THE SENSES 549 



superior oblique, the external rectus being supplied by the sixth 

 pair, or abducens, and the superior oblique by the fourth pair, 

 or pathetic ; so that three pairs of cranial nerves supply seven 

 ocular muscles. 



The chief movements of the eyeballs are backwards and for- 

 wards, corresponding to the directions described as outwards 

 and inwards in man. During these movements it is evident that 

 the external rectus of one eye is acting in conjunction with the 

 internal rectus of its fellow, and such is always the case in mon- 

 ocular vision. Animals with the eyes not too laterally placed 

 have, however, the power of monocular and also of binocular 

 vision, but the latter is only produced by an internal squint, 

 and the movements of the muscles are now no longer conjugate, 

 for both internal recti are acting together (Fig. 167). Sometimes, 

 then, the group of muscles employed in moving the eyeballs is 

 the same in each eye, at other times it is not. The torsion pro- 

 duced by the superior and inferior oblique muscles is of value 

 in the binocular vision of animals and in the vertical movements 

 of the head. When the muzzle is raised, as previously described, 

 the superior oblique muscle rotates the eyeball in its socket 

 until the pupil is horizontal. The explanation of the cornea 

 partly disappearing under the lower lid, and the sclerotic show- 

 ing extensively above, appears to be due to a conjugate action 

 of the inferior rectus muscle whenever the superior oblique is 

 so employed. The inferior oblique is mainly employed with the 

 internal rectus in pulling the eyes inwards for binocular vision ; 

 also, as mentioned above, for maintaining the horizontal pupil 

 when the head is depressed or raised. 



Monocular and Binocular Vision. — The question of whether an 

 animal is able to see a single object with both eyes depends entirely 

 on the position of the eyes in the head. Eyes laterally placed, as, 

 for example, in fishes, the hare or guinea-pig, are incapable of looking 

 forward, owing to the anatomical arrangement of the parts lying 

 between the two eyes. A laterally - placed eye possesses only 

 panoramic vision ; two eyes, each possessing only monocular vision, 

 command a very wide visual field. Eyes placed towards the centre 

 of the face in which the visual axes are practically parallel, as ka 

 the case of man, monkey, cat, etc., enjoy binocular vision, which, 

 though it loses in range, gains in quality, for in such eyes the per- 

 ception of relief exists; whereas eyes laterally placed, though 

 enjoying keen sight, can only furnish relief and depth of objects in 

 an imperfect manner. Between these two extremes of monocular 

 and binocular visions is a third class, in which the vision, though 

 mainly monocular, may also be binocular, and of these the horse 

 furnishes an example. When dealing with the arrangement of the 

 fibres in the optic nerve, it was pointed out that in some animals 

 the decussation was complete — viz., all the fibres, say, from the 

 right eye, passed to the left brain ; this is what exists for all eyes so 

 laterally placed in the head that only monocular vision is possible. 



