elasticity of the parts concerned, the dense elastic 

 tissues around its outlet being able to resist the elastic force 

 exerted by the viscera and the walls of the bladder upon its 

 contents. Thus, where no active muscular forces can possibly 

 coine into play, as in the case of the dead subject, or in complete 

 paralysis following destruction of the spinal cord, a considerable 

 amount of urine is retained. But when a certain pressure is 

 attained by the gradual accumulation of urine within the bladder, 

 the elasticity of the sphincter and the other tissues around the 

 outlet is overcome by the elasticity of the bladder wall, and the 

 urine slowly dribbles away. 



In the normal condition there are two sets of muscular 

 mechanisms which aid the elastic forces just named. 



They may be regarded as antagonistic the one, the sphincter 

 muscle, by contracting, strengthens the elastic power of the tissues 

 around the urethra which retains the urine; the other, formed by 



