Fig. 1, Plate 56, represents by section the natural forms of the urethra and bladder. The general direction of the urethra measured during its relaxed state from the vesical orifice to the glans is usually described as having the form of the letter S laid procumbent to the right side [capital S rotated 90 degrees right] or to the left [capital S rotated 90 degrees left]. But as the anterior half of the canal is moveable, and liable thereby to obliterate the general form, while the posterior half is fixed, I shall direct attention to the latter half chiefly, since upon its peculiar form and relative position depends most of the difficulty in the performance of catheterism. The portion of the urethra which intervenes between the neck of the bladder, K, and the point E, where the penis is suspended from the front of the symphysis pubis by the suspensory ligament, assumes very nearly the form of a semicircle, whose anterior half looks towards the forepart, and whose posterior half is turned to the back of the pubis. The pubic arch, A, spans crossways, the middle of this part of the urethra, G, opposite the bulb H. The two extremes, F K, of this curve, and the lower part of the symphysis pubis, occupy in the adult the same antero-posterior level; and it follows, therefore, that the distance to which the urethra near its bulb, H, is removed from the pubic symphysis above must equal the depth of its own curve, which measures about an inch perpendicularly. The urethral aperture of the triangular ligament appears removed at this distance below the pubic symphysis, and that portion of the canal which lies behind the ligament, and ascends obliquely backwards and upwards to the vesical orifice on a level with the symphysis pubis in the adult should be remembered, as varying both in direction and length in individuals of the extremes of age. In the young, this variation is owing to the usual high position of the bladder in the pelvis, whilst in the old it may be caused by an enlarged state of the prostate. The curve of the urethra now described is permanent in all positions of the body, while that portion of the canal anterior to the point F, which is free, relaxed, and moveable, can by traction towards the umbilicus be made to continue in the direction of the fixed curve F K, and this is the general form which the urethra assumes when a bent catheter of ordinary shape is passed along the canal into the bladder. The length of the urethra varies at different ages and in different individuals, and its structure in the relaxed state is so very dilatable that it is not possible to estimate the width of its canal with fixed accuracy. As a general rule, the urethra is much more dilatable, and capable consequently of receiving an instrument of much larger bore in the aged than in the adult.