952 



A MANUAL OF PHYSIOLOGY 



The Extrinsic Muscles of the Eyes. The eyeball is acted 

 upon by six muscles arranged in three pairs, which may be 

 considered, roughly speaking, as antagonistic sets. These are 

 the internal and external recti, the superior and inferior recti, 

 and the superior and inferior obliqui. 



Although the movements of the eye have been very fully 

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

 ledge of the manner in which any given movement is brought 

 about, and of 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 in- 

 ferred 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 ac- 

 tion is to be investi- 

 gated, and through the 

 centre 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 con- 

 tracts 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 practi- 

 cally coincides with the vertical axis of the eyeball (Fig. 418) 

 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 axis of the 

 superior and inferior recti, @, lies in the horizontal visual plane 

 in the primary position, but makes an angle of about 20 with 

 the transverse axis, its inner end being tilted forwards. The 

 consequence is that contraction of the superior rectus turns the 

 eye up, and contraction of the inferior rectus turns it down, 



FIG. 418. 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 /3 at right angles to the plane of the paper. 



