. Animal physiology. Physiology, Comparative; Physiology, Comparative. MUSCLES AND MOVEMENTS OF THE EYE. 405 fourth, the external rectus, (the cut extremity of which is seen at /), turns it outward. Be- h e sides these, there is a remark- able muscle, the superior oblique (A), which originates at the back of the orbit, comes for- wards to the front, where its tendon passes through a pulley, and then turns backwards to be inserted into the sclerotic coat, at a point considerably d behind the pulley. The action Fig. 198.—Vertical Section of the Ormt, o ,, . , .11 i , ii showing the globe of the
. Animal physiology. Physiology, Comparative; Physiology, Comparative. MUSCLES AND MOVEMENTS OF THE EYE. 405 fourth, the external rectus, (the cut extremity of which is seen at /), turns it outward. Be- h e sides these, there is a remark- able muscle, the superior oblique (A), which originates at the back of the orbit, comes for- wards to the front, where its tendon passes through a pulley, and then turns backwards to be inserted into the sclerotic coat, at a point considerably d behind the pulley. The action Fig. 198.—Vertical Section of the Ormt, o ,, . , .11 i , ii showing the globe of the eye and its append- of this muscle will be to roll ages. a> cornea. b>} opticnerve; the globe downwards and Some- d> inferior rectus muscle ; e, superior rectus ; & ft cut extremity of the external rectus ; g, what Outwards. The Sixth milS- end of the inferior oblique; h, superior ob- cle, termed the inferior ollique X^ST' * "" "PperM; *''*" (^), does not arise like the rest from the back of the orbit, but from its lower border ; and its action is to roll the eye upwards and inwards. Of these muscles, the superior, inferior, and internal recti, together with the inferior oblique, and also the elevator muscle (i) of the upper eyelid, are supplied with motor nerves by the third pair (§. 458) ; whilst the superior oblique has a nerve to itself, the fourth ; and the external rectus has another nerve to itself, the sixth. 539. The relative actions of these muscles and nerves are not yet fully understood. There is this very peculiar circum- stance attending the movements of the two eyes,—that although they are harmonious, they are seldom symmetrical. Thus, when we direct our eyes towards an object on one side of us, they move harmoniously, that is, with a common purpose ; but their movement is not symmetrical, for one globe is rolled inwards by the internal rectus, whilst the other is rolled out- wards by the external rectus. These two
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