114 MOTIONS OF THE EYEBALLS. 



The muscles of the eyeball are divisible in respect of 

 their action, into two groups, of which one comprises 

 the rectus intermix and r. externus ; the other, the 

 obliquus inferior, and o. superior, which act in concert with 

 the rectus superior and r. inferior. The axis round which 

 any muscle rotates the eyeball is called its proper axis of 

 rotation. Those of the internal and external recti nearly 

 coincide with the vertical axis of the eyeball. Consequently 

 these muscles, acting antagonistically, rotate the eyes 

 directly to right or left (secondary positions). The com- 

 bined axis of the oblique muscles (the axes of the two 

 being approximately identical) is horizontal, but cuts the 

 equator of the eye at an angle of 60. The combined axis 

 of the superior and inferior recti is neither horizontal nor 

 transverse, but has its inner end lower, as well as further 

 forwards than the outer. If, however, the rectus sup. 

 acts with the obliquus inf., they together rotate the eye 

 round an axis which lies between their own axes, and 

 nearly coincides with the horizontal axis of the eyeball : 

 the obliquus superior acts similarly in conjunction with the 

 rectus inferior. Thus rotations of the eyeball from the 

 rest position into secondary positions are performed, if to 

 right or left, by the internal and external recti, if upwards 

 and downwards, by the combined action of the other four 

 muscles. 



When the eyes are so fixed on any point that it lies in 

 the visual axis of both of them, the point so contemplated 

 is seen singly and perfectly ; and all other points are seen 

 singly, but not perfectly, which are received by corre 

 spending points of the two retinae that is by points which 

 would exactly cover each other if it were possible for both 

 eyes to occupy the same position, the vertical and horizon- 

 tal meridians respectively coinciding. 



Mental Interpretations or Judgments of Visual Sensa- 

 tions. I. Binocular blending of colours. When rays of two 

 colours enter the two retinse simultaneously, the sensation 



