648 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



3. The Mechanism of Micturition. — The secretion of urine, like 

 the bile, is constant, and if the ureters be divided it will be found that 

 there will be a steady flow of urine, drop by drop. The urin,e has been 

 described as" a pure excretion — that is, it is composed solely of sub- 

 stances which no longer have any office to fulfill in the economy, and 

 which are deleterious and must be removed. Arrest of the renal secre- 

 tion, or so-called suppression of urine, by preventing the elimination of 

 these substances invariably leads to a fatal result. 



The urinary constituents eliminated from the blood through the 

 action of the glomeruli and epithelial cells of the uviniferous tubules pass 

 drop by drop through the straight canals into the pelvis of the kidney, 

 and from there through the peristaltic contractions of the muscular walls 

 of the ureters into the bladder. 



The bladder is a muscular organ composed of an internal mucous 

 coat and double muscular coat. The fibres, which are of the unstriped 

 variety, are arranged in oblique and circular layers, the latter being es- 

 pecially developed at the neck of the bladder. Externally situated is a 

 fibrous membrane, while the upper portion of the bladder is covered by 

 the peritoneum. The ureters pierce the vesical walls obliquely, and at the 

 orilice of entrance of the ureters into the bladder is located a valvular 

 fold of mucous membrane. As the bladder fills the increased pressure 

 on its walls tends to obliterate the orifice of entrance of the ureters, 

 and so prevent regurgitation through the ureters back to the .kidneys. 

 Sometimes, as a consequence of obstruction to the flow of urine from the 

 bladder, it will be found that the ureters are then the seat of considera- 

 ble distention. Such distention is not, however, caused by a reflux from 

 the bladder, but is only produced when, the bladder becoming distended 

 to its full capacity, the constant secretion of urine still continues to 

 collect in the ureters behind the bladder. As the urine accumulates in 

 the bladder it rises from the cavity of the pelvis to occupy the lower 

 portion of the abdominal region, where in man, when fully distended, it 

 may be recognized by percussion, and extends from eight to ten centi- 

 meters above the symphysis of the pubes. 



The urine is retained in the bladder by the normal tonic contraction 

 of the circular sphincter of the bladder, aided by the tonic contraction 

 of the sphincter urethrae and the elastic fibres surrounding the urethra. 

 As the bladder becomes distended the sphincter becomes relaxed, and 

 the contact of the escaping urine with the upper part of the membranous 

 portion of the urethra causes the desire to urinate. Escape of urine may 

 at this time be prevented by the contraction of the sphincter urethrae 

 muscle, which is a red, striped, voluntary muscle. 



In animals and infants this contact of the urine with the mucous 

 membrane of the urethrae starts the process of micturition, which, in 



