THE SENSES 



547 



rapidly turned in any direction. The movements are somewhat 

 complex, for in some of the lower animals — for example, the horse 

 — the eyes are laterally placed in the head, so that vision is 

 commonly single-eyed, and not binocular, as in man. The eye 

 that is viewing an object situated to one side and moving to and 

 fro is being followed in this muscular movement by the eye 

 which does not see ; but this does not apply to all animals, as shown 

 by Harris. The movements in the horse are conjugate, but this 

 only occurs so long as monocular vision is practised. If both 



C 



V- ~<fr$ 



In 



In 



In 



B B 



Fig. 165. — Diagram to illustrate the Rotation of an Eyeball. 

 A, B, Middle line of the face ; C, the eye. 

 is supposed to be in the ordinary upright position and the pupil 



1 the head 

 horizontal 



2 the head 

 unaltered, 

 zontal, as 



3 the head 

 change, it 

 maintains 



is depressed as in grazing ; if the direction of the pupil remained 

 it would be as represented — viz., oblique. It is, however, hori- 

 indicated by the dotted line y y. 



is elevated. If the direction of the pupil in 1 had undergone no 

 would now be nearly vertical, as shown, but the torsion of the globe 

 its horizontal direction y y. 



eyes be directed to an object situated to the front binocular 

 vision becomes possible, and now the movements are no longer 

 conjugate, but opposite, for while the left eye is inclined to the 

 right, the right eye is inclined to the left. Another complication 

 in the ocular muscles is due to the movement of the head ; it was 

 first pointed out by Lang and Barrett that in the rabbit and 

 guinea-pig, no matter what position the head occupied, the pupil 

 was always kept vertical. If the head of the horse or ox be raised 

 or depressed to the fullest possible extent, the muzzle being at 

 one time on the ground, at the next high in the air, it will be 



