786 



ACTION OF THE OCULAR MUSCLES. 



and the axis of rotation of the eyebalL The plane of traction is found by the 

 plane lying in the middle of the origin and insertion of the muscle and the point 

 of rotation of the eyeball. The axis of rotation is always at right angles to the 

 plane of traction in the point of rotation of the eyeball. 



1. The rectus interims (I) and externus (E) rotate the eye almost exactly inwards 

 and outwards (fig. 566). The plane of traction lies in the plane of the paper ; Q, 

 E, is the direction of the traction of the external rectus, '.Qj, I, that of the internal. 

 The axis of rotation is in the point of rotation, O, at right angles to the plane of 

 the paper, so that it coincides with the vertical axis of the eyeball. 2. The axis 

 of rotation of the R. superior and inferior (the dotted line, R. sup., R. inf.), 

 lies in the horizontal plane of separation of the eye, but it forms an angle of about 

 20 with the transverse axis (Q, Qj) ; the direction of the traction for both muscles 

 is indicated by the line, s, i. By the action of these muscles, the cornea is turned 

 upwards and slightly inwards, or downwards and slightly inwards. 3. The axis 

 of rotation of both oblique muscles (the dotted lines, Obi. sup. and Obi. inf.) also lies 

 in the horizontal plane of separation of the eyeball, and it forms an angle of 60 with 

 the transverse axis. The direction of the traction of the inferior oblique gives the 

 line, a, b ; that of the superior, the line, c, d. The action of these muscles, there- 

 fore, is in the one case to rotate the cornea outwards and upwards, and in the other 

 outwards and downwards. These actions, of course, only obtain when the eyes are in 

 the primary position in every other position the axis of rotation of each muscle 

 changes. 



When the eyes are at rest, the muscles are in equilibrium. Owing to the power 

 of the internal recti, the visual axes converge and would meet, if prolonged 40 

 centimetres in front of the eye. In the movements of the eyeball, one, two, or 

 three muscles may be concerned. One muscle acts only when the eye is moved 

 directly outwards or inwards, especially the internal and external rectus. Two 

 muscles act when the eyeball is moved directly upwards (superior rectus and inferior 

 oblique) or downwards (inferior rectus and superior oblique). Three muscles are 

 in action when the eyeballs take a diagonal direction, especially for inwards and 

 upwards, by the internal and the superior rectus and inferior oblique ; for inwards 

 and downwards, the internal and inferior rectus and superior oblique ; for outwards 

 and downwards, the external and inferior rectus and superior oblique ; for outwards 

 and upwards, the external and superior rectus and inferior oblique. 



[The following table shows the action of the muscles of the eyeball : 



Inwards, 

 Outwards, 



Upwards, 



Downwards, . 



Inwards and 

 upwards, 



Rectus intemus. 

 Rectus externus. 

 f Rectus superior. 

 \ Obliquus inferior. 

 / Rectus inferior. 

 \ Obliquus superior. 

 ( Rectus internus. 

 -J Rectus superior. 

 ^ Obliquus inferior. 



Inwards and 

 downwards, 



Outwards and 

 upwards, 



Outwards and 

 downwards, 



Rectus internus. 

 Rectus inferior. 

 Obliquus superior. 

 Rectus externus. 

 Rectus'superior. 

 Obliquus inferior. 

 Rectus externus. 

 Rectus inferior. 

 Obliquus superior.] 



Ruete imitated the movements of the eyeballs by means of a model, which he called the 

 ophthalmotrope. 



The size of the eyeball and its length diminish with age. The mobility is less in the vertical 

 than in the lateral direction, and less upwards than downwards. The normal and myopic eye 

 can be moved more outwards, and the long-sighted eye more inwards, the external and internal 

 recti act most when the eye is moved outwards, the obliqui when it is rotated inwards. An 

 eye can be turned inwards to a greater extent when the other eye at the same time is turned 

 outwards than when the other is turned inwards. During near vision, the right eye can be 

 turned less to the right, and the left to the left, than during distant vision (Hering). 



Simultaneous Ocular Movements. Both eyes are always moved simultaneously. 

 Even when one eye is quite blind, the ocular muscles move when the whole eyeball 

 is excited. When the head is straight, the movements always take place so that 



