948 THE OCULAR MUSCLES. [BOOK in. 



take place without any such rotation, a slight amount does 

 intervene during convergence of the visual axes, as when we 

 turn our eyes from a distant to a near object. On careful 

 examination, however, it appears that such an amount of swivel 

 rotation as does take place is after all for the purpose of secur- 

 ing the end that corresponding parts of the two retinas should 

 be affected by the same external object ; and, though we cannot 

 here enter more fully into the subject, we may say that not only 

 the more general movements of the eye which obey Listing's 

 law, but also those which form an exception to it, appear to be 

 carried out in the interests of binocular vision. We may now 

 turn to the study of the ocular muscles, by the carefully coordi- 

 nated contractions of which the various movements, on which 

 we have dwelt, are brought about. 



592. The muscles of the eyeball or ocular muscles. The 

 eyeball is moved by six muscles, four of which are straight, 

 musculi recti, inferior, superior, internus or medialis and externus 

 or later alis, and two oblique, musculi obliqui, inferior and supe- 

 rior. The four straight muscles, taking origin from the back 

 of the orbit around the sphenoidal fissure and the entrance of 

 the optic nerve, are directed, as their name indicates, straight 

 forward, (the superior rectus, however, having a peculiar bend,) 

 and are inserted in positions corresponding to their several 

 names into the sclerotic, behind the cornea, the bundles of 

 fibres of the tendons being interwoven with those of the scle- 

 rotic. The tendon of the internal rectus oji the median or 

 nasal side of the eyeball is the broadest of the four ; that of the 

 superior rectus on the upper surface being somewhat narrower, 

 and those of the inferior rectus on the under surface and of the 

 external rectus on the lateral or temporal side, still narrower 

 (Fig. 159). The insertion of the superior rectus lies nearer to 

 that of the external rectus than to that of the internal rectus ; 

 its position therefore is not exactly median, indeed for two- 

 thirds of its width it lies in the upper lateral quadrant of the 

 sclerotic ring. The insertions of the external and of the 

 internal rectus are both median. The insertion of the in- 

 ternal rectus is the one closest to, and that of the superior 

 rectus the one farthest away from the cornea, and the latter 

 slants so as to be nearer the cornea at its median than at its 

 lateral end. 



The superior oblique muscle, or trochlear or pathetic muscle, 

 taking origin from the back of the orbit near the origin of the 

 straight muscles and running forward internal to the superior 

 rectus, ends in a tendon, which changing its direction by means 

 of a pulley (trochlea), and passing beneath the superior rectus 

 is inserted into the sclerotic in the upper region of the bulb 

 towards its hind part. The line of insertion of the tendon 

 (Fig. 159) runs obliquely from the temporal towards the nasal 



