210 THE ANATOMY OF THE HORSE. 



optic nerve. In form it is funnel-shaped, forming a kind of sheath to 

 the optic nerve ; but frequently it is divided into four distinct fasciculi, 



Fig. 23. 

 Muscles of the Eyeball. 



1. Inferior oUique ; 2. Inferior rectus ; 3. External rectus ; 4. Internal rectus ; 5. Superior rectus ; 

 0, 6. Retractor ; 7. Nerve to inferior oblique. 



one lying beneath each of the recti. Its origin is from the margin of 

 the optic foramen, and it is inserted into the sclerotic. 



The Superior Oblique, or Trochlearis, has a i-emarkable disposition. 

 It arises at the back of the orbit, and passes inwards to a fibrous arch, 

 or pulley, through which it plays at the inner wall of the orbit, below 

 the root of the supraorbital process. Having passed through this arch, 

 the muscle is directed outwards above the eyeball ; and it is continued 

 by a tendon which passes beneath the superior rectus to be inserted into 

 the sclerotic on its outer side, between the insertions of the superior 

 and external recti. 



The Inferior Oblique. This muscle has its origin from the lachry- 

 mal fossa at the floor of the orbit. It passes outwards below the eye- 

 ball, and becomes inserted into the sclerotic between the insei-tions of the 

 inferior and external recti. 



Action of the Muscles of the Eyeball. — The superior and the inferior rec- 

 tus rotate the eye around a horizontal transverse axis, the former rolling 

 it upwards, the latter downwards. The external and the internal rec- 

 tus rotate the eye around a vertical axis, the first rolling it outwards, 

 the second inwards. The oblique muscles rotate the eye around an 



