200 Neuroanatomy: Draw It to Know It
Extraocular Movements
Here, we will draw the six cardinal positions of gaze and
create a table for the complete extraocular muscle actions.
Cranial nerve 3 innervates the majority of the extraocu-
lar muscles: the medial rectus, superior rectus, inferior
rectus, and inferior oblique; cranial nerve 6 innervates
the lateral rectus; and cranial nerve 4 innervates the
superior oblique. We will draw the cardinal positions in
coronal section, so fi rst label the superior–inferior and
lateral–medial axes and then identify our perspective as
from the left eye. Show that the lateral rectus directs the
eye laterally (called abduction) and that the medial
rectus directs the eye medially (called adduction). Th en,
show that when the eye is abducted, the superior rectus
directs the eye superiorly and the inferior rectus directs
the eye inferiorly. Next, show that when the eye is
adducted, the superior oblique directs the eye inferiorly
and the inferior oblique directs the eye superiorly.
Now, in order to understand the complete move-
ments, beyond just the six cardinal directions of gaze,
let’s fi rst learn the relationship between the eye and the
eye muscle plane. Draw an axial view of the eye within
the orbit. Show the optical axis of the eye and then the
eye muscle plane. Th e muscle plane originates from the
orbital apex, which lies anterior to the orbital opening of
the optic canal; all of the extraocular muscles except for
the inferior oblique attach at the orbital apex in a
common tendinous ring : the annulus of Zinn. Th e infe-
rior oblique, instead, attaches to the medial orbital fl oor.
Now, show that the primary position of the eye, the posi-
tion of the eye when it is looking straight ahead, is
23 degrees nasal to the eye muscle plane. Th is angle
results in the vertical eye muscles (superior and inferior
rectus and superior and inferior oblique) all having three
diff erent actions on the eye in primary position: vertical,
rotational, and horizontal.
Next, let’s complete our eye muscle actions table.
Across the top row, write cranial nerve (CrN), muscle,
primary action, secondary action, and tertiary action.
Th e muscle’s chief action in primary position (ie, looking
straight ahead) is its primary action, and the muscle’s
secondary and tertiary actions are its additional rota-
tional eff ects on the eye. Begin with the medial rectus
muscle, innervated by cranial nerve 3, and show that its
primary action is adduction and that it does not have
secondary or tertiary actions. Th en, show that the cra-
nial nerve 6-innervated lateral rectus muscle’s primary
action is abduction, and show that it also does not have
either secondary or tertiary actions. In the next row,
write that the cranial nerve 3-innervated superior rectus
muscle’s primary action is elevation, its secondary action
is intorsion, and its tertiary action is adduction. Th en,
write that the cranial nerve 3-innervated inferior rectus
muscle’s primary action is depression, its secondary
action is extorsion, and its tertiary action is adduction.
Next, show that the cranial nerve 4-innervated superior
oblique muscle’s primary action is intorsion, its second-
ary action is depression, and its tertiary action is abduc-
tion. Finally, show that the cranial nerve 3-innervated
inferior oblique muscle’s primary action is extorsion, its
secondary action is elevation, and its tertiary action is
abduction.
Now, include the following two mnemonics:
“SUPERIOR people do NOT extort,” which means that
the superior muscles are both intorters, and “OBLIQUE
muscles rotate the eye OUT,” which means that the
oblique muscles are abductors.^8 , 9