408 MANUAL OF PHYSIOLOGY. 



owing to spasm of the renal arterioles and fall of blood-pressure 

 in the glomerular capillaries, may be brought to a standstill. 

 Section of the spinal cord at the 7th cervical vertebra stops the 

 flow, because it so reduces the general blood-pressure that the 

 pressure in the renal vessels falls below that necessary for the fil- 

 tration of the urine. 



PASSAGE OF THE URINE TO THE BLADDER. 



The pressure exerted by the blood in the glomerular capillaries 

 is quite sufficient to make the urine flow from the pelvis of the 

 kidneys into the bladder, because when the ureters are tied they 

 become distended above the ligature by the urine flowing from 

 the pelvis, where a pressure may be produced of some forty milli- 

 metres 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 ex- 

 erted by the viscera and the walls of the bladder upon its con- 



