CH. LVL] POSITIONS OF THE EYEBALLS 805 



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 brain 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 palpebrae 

 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 



FIG. 595. Diagram of the axes of rotation to the eye. The thin lines indicate axes of rotation, the 

 thick the position of muscular attachment. 



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. 



