The tube that conveys the urine from the kidney to the bladder is like a white, round cord, about the size of a goose quill, prolonged from the pouch on the lower surface of the kidney backward beneath the loins, then inward, supported by a fold of thin membrane, to open into the bladder just in front of its neck. The canal passes first through the middle (muscular) coat of the bladder, and then advances perceptibly between that and the internal (mucous) coat, through which it finally opens. By this arrangement in overfilling the bladder this opening is closed like a valve by the pressure of the urine, and the return of liquid to the kidney is prevented. The bladder (Pl. IX, fig. 2) is a dilatable, egg-shaped pouch, closed behind by a strong ring of muscular fibers encircling its neck, and enveloped by looped, muscular fibers extending on all sides around its body and closed anterior end. Stimulated by the presence of urine, these last contract and expel contents through the neck into the urethra. This last is the tube leading backward along the floor of the pelvic bones and downward through the penis. In the bull this canal of the urethra is remarkable for its small caliber and for the S-shaped bend which it describes in the space between the thighs and just above the scrotum. This bend is attributable to the fact that the retractor muscles are attached to the penis at this point, and in withdrawing that organ within its sheath they double it upon itself. The small size of the canal and this S-shaped bend are serious obstacles to the passing of a catheter to draw the urine, yet by extending the penis out of its sheath the bend is effaced, and a small, gum-elastic catheter, not more than one-fourth of an inch in diameter, may with care be passed into the bladder. In the cow the urethra is very short, opening in the median line on the floor of the vulva about 4 inches in front of its external orifice. Even in her, however, the passing of a catheter is a matter of no little difficulty, the opening of the urethra being very narrow and encircled by the projecting membranous and rigid margins, and on each side of the opening is a blind pouch (canal of G‰rtner) into which the catheter will almost invariably find its way. In both male and female, therefore, the passing of a catheter is an operation which demands special skill.