absence of a complete neural arch of the fifth sacral vertebra. The four ante-
rior(answer a)and posterior (answer d)sacral foramina on either side of
the midline join the intervertebral foramen (answer c)and provide egress
for the anterior and posterior primary rami of the sacral spinal nerves. The
level to which the anesthesia blocks the spinal nerves is a function of the
amount of anesthetic delivered.
434.The answer is b.(Moore and Dalley, pp 387–388.)The uterine artery
(answer a)crosses anterior and superior to the ureter near the lateral
fornix of the vagina and is deliberately clamped and removed during a hys-
terectomy. Some clinicians and anatomists once referred to the uterine
artery as “the bridge over troubled waters.” Because of its close proximity
to the artery, the ureter may be accidentally ligated or severed while tying
off of the artery. Of course, the bladder (answer c)and the kidney (answer e)
are large structures and should notbe mistakenly clamped. The urethra
(answer d)should be far out of the operating field.
435.The answer is d.(Moore and Dalley, pp 449–451.)Pus from the
abscessed diverticuli in that space can extend anteriorly deep to the per-
ineal membrane, but inferior to the urogenital diaphragm. The ischioanal
fossa (asterisk on CT accompanying the question) is a fat-filled space [thus
not muscle (answer b)] that extends from below the levator ani muscle
(puborectalis, pubococcygeus, and iliococcygeus muscles). It also extends
anteriorly in the area between the pelvic diaphragm (superiorly) and the
perineal membrane (inferiorly). It cannot extend superiorly above the
pelvic diaphragm [thus not (answer c)] and, therefore, cannot extend
superiorly anterior to the sacrum (answer d). The paracolic gutter
(answer a)innoton the image.
436.The answer is b.(Moore and Dalley, p 387.)The obturator usually
arises from the anterior trunk of the internal iliac artery. However, in 25%
of the population, it arises from the inferior epigastric or the external iliac
artery. There is considerable variation as to the origins of the branches of the
posterior and anterior trunks of the internal iliac artery. The internal puden-
dal artery (answer a),umbilical artery (answer d),and uterine artery
(answer e)almost always arise from the anterior trunk. The superior
gluteal artery (answer c)usually arises from the posterior trunk.
Pelvis Answers 553