784 



SIGHT. 



passing through the centre of rotation of the eyeball parallel to the median 

 plane of the head when the head is vertical. Thus the latter pair acting 

 alone would turn the eye from side to side, the other straight pair acting 

 alone would move the eye up and down, while the oblique muscles acting 

 alone would give the eye an oblique movement. The rectus externus acting 

 alone would turn the eye to the malar side, the internus to the nasal side, 

 the rectus superior upward, the rectus inferior downward, the oblique superior 

 downward and outward, and the inferior upward and outward. The recti 

 superior and inferior in moving the eye up and down also turn it somewhat 

 inward and at the same time give it a slight amount of rotation ; but this 

 is corrected if the oblique muscles act at the same time ; and it is found 

 that the rectus superior acting with the obliquus inferior moves the eye up- 

 ward, and the rectus inferior with the obliquus superior downward in a 

 vertical direction. In oblique movements also, the obliqui are always asso- 

 ciated with the recti. Hence the various movements of the eyeball may be 

 arranged as follows: 



f I ,[ 



OQ o 



s 



Elevation. 

 Depression. 

 Adduction to 



nasal side. 

 Adduction to 



malar side. 

 Elevation with 



adduction. 



Depression 

 with adduction. 

 Elevation with 



abduction. 



Depression 

 with abduction. 



Rectus superior and obliquus inferior. 

 Rectus inferior and obliquus superior. 



Rectus internus. 



Rectus externus. 



Rectus superior and internus with obliquus 



inferior. 

 Rectus inferior and internus with obliquus 



superior. 

 Rectus superior and externus with obliquus 



inferior. 

 Rectus inferior and externus with obliquus 



superior. 



684. Coordination of visual movements. Thus even in the movements 

 of a single eye a considerable amount of coordination takes place. When 

 the eye is moved in any other than the vertical and horizontal meridians, 

 impulses must descend to at least three muscles, and in such relative energy 

 to each of the three as to produce the required inclination of the visual 

 axis. But the coordination observed in binocular vision is more striking 

 still. If the movements of any person's eyes be watched it will be seen that 

 the two eyes move alike. If the right eye moves to the right, so does also 

 the left ; and, if the object looked at be a distant one, exactly to the same 

 extent ; if the right eye looks up, the left eye looks up also, and so in every 

 other direction. Very few persons are able by a direct effort of the will to 

 move one eye independently of the other ; though some, and among them 

 one distinguished both as a physiologist and an oculist, have acquired this 

 power. In fact, the movements of the two eyes are so arranged that in the 

 various movements the images of any object should fall on the corresponding 

 points of the two retinas, and that thus single vision should result. We 

 cannot by any direct effort of our will place our eyes in such a position that 

 the rays of light proceeding from any object shall be brought to a focus on 

 parts of the two retinas which do not correspond, and thus give rise to two 

 distinct visual images. We can bring the visual axes of the two eyes from 

 a condition of parallelism to one of great convergence, but we cannot, with- 

 out special assistance, bring them from a condition of parallelism to one of 

 divergence. The stereoscope will enable us to create a divergence. If in a 

 stereoscopic picture the distance between the pictures be increased very 

 gradually so as carefully to maintain the impression of a single object, the 



