ig. 3,
Plate 50, and assumes a form corresponding with the bag of the
superficial fascia which encloses the structures connected with the
urethra. The anus is centrally situated in the depression formed between
D D, the ischiatic tuberosities, and the double folds of the nates.

The perinaeum, Fig. 3, Plate 50, is, for surgical purposes, described as
divisible into two spaces (anterior and posterior) by a transverse line
drawn from one tuber ischii, D, to the other, D, and crossing in front
of the anus. The anterior space, A D D, contains the urethra; the
posterior space, D D C, contains the rectum. The central raphe, A B C,
traverses both these spaces. The anterior or urethral space is (while
viewed in reference to its osseous boundaries) triangular in shape, the
apex being formed by the pubic symphysis beneath A, whilst two lines
drawn from A to D D, would coincide with the ischio-pubic rami which
form its sides. The raphe in the anterior space indicates the central
position of the urethra, as may be ascertained by passing a sound into
the bladder, when the shaft of the instrument will be felt prominently
between the points A B. Behind the point B, the sound or staff sinks
deeper in the perinaeum as it follows the curve of the urethra towards
the bladder, and becomes overlaid by the bulb, &c.

The ischiatic tuberosities, D D, Fig. 3, Plate 50, are, in all subjects,
sufficiently prominent to be felt through the integuments, &c.; and the
line which, when drawn from one to the other, serves to divide the two
perinaeal spaces, forms the base of the anterior one. In well-formed
subjects, the anterior space is equiangular, the base being equal to
each side; but according as the tuberosities approach the median line,
the base becomes narrowed, and the triangle is thereby rendered acute.
These circumstances influence the direction in which the first incision
in the lateral operation of lithotomy should be made. When the
tuberosity of the left ischium stands well apart from the perinaeal
centre, the line of incision, B E, Fig. 3, Plate 50, is carried
obliquely from above downwards and outwards; but in cases where the
tuberosity approaches the centre, the incision must necessarily be made
more vertical. The posterior perinaeal space may be described on the
surface by two lines drawn from D D, the ischiatic tuberosities, to C,
the point of the coccyx, whilst the transverse line between D and D
bounds it above.

By removing th