THE SENSE OF SIGHT 



709 



axis, around which the movements of elevation (up to 34 degrees) and de- 

 pression (down to 57 degrees) take place. At right angles to both of these 

 lines there is the vertical axis, around which the movements of adduction 

 (toward the nose up to 45 degrees) and abduction (toward the temple up 

 to 42 degrees) occur. The six muscles may be divided into three pairs, each 

 of which has a common axis around which it tends to move the eyeball. 

 But only the common axis of the internal and external recti coincides with 

 one of three axes before mentioned namely, with the vertical axis thus 

 moving the ball only inwardly or outwardly respectively. The other two 

 pairs, however, have their own axes of action, and their movements of the 

 ball must be, therefore, analyzed with regard to all the three axes, each 

 of these four muscles producing rotation, elevation, and depression, and 

 abduction or adduction. The superior and inferior recti muscles, form- 

 ing one pair, move the eye around a horizontal axis which intersects the 

 median plane of the body in front of the eyes at an angle of 63 degrees, 

 and the superior and inferior oblique muscles forming the third pair rotate 

 the globe around a horizontal axis which cuts the median plane of the body 

 behind the eyes at an angle of 39 degrees. Thus it is that each muscle 

 moves the eye as follows, the movement for practical purposes being referred 

 to the cornea : The rectus externus draws the cornea simply to the temporal 

 side, the rectus internus simply to the nose; the superior rectus displaces the 

 cornea upward, slightly inward, and turns the upper part toward the nose 

 (medial torsion); the inferior rectus moves the cornea downward, slightly 

 inward, and twists the upper part away from the nose (lateral torsion); 

 the superior oblique displaces the cornea downward, slightly outward, and 

 produces medial torsion; while the inferior oblique moves the cornea upward, 

 slightly outward, and produces lateral torsion. These facts show that for 

 certain movements of the eye at least three muscles are necessary (see 

 following table): 



If both eyes have their line of vision in the horizontal plane parallel 

 with each other and with the median plane of the body, they are said to be 

 in the primary position. All other positions are called secondary and tertiary. 

 Both eyes always move simultaneously, which is called the associated move- 

 ment oj the eyes. There are three forms of associated movements: (i) move- 

 ment of both eyes in the same direction; (2) movements of convergence by 

 which the visual lines are converged on a point in the middle line of the body; 

 (3) movements of divergence, by which the eyes are brought back from 

 convergence to parallelism, or even to divergence, as in certain stereoscopic 

 exercises. A combination of (-1) and (2) or of (i) and (3) takes place for 

 certain positions of the object looked at. 



Color-perception. A beam of sunlight passed through a glass pns 

 is decomposed into a series of colors red, orange, yellow, green, blue, and 



