202 Neuroanatomy: Draw It to Know It
Extraocular Muscles: Recti Muscles
Here, we will draw the actions of the recti muscles: hori-
zontal (ie, medial and lateral) and vertical (ie, superior
and inferior). First, establish the anterior–posterior and
medial–lateral axes. Th en, draw two eyeballs in primary
position in axial section. On one, attach a medial rectus
muscle; on the other, attach a lateral rectus muscle.
Indicate that the medial rectus, which is innervated by
cranial nerve 3, rotates the eye medially and that the lat-
eral rectus, which is innervated by cranial nerve 6, rotates
it laterally. Next, draw axial sections with the eyes rotated
medially (adducted) and also laterally (abducted), and
show that in both of these positions, the medial rectus
still adducts the eye and the lateral rectus still abducts it.
Th e horizontal recti only move the eye within the hori-
zontal plane. As a clinical corollary, when the lateral
rectus fails, the eye has trouble turning outward and the
patient has diffi culty with far vision, whereas when the
medial rectus fails, the patient has diffi culty with eye
convergence and near vision.
Now, we will use our hands to feel the horizontal recti
rotational pull on the eye. Th e medial and lateral recti
muscles are the simplest. Make a fi st with your right hand
to represent your right eyeball. Your thumb is your right
medial rectus. Grip your thumb with your left hand and
pull on it (not so hard that you pull the eye out of its
orbit!). Notice that whether your fi st is directed straight
ahead, adducted, or abducted, the force from your left
hand always rotates your wrist medially along the horizon-
tal plane. To demonstrate the lateral rectus rotational pull,
fl ip your fi st over and pull your thumb backward. Again,
whether your fi st is directed straight ahead, adducted, or
abducted, the lateral rectus rotates it laterally.
Next, let’s learn the vertical recti muscles: superior
and inferior. For this drawing , add the superior–inferior
axis. At the extremes of horizontal gaze, the vertical recti
produce pure actions, but in primary position, the actions
are mixed. Start with the eyes in abduction: superior
rectus inserts on the top of the eyeball and inferior rectus
inserts on the bottom. Show that in this position, supe-
rior rectus rotates the eye up and inferior rectus rotates it
down. Next, show that in adduction, superior rectus
rotates the eye internally around the anterior–posterior
axis: it intorts it, meaning the medial aspect of the eye
lowers and the lateral aspect of the eye elevates. Th en,
show that in adduction, inferior rectus externally rotates
the eye: it extorts it, meaning the medial aspect of the
eye raises and the lateral aspect of the eye lowers. Finally,
in primary position, show that superior rectus produces
elevation, intorsion, and adduction, and that inferior
rectus produces depression, extorsion, and adduction.
Now, let’s use our hands again to feel the vertical recti
rotations. Use your right fi st as your right eye and hook
your left index fi nger (the right superior rectus) over the
top of it at an angle. First, with your hand fully abducted
pull on your wrist: the fi st rotates superiorly. Th en posi-
tion your fi st straight ahead and pull: you can feel your
fi st adduct (note, however, that superior rectus has mul-
tiple actions in this position). Finally, position your fi st
in full adduction and continue to pull. Th e fi st is unable
to rotate medially any farther, and instead it intorts.
Note that the eye muscle always overpowers the eye (ie,
your left fi nger should overpower your right fi st). Next,
let’s do the same for the inferior rectus. Flip your right
hand over (knuckles down) and hook your left index
fi nger underneath your fi st. Fully abduct your fi st and
pull; the downward/backward force of the inferior rectus
depresses the eye. Th en, direct your fi st straight ahead
and pull to feel your fi st adduct (note, however, that
inferior rectus has multiple actions in this position).
Finally, in full adduction, continue to pull down and
back: the fi st is unable to rotate medially any farther, and
instead it extorts.^6 , 8 , 9