The Senses. 307 



backwards into the socket. The following table shows the 

 action of the ocular muscles : 



Eye turned upwards aud inwards ... Sup. rectus, int. rectus, 



and inf. oblique. 

 Eye turned downwards and inwards ... Inf. rectus, int. rectus, 



and inf. oblique. 

 Eye turned upwards aud outwards .. Sup. rectus, ext. rectus, . 



and sup. oblique. 

 Eye turned downwards and outwards ... Ext. rectus, inf. rectus, 



and sup. oblique. 



When the head is elevated the superior oblique renders 

 the pupil horizontal, when depressed the pupil is horizontally 

 placed by the inferior oblique. 



With the exception of the external, all the recti muscles, 

 the inferior oblique, and the retractor, are supplied with 

 nerves by the third pair ; the fourth pair supplies the 

 external rectus, and the sixth pair the superior oblique. 

 The orbicularis palpebrarum, which closes the eye, is sup- 

 plied by the seventh nerve, and the muscle which raises 

 the upper lid derives its nerve supply from the third pair. 



It will be observed that the muscles of the two eyes em- 

 ployed in any movement may be the corresponding muscles 

 in both eyes or not ; both eyeballs may, and commonly are, 

 directed to the front, producing what would be known 

 in man as a decided internal squint : in this case the same 

 group of muscles in each orbit are employed ; but if the 

 left eye moves backwards, the right eye goes forwards, and 

 vice versa. Here the groups of muscles employed are 

 different in each eye, for Avhile the external rectus is acting 

 on the left eye and drawing it back, the internal rectus is 

 drawing the right eye forwards.* 



Monocular and Binocular Vision. — When a horse looks 

 direct to the front (Fig. 34), he is capable of seeing an 

 object with both eyes ; he produces a marked internal 

 double squint, the eyes being rotated inwards and upwards 

 by the combined action of the superior and internal rectus, 



* The terms employed here of backwards and forwards, express the 

 same condition as outwards and inwards when applied to ourselves. 



20—2 



