632 TEXT-BOOK OF PHYSIOLOGY. 



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 depres- 

 sion, and abduction or adduction. The superior and inferior recti 

 muscles, forming 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 tor- 

 sion. These facts show that for certain movements of the eye at 

 least three muscles are necessary (see following table) : 



Inward, ______ Rectus internus. 



Outward, _____ Rectus externus. 



*tus sueior ' 



Upward, ____ ; .' 



( Obliquus infenor. 



f Rectus inferior. 



Downward,..^ u: 



( Obliquus superior. 



Inward and ( Rectus internus. 



upward, -| Rectus superior. 



( Obliquus inferior. 



Inward and ( Rectus internus. 



downward, -J Rectus inferior. 



( Obliquus superior. 

 Outward and ( Rectus externus. 



upward, -^ Rectus superior. 



I Obliquus inferior. 

 Outward and ( Rectus externus. 



downward, Rectus inferior. 



( Obliquus superior. 



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. Both eyes always move simultaneously, which is called 

 the associated movement o] the eyes. There are three forms of asso- 

 ciated movements: (i) movement of both eyes in the same direction; 

 (2) movements of convergence by which the visual lines are con- 

 verged 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 (i) 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 

 prism is decomposed into a series of colors red, orange, yellow, 



