464 PHYSIOLOGY. 



equalizing pressure within that organ from damming back of the 

 urine. 



The excretory apparatus ^comprises the ureters, bladder, and 

 urethra. 



THE URETERS are two cylindrical membranous tubes of the 

 diameter of a goose-quill and about fifteen inches long. They extend 

 from the pelvis of the kidney to the bladder, to which viscus runs 

 the urine from the kidneys. The general course of each ureter is 

 downward and inward toward the median line,, to empty into the 

 base of the bladder by a constricted, slitlike orifice. The ureter runs 

 for nearly an inch between the muscular and mucous coats of the 

 bladder before it makes its exit upon the inner wall of the organ. 



Structure. The ureter is composed of three coats, or layers: 

 serous, or adventitia; muscular; and mucous. 



The adventitia is continuous with the capsule of the kidney at 

 one end and with the fibrous layer of the bladder at the other. In 

 it are found its larger vessels and nerves. 



The muscular coat comprises the two usually distinct muscular 

 layers: an external longitudinal; an internal, circular one. 



The mucous coat, continuous with that of the bladder, lines the 

 ureter. It is composed of stratified epithelial cells. 



Movement of the Urine. The urine flows into the tubules by the 

 vis-a-tergo pressure of the blood in the afferent capillaries. This 

 averages from 120 to 140 millimeters of mercury. This force, which 

 is capable of making the urine flow through the tubules, is incapable 

 of forcing the urine through the ureters. By reason of the ureters 

 taking a diagonal course through the vesical wall, the weight of the 

 urine already in the bladder must exert a certain amount of pressure 

 upon this portion of each ureter. To overcome this some auxiliary 

 force must be called into action, which is the peristaltic contraction 

 of the ureters. This movement begins at the kidneys and is trans- 

 mitted (with a speed of from 20 to 30 millimeters per second) down- 

 ward into the bladder. With the completion of each peristaltic 

 movement there exudes into the bladder a drop of urine. The move- 

 ments of the two ureters are not synchronous; they are reflex, being 

 caused by the presence of urine in the lumen of the ureter. 



In a case of Dr. W. Easterly Ashton's, where the ureters opened 

 on the abdominal surface, I counted an emission of urine by the 

 ureter every twenty-four seconds. 



