site of the spinal tap to stimulate closing of the hole in the meninges.) This
may lead to sliding of the brain down the foramen magnum, which is a rare
consequence of spinal taps. Since cranial nerve VI exits the brain at the
junction of the medulla and the pons and then enters the Dorello’s dural
cave along the clivus it cannot slide down with the brain and thus would
be subjected to stretching. Stretching of the abducent nerve would likely
lead to bilateral compromised ability to look laterally since noother cranial
nerves can abduce the eyes. The viral meningitis (answer a)wouldnot
cause a selective loss of both abducent cranial nerves. Excess cerebral
spinal fluid and cause CN VI palsy, but generally notCN III palsy (answer
c).Bilateral tumors at the superior orbital fissures (answer d)would be
both unlikely, and also would compromise other cranial nerves as well.
Loss of the ability to look laterally is notpart of a normal hospitalization
(answer e).
318.The answer is i.(Moore and Dalley, p 1149.)The cranial nerve that
provides the afferent limb of the gag reflex is the glossopharyngeal, IX cra-
nial nerve. Cranial nerve IX provides general sensation from the posterior
one-third of the tongue and palatine tonsil area. The response to touching
this area is to contract the soft palate and pharynx in a protective manor or
gag reflex. The motor aspects of this reflex are mainly mediated by the
vagus, cranial nerve X.
319.The answer is a.(Moore and Dalley, pp 973, 1137.)This condition is
most likely due to an aneurysm of the left posterior cerebral artery com-
pressing cranial nerve III. The woman’s symptoms, ptosis of the upper eye-
lid, an eye that is rotated down (because superior oblique muscle,
innervated by CN IV is still functioning) and out (because the lateral rectus
muscle, innervated by CN VI is still functioning) with a dilated pupil
(because sympathetics, which innervate the dilator pupili muscles are still
V 3 functioning) are all consistent with loss of function of cranial nerve III.
An aneurysm in either the posterior cerebral or superior cerebellar artery
often can compress cranial nerve III as it exits the midbrain. An aneursym
of the right anterior cerebral artery (answer b)would be very unlikely to
cause a problem for the left third cranial nerve. A tumor within the left optic
canal(answer c)would effect the left optic nerve which passes through it.
Cranial nerve III passes into the orbit through the superior orbital fissure
Head and Neck Answers 453