850 THE EYE AtfD VISION [CH. LV1II. 



starvation; and (4) on section or paralysis of the cervical sympa- 

 thetic nerves. 



The most important movements, however, are those produced by 

 the six ocular muscles. 



The internal rectus turns the eyeball inwards, the external rectus 

 turns it outwards. If the superior rectus acted alone, it would turn 

 the eyeball not only upwards, but owing to the sloping direction of 

 the muscle, the eyeball would be turned inwards also ; in turning 

 the eyeball directly upwards, this inward movement is arrested by 

 the outward tendency of the inferior oblique. Similarly, in turning 

 the eyeball directly downwards, the inferior rectus acts in conjunc- 

 tion with the superior oblique. Movements in intermediate directions 

 are produced by other combinations of the muscles. 



These muscles are all supplied by the third nerve except the 

 superior oblique, which is supplied by the fourth, and the external 

 rectus by the sixth nerve. 



The muscles of the two eyes act simultaneously, so that images 

 of the objects looked at may fall on corresponding points of the 

 two retinae. The inner side of one retina corresponds to the 

 outer side of the other, so that any movement of one eye inwards 

 must be accompanied by a movement of the other eye outwards. 

 If one eyeball is forcibly fixed by pressing the finger against it so 

 that it cannot follow the movement of the other, the result is 

 double vision (diplopia), because the image of the objects looked at 

 will fall on points of the two retinae which do not correspond. The 

 same is experienced in a squint, until the subject learns to disregard 

 the image from one eye. 



If the external rectus is paralysed, the eye will squint inwards ; 

 if this occurs in the right eye the false image will lie on the left side 

 of the yellow spot, and appear in the field of vision to the right of 

 the true image. If the third nerve is paralysed, the case is a more 

 complicated one: owing to the paralysis of the levator palpebrse 

 superioris, the patient will be unable to raise his upper lid (ptosis), 

 and so in order to see will walk with his chin in the air. If the 

 paralysis is on the right side, the eyeball will squint downwards and 

 to the right ; the false image will be formed below and to the right 

 of the yellow spot, and the apparent image in the field of vision will 

 consequently appear above and to the left of the true image, and 

 owing to the squint being an oblique one, the false image will slant 

 in a corresponding direction. 



Various Positions of the Eyeballs. 



All the movements of the eyeball take place around the point of 

 rotation, which is situated 177 mm. behind the centre of the visual 

 axis, or 10'9 mm. behind the front of the cornea. 



