EXTRAOCULAR MUSCLES

A set of six extraocular muscles (4 recti and 2 obliques)
control the movements of each eye (Fig. 13.1). Rectus
muscles are superior (SR), inferior (IR), medial (MR)
and lateral (LR). The oblique muscles include superior
(SO) and inferior (IO).
Origin and insertion
The rectus muscles originate from a common
tendinous ring (the annulus of Zinn), which is
attached at the apex of the orbit, encircling the optic
foramina and medial part of the superior orbital fissure
(Fig. 13.2). Medial rectus arises from the medial part
of the ring, superior rectus from the superior part and
also the adjoining dura covering the optic nerve,
inferior rectus from the inferior part and lateral rectus
from the lateral part by two heads which join in a ‘V’
form.
All the four recti run forward around the eyeball
and are inserted into the sclera, by flat tendons
(about 10-mm broad) at different distances from the
limbus as under (Fig. 13.3):
Medial rectus : 5.5 mm
Inferior rectus : 6.5 mm
Lateral rectus : 6. 9 mm
Superior rectus : 7.7 mm
The superior oblique muscle arises from the bone
above and medial to the optic foramina. It runs
forward and turns around a pulley — ‘the trochlea’
(present in the anterior part of the superomedial angle
of the orbit) and is inserted in the upper and outer
part of the sclera behind the equator (Fig. 13.3C).
The inferior oblique muscle arises by a rounded
tendon from the orbital plate of maxilla just lateral to
the orifice of the nasolacrimal duct. It passes laterally
and backward to be inserted into the lower and outer
part of the sclera behind the equator (Fig. 13.3C).
Nerve supply
The extraocular muscles are supplied by third, fourth
and sixth cranial nerves. The third cranial nerve
(oculomotor) supplies the superior, medial and inferior
recti and inferior oblique muscles. The fourth cranial
nerve (trochlear) supplies the superior oblique and
the sixth nerve (abducent) supplies the lateral rectus
muscle.
Actions
The extraocular muscles rotate the eyeball around
vertical, horizontal and antero-posterior axes. Medial
and lateral rectus muscles are almost parallel to the
optical axis of the eyeball; so they have got only the
main action. While superior and inferior rectus
muscles make an angle of 23o (Fig. 13.4) and reflected
tendons of the superior and inferior oblique muscles
of 51o (Fig. 13.5) with the optical axis in the primary
position; so they have subsidiary actions in addition
to the main action. Actions of each muscle (Fig. 13.6)
are shown in Table 13.1.
Table 13.1: Actions of extraocular muscles
Muscle Primary Secondary Tertiary
action action action
MR Adduction — —
LR Abduction — —
SR Elevation Intorsion Adduction
IR Depression Extorsion Adduction
SO Intorsion Depression Abduction
IO Extorsion Elevation Abduction