CHAP, in.] SIGHT. 143 



therefore really proves that the cross of the negative image 

 undergoes no twisting while the eye is being directed from the 

 centre of the board to the corner, though at first sight it seems, 

 and once was thought, to prove the contrary. 



In the ordinary movements of the eye then, a swivel rotation 

 round the visual axis does not take place ; and this limitation, 

 since it holds good for the two eyes used together, as well-as Jor 

 one eye used by itself, serves to secure single vision with two eyes 

 inasmuch as it avoids changes which might cause the images of 

 external objects to fall on the parts of the two retinas which were 

 not " corresponding parts." In certain movements of the eyes, 

 however, a certain amount of swivel rotation does take place. 

 This is especially seen in somewhat unusual movements. For 

 instance when the head is turned down to the shoulder, or again 

 when in directing vision to any object, the head is moved from 

 side to side, the eyes do not move with the head ; 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 ; and these compensating movements are 

 found, on careful examination, to include a certain amount of 

 swivel rotation round the visual axes. In certain other more 

 usual movements some amount of such a swivel rotation is also 

 present; and indeed, though so long as the visual axes remain 

 parallel, movement in any direction may take place without any 

 such rotation, a slight amount does intervene during convergence 

 of the visual axes, as when we turn our eyes from a distant to a 

 near object. On careful examination, however, it appears that 

 such an amount of swivel rotation as does take place is after all 

 for the purpose of securing the end that corresponding parts of 

 the two retinas should be affected by the same external object; 

 and, though we cannot here enter more fully into the subject, 

 we may say that not only the more general movements of the 

 eye which obey Listing's law, but also those which form an ex- 

 ception to it, appear to be carried out in the interests of binocu- 

 lar vision. We may now turn to the study of the ocular muscles, 

 by the carefully coordinated contractions of which the various 

 movements, on which we have dwelt, are brought about. 



790. The muscles of the eyeball or ocular muscles. The eye- 

 ball is moved by six muscles, four of which are straight, musculi 

 recti, inferior, superior, internus or medialis, and externus or later- 

 alis, and two oblique, musculi obliqui, inferior and superior. 

 The four straight muscles, taking origin from the back of the 

 orbit around the sphenoidal fissure and the entrance of the optic 

 nerve, are directed, as their name indicates, straight forward, 

 (the superior rectus however having a peculiar bend) and are 



