The inferior oblique is inserted back of the equator of the eye. Hence it pulls the back part of the eye down and consequently throws the front part up. It is thus an elevator of the eye, reinforcing the action of the superior rectus. Owing to the way in which it runs, from the front backward and outward, its elevating action is greatest when the eye is adducted, and diminishes to little or nothing when the eye is abducted. The superior oblique, so far as its action on the eyeball is concerned, may be regarded as arising from the trochlea. From this point it runs backward and outward and is inserted back of the equator of the eye. It there pulls up the back part of the eye and consequently throws the front part down. It is thus a depressor, reinforcing the action of the inferior rectus. Owing to the oblique way in which it runs, its depressing action is greatest when the eye is adducted, and diminishes to little or nothing when the eye is abducted. SUBSIDIARY ACTIONS Besides these actions, rightly regarded as the main action of the ocular muscles, there are various subsidiary actions, due to the oblique way in which the superior and inferior recti and the two obliques run. Thus, both the superior and inferior recti adduct the eye, their action being most pronounced when the eye is already adducted. The two obliques, on the other hand, abduct the eye and do so most effectively when the eye is already abducted. The superior rectus and superior oblique rotate the top of the vertical meridian of the eye inward (intorsion); while the inferior oblique and inferior rectus rotate it outward (extorsion). The superior and inferior recti act thus on the vertical meridian mainly when the eye is adducted; the oblique, on the other hand, when the eye is abducted. Hence the eye is adducted by the internal rectus, assisted toward the end of its course by the superior and inferior recti. It is abducted by the external rectus, assisted toward the end of its course by the two obliques. It is carried straight up by the superior rectus and inferior oblique, up and out by the superior rectus and external rectus (the inferior oblique helping to carry it out, but not up; and in, mainly by the inferior oblique and internal rectus). The superior rectus assists in carrying it in, but hardly up at all. The eye is likewise carried straight down by the inferior rectus and the superior oblique; down and out by the inferior and external recti, and down and in by the superior oblique and internal recti. FIELD OF ACTION OF MUSCLES