VISION. 593 
We may speak of that position of the eye when, with the 
head vertical in the standing position, the distant horizon is 
viewed as the primary 
position and all others as 
secondary positions. 
Fig. 434 is meant to 
illustrate diagrammatical- 
ly the movements of the 
eyeball. 
While the several recti 
muscles elevate or depress 
the eye, and turn it inward 
or outward, and the oblique 
muscles rotate it, the move- 
ments produced by the su- 
perior and inferior recti 
are always corrected by the 
assistance of the oblique 
muscles, since the former 
tend of themselves to turn 
the eye somewhat inward. 
In like manner the oblique 
muscles are corrected by 
the recti. The following tab- 
ular statement will express 
sno pousaqul 
the conditions of muscular 
contraction for the various 
movements of the eye: 
( Elevation.............. 
Fic. 434.—Diagram intended to illustrate action of 
extrinsic ocular muscles (after Fick). The heavy 
lines represent the muscles of the eyeball, and 
the fine lines the axes of movement. 
. .Rectus superior and obliquus inferior. 
Straight Depression.... .......... Rectus inferior and obliquus superior. 
move- ‘ Adduction to nasal side. ..Rectus internus. 
ments. | Adduction to malar side.. .Rectus externus. 
Elevation with adduction..Rectus superior and internus, with obli- 
quus inferior. ; 
Depression with adduction.Rectus inferior and internus with obliquus 
Oblique superior. 
move- \ Blevation with abduction..Rectus superior and externus with obliquus 
sila : inferior. 
Depression with abduction. Rectus inferior and externus, with obliquus 
ce superior. 
What is the nervous mechanism by which these “ associ- 
ated ” movements of the eyes are accomplished ? It has been 
found, experimentally, that when different parts of the corpora 
quadrigemina are stimulated, certain movements of the eyes 
38 
