OCULAK MOVEMENTS 



389 



(&) More perfect inspection of space, since we can look at an 

 object from two fixed points, which makes stereoscopic or tri- 

 dimensional vision possible ; 



(c) Facilitation and greater accuracy in judging the distance 

 and real size of objects ; 



(d) Possibility of correcting congenital and acquired defects 

 n one eye by means of the other. 



Before commencing the study of visual perceptions and 

 judgments we must therefore examine the movements of the eye- 



rie. 182. Diagram of the motor apparatus of the eye. Natural size. (Motais.) The left figure 

 shows more particularly the attachments and course of the muscles ; the right, the relations 

 of Tenon's capsule. Explanation of letters in text. 



ball produced by its external muscles, since this is essential to 

 the right understanding of the phenomena of binocular vision. 



I. The eye-ball is contained in the orbital cavity framed by 

 bony walls, which is in the shape of a quadrangular pyramid 

 with the point tilted backwards. The space comprised between 

 the walls of the cavity and the eye-ball is occupied by loose 

 adipose tissue, by the external eye-muscles, and by the lachrymal 

 glands all of which are provided with vessels and nerves. 



The nasal walls of the orbit are almost parallel to each other 

 (Fig. 182), while the temporal walls diverge from behind forwards 

 at about 24-30 from the axes of the two cavities. The anterior 

 margins of the orbital cavity are curved inwards above and at 



2 C i 



