160 VETERINARY PHYSIOLOGY 



Tlie internal rectus rotates the pupil inwards. 



external „ ,, ,, outwards. 



r upwards and in- 

 superior „ „ „ | ^^^^^^^ 



inferior 



( downwards and in- 



( wards. 



, ,. f downwards and out- 



superior oblique ,, " \ wards. 



... f upwards and out- 



^^^^"^' " " " I wards. 



In directing the eyes to the right, the external rectus of 

 the right eye acts along with the internal rectus of the left. 

 In directing the eyes straight upwards, the superior rectus 

 and inferior oblique of each eye act together ; and in looking 

 downwards, the inferior rectus and superior oblique come into 

 play (fig. 79). Every movement of the eye involves the 

 co-ordinated excitation and inhibition of muscles (see p. 193). 



In the horse, dog, and other similar animals the eye is set 

 more nearly in the axis of the orbit, and the obliques do not 

 pass backwards upon the ball, but act more purely as rotators; 

 the superior oblique swinging the outer angle of the pupil 

 upwards and inwards, the inferior oblique downwards and 

 inwards. The superior and inferior recti move the pupil 

 more directly upwards and downwards. 



In the horse and other herbivora a retractor oculi muscle 

 is inserted all round the ball inside these muscles just 

 described, and it can retract the eye in the orbit, and at 

 the same time pushes forward the fatty tissue to which the 

 nictitating membrane is attached and thus thrusts this over 

 the front of the eye. 



4. "Glance" Movements of the Eyes. — When the eyes are 

 allowed to sweep over a landscape or any series of objects, 

 or when these move rapidly past the eyes, or the eyes rapidly 

 past them, as in travelling by train, the axes are directed in 

 a series of glances to different points, and the succession of 

 pictures thus formed gives the idea of a continuous series of 

 objects. The jerking movement of the eyes may be well seen in 

 a passenger looking out of a railway carriage in motion. This 

 " glance vision " is taken advantage of in the cinematograph. 



