to the anatomical fact, that the urethra at and close to the bulb is the
most dependent part of the curve, F K, Fig. 1, Plate 56; and hence, that
instruments descending to this part from before push forcibly against
the urethra, and are more apt to protrude through it than to have their
points turned so as to ascend the curve towards the neck of the bladder.
If it be also true that strictures happen here more frequently than
elsewhere, this circumstance will of course favour the accident. An
additional cause why the catheter happens to be frequently arrested at
this situation and to perforate the canal, is owing to the fact, that
the triangular ligament is liable to oppose it, the urethral opening in
this structure not happening to coincide with the direction of the point
of the instrument. In the figure, part of a bougie traverses the urethra
through both strictures and lodges upon the enlarged prostate. Another
instrument, after entering the first stricture, occupies a false passage
which was made in the canal between the two constricted parts.

[Illustration]
Plate 58.--Figure 2.

Fig. 3, Plate 58.--A calculus is here represented lodging in the urethra
at the bulb. The walls of the urethra around the calculus appear
thickened. Behind the obstructing body the canal has become dilated,
and, in front of it, contracted. In some instances the calculus presents
a perforation through its centre, by which the urine escapes. In others,
the urine makes its exit between the calculus and the side of the
urethra, which it dilates. In this latter way the foreign body becomes
loosened in the canal and gradually pushed forwards as far as the
meatus, within which, owing to the narrowness of this aperture, it
lodges permanently. If the calculus forms a complete obstruction to the
passage of the urine, and its removal cannot be effected by other means,
an incision should be made to effect this object.

[Illustration]
Plate 58.--Figure 3.

Fig. 4, Plate 58, represents the neck of the bladder and neighbouring
part of the urethra of an ox, in which a polypous growth is seen
attached by a long pedicle to the veru montanum and blocking up the neck
of the bladder. Small irregular tubercles of organized lymph, and
tumours formed by the lacunae distended by their own secretion, their
orifices being closed by inflammation, are also found to obstruct the
urethral canal.

[Illustration]
Plate 58.--Figure 4.

Fig. 5, Plate 58