392 PHYSIOLOGY CHAP. 



plane is raised or lowered is determined by the angle formed with 

 the visual plane of the primary position ; the lateral deviation of 

 the visual axis is determined by the angle formed with the 

 median line of the plane of vision. The first is the angle of 

 elevation or depression ; the second, the angle of lateral deviation 

 (adduction or abduction), which brings the eye into the secondary 

 position. 



The eyeball can also execute more complicated movements, in 

 which the lines of sight converge or diverge, and are at the same 

 time directed upwards or downwards, by simultaneous rotation 

 round the vertical and the transverse axes. These oblique move- 

 ments bring the eye into the tertiary positions. The eyeball can 

 also rotate round the visual axis by a wheel movement. This 

 rotary movement is always associated with a tertiary position, 

 and never occurs independently owing to the co-ordinated in nerva- 

 tion of the eye-muscles. 



II. The most accurate measurements which it is possible to 

 take of the points of origin and attachment of the several muscles 

 of the eye, relatively to the co-ordinates described above, show 

 that the three pairs of muscles are not perfectly antagonistic, 

 since the axis of rot at inn l>y the superior rectus does not coincide 

 with that by the interior rectus, nor that by the internal rectus 

 with that by the external rectus, nor that by the superior oblique 

 with that by the inferior oblique. But the differences in the 

 angle made by the axes of rotation of each pair of muscles with 

 the co-ordinates are insignificant ; it is only the angle formed by 

 the two axes of rotation of the obliques with the sagittal axis 

 that amounts to the perceptible value of 6. These differences 

 may therefore, for simplicity, be neglected, and we may assume 

 with Volkmaun that each pair of muscles moves the eyeball by 

 antagonistic action to the others round a common axis. 



To determine the action of each pair of muscles it is necessary 

 to ascertain the plane of traction in which they work, and the 

 common axis of the rotation round which they move the eyeball 

 in antagonism. The former is easily discovered if we imagine a 

 plane passing through the points of origin and attachment of the 

 muscles, and the centre of rotation of the eye ; the latter is repre- 

 sented by the perpendiculars dropped from the centre of rotation 

 upon the plane of traction. 



The measurements carried out by Euete and A. Fick gave the 

 following results : 



(a] The internal and external recti rotate the eye inward 

 (adduction) and outward (abduction). As shown in Fig. 183 their 

 plane of traction coincides with the plane of the page ; QE shows 

 the traction of the external rectus, $'/that of the internal rectus; 

 the axis of rotation coincides with the vertical axis of the eye, 

 which is perpendicular to the centre of rotation 0. 



