313.The answer is d.(Moore and Dalley, p 1137.)Cranial nerve IV has a
long intracranial course, so it has an increased chance of injury from. The
nerve originates from the trochlear nucleus in the midbrain and is the only
cranial nerve to exit the brain on the dorsal rather than ventral surface. It
exits the middle cranial fossa by passing out of the superior orbital fissure
(along with CN III, V 1 , and VI). The nerve also does notpass through the
common tendineous ring to reach the superior oblique muscle, the only
extraocular muscle to pass through a pulley, the trochlea. A patient with a
cut CN IV tends to tilt her/his head towards the unaffected side (in this
case to the right) [thus not (answer c)]. You examine individual extraoc-
ular muscle function by performing the “H” test (see High Yield Facts,
page 67). To test the function of the superior oblique muscle (innervated
by the trochlear nerve) you have the patient first look medial (adduction)
and then inferior (towards the nose). Cranial nerve III (answers a and b)
is involved with the bulk of the other extraocular eye movements and is
normal. Cranial nerve VI (answer e)innervates the lateral rectus muscle,
which is responsible for abducting each eye, and is normal in this patient.
314.The answer is d.(Moore and Dalley, p 986.)The medial pterygoid
muscle, which originates on the medial side of the lateral pterygoid plate,
and the masseter muscle, which originates from the zygomatic arch, pass
medially and laterally to the ramus of the mandible to form a sling about
the angle of the mandible. These muscles are powerful elevators of the jaw.
The muscle bundles of the anterior portion of the temporalis muscle run
nearly vertically into the coronoid process of the mandible, acting as a jaw
elevator. The lateral pterygoid muscles (answer c)run from the lateral side
of the pterygoid plate and from the infratemporal fossa to the head of the
mandible and the articular disk of the temporomandibular joint. Contrac-
tion of the lateral pterygoid muscles bilaterally protrudes the jaw. Unilateral
contraction swings the jaw toward the opposite side. The submental mus-
cles, assisted by gravity, are the primary depressors of the jaw. These include
the geniohyoid (answer b)and mylohyoid muscles as well as the anterior
belly of the digastric muscle, all of which function in conjunction with the
infrahyoid strap muscles. The posterior muscle bundles of the temporalis
(answer e)originate over the temporal region and pass nearly horizontally
into the coronoid process of the mandible and, therefore, function as jaw
retractors. The buccinator muscle (answer a)fibers are horizontal between
the maxilla and mandible so that they cannot act on the mandible. This is
Head and Neck Answers 451