CH. LV.] MUSCLES OF THE EYKBALL 



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 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 

 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 1-77 mm. behind the centre of the 

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



The three axes around which the movements occur are : 



1. The visual or autero-posterior axis. 



2. The transverse axis, which connects the points of rotation of 



the two eyes. 



3. The vertical axis, which passes at right angles to the other 

 two axes through their point of intersection. 



The line which connects the fixed point in the outer world at 

 which the eye is looking to the point of rotation is called the 

 visual line. The plane which passes through the two visual lines 

 is called the visual plane. 



The various positions of the eyeballs are designated primary, 

 secondary, and tertiary. 



The primary position occurs when both eyes are parallel, the 

 visual lines being horizontal (as in looking at the horizon). 



Secondary positions are of two kinds : 



(1) The visual lines are parallel but directed either upwards or 

 downwards from the horizontal (as in looking at the sky). 



(2) The visual lines are horizontal, but converge towards one 



