BINOCULAR VISION. 



783 



FIG. 180. 



oblsup. 



liar way. The shoulder-joint is also a ball-and-socket joint ; and we know that 

 we can* not only move the arm up and down round a horizontal axis passing 

 through the centre of rotation of the head of the humerus, and from side to 

 side round a vertical axis, but we can 

 also rotate it round its own longitudi- 

 nal axis. When, however, we come to 

 examine closely the movements of the 

 eyeball we find that though we can 

 move it up and down round a horizon- 

 tal axis, as when with fixed head we 

 direct our vision to the heavens or to 

 the ground, and from side to side, as 

 when we look to left or right, and 

 though by combining these two move- 

 ments we can give the eyeball a variety 

 of inclinations, we cannot, by a volun- 

 tary effort, rotate the eyeball round 

 its longitudinal visual axis. The ar- 

 rangement of the muscles of the eye- 

 ball will permit of such a movement, 

 but we cannot by any direct effort of 

 will bring it about by itself. In cer- 

 tain movements of the eye rotation of 

 the eyeball does take place and by 

 bringing about these movements we 

 can indirectly cause rotation ; but we 

 cannot rotate the eyeball except thus 

 indirectly as a part of these movements. 



If, when vision is directed to any 

 object the head be moved from side to 

 side, the eyes do not move with it ; they 

 appear to remain stationary, very 

 much as the needle of a ship's compass remains stationary when the head 

 of the ship is turned. The change in the position of the visual axes to which 

 the movement of the head would naturally give rise is met by compensating 

 movements of the eyeballs ; were it not so, steadiness of vision would be 

 impossible. 



682. There is one position of the eyes which has been called the primary 

 position. It corresponds to that which may be attained by looking at the 

 distant horizon with the head vertical and the body upright ; but its exact 

 determination requires special precautions. The visual axes are then parallel 

 to each other and to the median plane of the head. All other positions of 

 the eyes are called secondary positions. 



683. Muscles of the eyeball. The eyeball is moved by six muscles, the 

 recti inferior, superior, internus, and externus, and the obliqui inferior and 

 superior. It is found by calculation from the attachments and directions of 

 the muscles, and confirmed by actual observation, that the six muscles may 

 be considered as three pairs, each pair rotating the eye round a particular 

 axis. The relative attachments and the axes of rotation are diagrammati- 

 cally shown in Fig. 180. The rectus superior and the rectus inferior rotate 

 the eye round a horizontal axis, which is directed from the upper end of the 

 nose to the temple ; the obliquus superior and obliquus inferior round a 

 horizontal axis directed from the centre of the eyeball to the occiput ; and 

 the rectus internus and rectus externus round a vertical axis (which, being 

 at right angles to the plane of the paper, cannot be shown in the diagram), 



r.ext. r.sup. r.inf. 

 r.inf. 



Diagram of the Attachments of the Mus- 

 cles of the Eye and of their Axes of Rota- 

 tion, the latter being represented by dotted 

 lines. 



The axis of rotation of the rectus externus 

 and internus, being perpendicular to the 

 plane of the paper, cannot be shown. (After 

 Fick.) 



