488 HANDBOOK OF PHYSIOLOGY. 



and the principal urinary solids are eliminated, is accomplished by the 

 cells of the convoluted tubes, anr] is sometimes (as in the case of the 

 elimination of urea and similar substances) accompanied by the elimina- 

 tion of copious fluid, produced by the chemical stimulation of the epi- 

 thelium of f 'ie same tubules. 



The Passage of Urine into the Bladder. 



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 

 through the ureter the urine passes into the bladder, into which its rate 

 and mode of entrance has been watched in cases of ectopia vesicce, i.e., 

 of such fissures in the anterior or lower part of the walls of the abdo- 

 men, and of the front wall of the bladder, as expose to view its hinder 

 wall together with 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. During fasting, two or three drops 

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

 the little papilla on which, in these cases, the ureter opens, and then 

 passing slowly through its orifice, which at once again closes like a 

 sphincter. In the recumbent posture, the urine collects for a little time 

 in the ureters, then flows gently, and, if the body be raised, runs from 

 them in a stream till they are empty. Its flow is aided by the peristaltic 

 contractions of the ureters, and is increased in deep inspiration, or by 

 straining, and in active exercise, and in fifteen or twenty minutes after 

 a meal. The urine collecting is prevented from regurgitation into the 

 ureters by the mode in which these pass through the walls of the blad- 

 der, namely, by their lying for between half and three-quarters of an inch 

 between the muscular 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. 

 In so far, however, as it is a voluntary act, it is performed by means of 

 the abdominal and other expiratory muscles, which in their contraction, 

 as before explained, press on the abdominal viscera, the diaphragm being 

 fixed, and cause the expulsion of the contents of those whose sphincter 

 muscles are at the same time relaxed. The muscular coat of the blad- 

 der co-operates, in micturition, by reflex involuntary action, with the 

 abdominal muscles; and the act is completed by the accelerator urince, 

 which, as its name implies, quickens the stream, and expels the last 

 drop of urine from the urethra. The act, so far as it is not directed by 

 volition, is under the control of a nervous centre in the lumbar spinal 

 cord, through which, as in the case of the similar centre for defalcation, 

 the various muscles concerned are harmonized in their action. It is 

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



