THE SENSES 835 



Although the movements of the eye have been very fully 

 studied, and are, upon the whole, well understood, our 

 knowledge of the manner in which any given movement is 

 brought about, and the exact action of the muscles which 

 take part in it, is by no means as copious and precise. And 

 from the nature of the case, the greater part of what we do 

 know has been inferred from the anatomical relations of the 

 muscles as revealed by dissection in the dead body rather 

 than gained from actual observation of the living eye. A 

 plane, called the plane of traction, is supposed to pass through 

 the middle points of the origin and insertion of the muscle 

 whose action is to be investigated, and through the centre 



FIG. 321. HORIZONTAL SECTION OF LEFT EYE. 



Arrows show direction of pull of the muscles. The axis of rotation of the external 

 and internal recti would pass through the intersection of a and ft at right angles to the 

 plane of the paper. 



of rotation of the eyeball. A straight line drawn at right 

 angles to this plane through the centre of rotation is evidently 

 the axis round which the muscle when it contracts will cause 

 the eye to rotate, provided that the fibres of the muscle are 

 symmetrically distributed on each side of the plane of 

 traction. The axes of rotation of the antagonistic pairs 

 almost, but not completely, coincide with each other. The 

 common axis of the external and internal recti practically 

 coincides with the vertical axis of the eyeball (Fig. 321) in the 

 primary position. The eye is turned towards the temple 

 when the external rectus alone contracts, towards the nose 

 when the internal rectus alone contracts. The common 



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