390 EXCRETION 



THE DISCHARGE OF THE URINE. 



As each portion of urine is secreted, it propels that which is already in the 

 uriniferous tubes onward into the pelvis of the kidney. Thence it passes 

 through the ureter into the bladder, from which at intervals it is discharged 

 to the exterior. The rate and mode of entrance of urine into the bladder 

 has been watched in cases of ectopia vesicae, i.e., cases in which fissures in 

 the anterior or lower part of the walls of the abdomen and of the front wall 

 of the bladder expose to view the orifices of the ureters. The urine does not 

 enter the bladder at any regular rate, nor is there a synchronism in its move- 

 ment through the two ureters. Ordinarily two or three drops enter the 

 bladder every minute, each drop as it enters first raising up the little papilla 

 on which the ureter opens, and then passing through the orifice, which at 

 once again closes like a sphincter. Its flow is aided by the peristaltic con- 

 tractions of the ureters, and is increased in deep inspiration or by straining. 

 The urine collected in the bladder is prevented from regurgitation into the 

 ureters by the mode in which these pass through the walls of the bladder, 

 namely, by their lying a half to three-quarters of an inch between the muscu- 

 lar and mucous coats before they turn rather abruptly forward and open 

 through the latter into the interior of the bladder. 



Micturition. The contraction of the muscular walls of the bladder 

 may by itself expel the urine with little or no help from other muscles. The 

 vesicular pressure is increased in the voluntary act by the contraction of 

 the abdominal and other expiratory muscles which bear on the abdominal 

 viscera, thus aiding in the expulsion of the contents of the bladder. The 

 diaphragm is at the same time fixed in contraction and the sphincter of the 

 bladder relaxes. The pressure within the bladder under the combined con- 

 tractions of these expulsive muscles sometimes amounts to 8 to 10 cm. of 

 mercury. The act is completed by the accelerator urinae muscle, which, as 

 its name implies, quickens the stream and expels the last drop of urine from 

 the urethra. The act is under the regulative control of a nervous center in 

 the lumbar spinal cord, through which, as in the case of the similar center for 

 defecation, the various muscles concerned are coordinated in their action. 

 It is well known that the act may be reflexly induced, e.g., in children who 

 suffer from intestinal worms or other such irritation. Generally the afferent 

 impulses which set up the reflexes leading to the desire to micturate are ex- 

 cited by overdistention of the bladder, or sometimes by a few drops of urine 

 passing into the urethra. This impulse passes up to the lumbar center or 

 centers, and reflexly produces on the one hand inhibition of the sphincter 

 and on the other contraction of the necessary muscles for the expulsion 

 of the contents of the bladder. In the voluntary act these motor centers 

 are stimulated to activity by impulses coming from the higher cerebral 

 centers. 



