THE SENSES 125 



The superior rectus rotates the pupil upwards and inwards, 

 inferior (downwards and in- 



\ wards. 



superior oblique (downwards and out- 



l wards. 

 inferior upwards and outwards. 



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. 64). 



When a distant object is looked at, the axes of the two 

 eyes may be considered as parallel; but as an object is 

 approached to the eyes, the axes converge. It is not possible 

 by voluntary effort to diverge the optic axis or to rotate 

 the eyes round antero-posterior axes. 



When the eyes are Nose 



allowed to sweep over a In 01. 



landscape or any series of ^ 



objects, or when these 



move rapidly past the */?.< [ ( ^ J > InR. 



eyes, or the eyes rapidly 

 past them, as in travelling 

 by train, the axes are -s-di,. 



directed in a Series Of FIG. 64. The Movements of the Pupil 



glances to different points caused by the various Muscles of the 



r ' Eye. (Eight Eye.) 



and the succession of pic- 

 tures thus got gives the idea of the continuous series of 

 objects. This jerking movement of the eyes may be well 

 seen in a passenger looking out of a railway carriage in 

 motion. 



A somewhat complex nervous mechanism presides over 

 these various movements of the eyes. All the muscles are 

 supplied by the third cranial nerve, except the superior 

 oblique, which is supplied by the fourth nerve, and the 

 external rectus, which is supplied by the sixth nerve (Fig. 

 65 ; see also Fig. 85, p. 158). 



The centres for the third and fourth nerves are situated 

 in the floor of the aqueduct of Sylvius under the corpora 



