Page 194 - Edited - Webster HEAD AND NECK - part 1

HN 160

EYE MOVEMENTS & EXTRAOCULAR MUSCLES: SIMPLE RULES

1. Lateral and medial RECTUS muscles are straightforward:

(a) Lateral rectus laterally rotates the eye i.e. abducts and is therefore supplied by the abducent
nerve (VI).
(b) Medial rectus medially rotates i.e. adducts.
2. Oblique muscles are untrustworthy, as their name implies. Thus they always:

(a) ABDUCT (i.e. steal) and
(b) in the vertical plane (elevation vs depression), oblique and shifty as their name implies, they
move the eyes in directions opposite to those suggested by their positions in the orbit.

Thus:
Superior oblique ABDUCTS and depresses the eye.
Inferior oblique ABDUCTS and elevates the eye.

3. The remaining two muscles are both rectus muscles. They are straight (“rectus” means “straight” -
hence “rectilinear”) up and down chaps. Thus they ALWAYS

(a) ADDUCT (bring back the stolen parties) and
(b) act in the vertical plane in just the way their names suggest.
Thus:
Superior rectus ADDUCTS and elevates the eye.
Inferior rectus ADDUCTS and depresses the eye.
Note also that:

(a) Elevation of gaze is effected by the right and left oculomotor nerves (CrNIII), acting together,
and the IIIrd cranial nerve (oculomotor) also conveniently innervates the major skeletal muscle
component of levator palpebrae superioris (raising the upper eyelids so as to clear the line of
gaze as you look upwards).
(b) Convergence of gaze is required during accommodation, together with changing the focal
length (curvature) of the lens for near-vision: both these functions are subserved by the
oculomotor nerves (HN 161. Fig. (B); 182, Fig. A; 183. Fig. D) acting together; changing the
curvature of the lens requires the parasympathetic component of CrNIII (HN 165). As
convergence occurs, the pupils also constrict (loc.cit.), which also requires the parasympathetic
component of the oculomotor nerves. Once the new gaze/focus is established, the size of the
pupils adjusts to that appropriate for the prevailing level of illumination.

(c) Only two extraocular muscles are not supplied by the oculomotor nerves:

(i) The trochlear nerve (CrNIV) supplies the muscle with a trochlea (Latin: a pulley), i.e.
superior oblique.
(ii) The abducent nerve (CrNVI) innervates the abductor muscle (lateral rectus).
(d) Overall, the oculomotor nerve subserves all functions which do not require abduction (lateral
rotation) of the eye. Paralysis of an oculomotor nerve therefore leaves the eye abducted, but
because the superior oblique muscle is still functional, the eye is also depressed (HN 176;
190). The pupil is dilated by the unopposed action of the radial muscle of the iris (HN 164,
Fig. C).

K.E.W

\NewCMedPhysSc\22 HN 160 EyeMove.