548 



A MANUAL OF VETERINARY PHYSIOLOGY 



found that the eyeballs rotate like a wheel, so that the pupil is 

 still kept horizontal ; if it were not for this the pupil in the up- 

 lifted head would be vertical and in the depressed head oblique 

 (Fig. 165). When the head is elevated the eyeball becomes 

 depressed to such an extent that the sclerotic shows largely 

 above, while the cornea partly disappears beneath the lower 

 eyelid. When the head is depressed to the ground, no more 

 sclerotic shows than when it is in the ordinary position ; the prob- 

 able cause of this will be mentioned presently. The horizontal 

 pupil behaves like a spirit-level, and maintains the animal's 

 position to the outside world by keeping the eyeball always 

 horizontal. 



The Muscles of the Eyeball (Fig. 166) are seven in number — 

 viz., four recti, two oblique, and one retractor. The use of the 



/Sup,: Rectus, 



^Retractor. 



•tr Reccus. 

 lnf:Ohliaue . 



Fig. 166. — The Muscles of the Left Eyeball of the Horse viewed 

 from the Temporal Side. 



recti is clear enough : they rotate the eye in four directions — 

 outwards, inwards, upwards, and downwards. The two oblique 

 muscles rotate the eye in opposite directions around its antero- 

 posterior axis. When the superior oblique contracts, it pulls 

 the temporal side of the eyeball upwards, and if it were not 

 counteracted by the inferior oblique it would continue to con- 

 tract until the pupil became vertical like that of the cat ; the 

 inferior oblique pulls the temporal side of the eyeball down- 

 wards — in other words, these oblique muscles produce a torsion 

 of the globe or swivel rotation, and their action is regulated 

 from the semicircular canals of the internal ear (see p. 484). The 

 retractor partly withdraws the eye in its socket. 



The nerves supplying the muscles of the eyeball with motor 

 power are the third pair to all excepting the external rectus and 



