Muscles of Orbit 79 



Similarly, the inferior oblique, passing outwards and backwards, also 

 turns the pupil outwards. 



If this imaginary pin be then thrust through the transverse axis of 

 the globe, the superior oblique, which is inserted behind the pin, coming 

 from above, turns the pupil downwards, whilst the other, coming from 

 below, turns it upwards. Thus the oblique muscles work in harmony 

 in turning the cornea outwards, but in antagonism in the upward and 

 downward movements. 



To turn the cornea downwards it would not do for the inferior 

 rectus to act alone, or internal strabismus would result ; so the 

 superior oblique is called on to help the inferior rectus, abducting the 

 eye-ball at the same time, so as to neutralise the adduction of the 

 inferior rectus. Thus, the two muscles, acting together, merely turn 

 the globe downwards. 



Conversely, the inferior oblique acts with the superior rectus, the 

 two muscles merely moving the eye to look upwards. 



Thus even a simple movement of the front of the globe is not left 

 under the control of a single muscle. Inversion is effected by the in- 

 ternal rectus, with the superior and inferior recti ; emersion by the 

 external rectus and the two obliques ; elevation by the superior rectus 

 and the inferior oblique ; and depression by the inferior rectus, with the 

 help of the superior oblique. 



When the external rectus is paralysed, abduction of the eye-ball is 

 limited. If the external rectus, say of the right eye, be paralysed, 

 there may be no double vision as the man looks towards the left, but 

 when he tries to look to the right, that is, to use his right external 

 rectus, the right eye-ball remains almost stationary, though the 

 left internal rectus directs the left eye-ball upon the object ; the result 

 is double vision. To save himself from this annoyance, the patient 

 keeps his head constantly in such a position that the useless muscle 

 may have no demand made upon it ; that is, he keeps his head con- 

 stantly turned towards the right, making, as it were, the left sterno- 

 mastoid do the work of the right external rectus. 



When the paralysis of the external rectus is only partial, the man 

 expends an unusual amount of energy in inducing it to act ; but a 

 certain amount of this energy necessarily passes into the associate 

 muscle, the opposite internal rectus, which then overacts its part and 

 produces ' secondary deviation ' of that eye inwards. 



When the right internal rectus is paralysed the face is apologeti- 

 cally turned to the left, so that the visual defect may not occur. So 

 also, due allowances being made, does it happen when a superior or 

 inferior rectus fails to act. 



When the inferior oblique is paralysed the cornea cannot readily 

 be turned upwards and outwards ; indeed, it falls somewhat downwards 

 and inwards, and thus there is double vision. To correct the double 

 vision, the subject makes up for the defect of the oblique muscle by 



