CHAP, ii.] SIGHT. 543 



protruded; but this trifling locomotion may be neglected. By 

 disease, however, the position of the eyeball in the socket may be 

 materially changed. 



Each eyeball is capable of rotating round an immobile centre of 

 rotation, which has been found to be placed a little (177 mm.) be- 

 hind the centre of the eye ; but the movements of the eye round 

 the centre are limited in a peculiar way. The sho alder-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 

 longitudinal 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 horizontal 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 movements we can give the eyeball a variety of in- 

 clinations, we cannot, by a voluntary effort, rotate the eyeball 

 round its longitudinal visual axis. The arrangement of the muscles 

 of the eyeball will permit of such a movement, but we cannot by 

 any direct effort of will bring it about by itself. In certain move- 

 ments 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. 



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 pre- 

 cautions. 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. 



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

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

 inferior and superior. It is found by calculation from the attach- 

 ments and directions of the muscles, and confirmed by actual obser- 

 vation, that the six muscles may be considered as three pairs, each 

 pair rotating the eye round a particular axis. The relative attach- 

 ments and the axes of rotation are diagrammatically shewn in 



