VIII 



OCULAR MOVEMENTS 



393 



The superior and inferior recti rotate the eye upward and 

 somewhat inward, or downward and somewhat inward. Their 

 axis of rotation (dotted line JR. sup. R. inf.) lies in the horizontal 

 plane of the eye, but forms with the transverse axis (QQ') an 

 angle of about 20 ; the direction of traction of both muscles is 

 shown by the line si. 



(c) The inferior and superior obliqui rotate the eye outward 

 and upward, or outward and downward. Their axis of rotation 

 (dotted line Obi. inf. Obi. -s^?.) also lies in the horizontal plane 

 of the eye, but forms an angle of about 60 with the transverse 

 axis ; the direction of 

 traction of the in- 

 ferior oblique is 

 shown by the line ab, 

 that of the superior 

 oblique by the line cd. 



Ruete, in 1846, so 

 as to imitate the eye- 

 movenieuts effected 

 by the isolated and 

 associated move- 

 ments of the three 

 pairs of muscles as 

 perfectly as possible, 

 constructed a special 

 model of the two eye- 

 balls with the corre- 

 sponding muscular 

 attachments, or 

 " ophthalmotfope" 

 which was subse- 

 quently modified by 

 Wundt, Ludwig, Lan- 



11, j .1 

 QOlt and Otliers. 



With the ophthal- 



niotrope Hering obtained an almost perfect diagram of the form 

 and direction of the movements of the visual axis in the field of 

 vision, when the (left) eyeball is moved by its respective muscles 

 from the primary to the secondary positions (Fig. 184). 



When the eyes are at rest, as is always the case in sleep, the 

 muscles are in a position of equilibrium, determined by the differ- 

 ences of their strength and tone. As the internal recti are the 

 strongest, the visual axes of the two eyes converge slightly, and 

 cross at a distance of about 40 cm. in the median line. We must 

 therefore conclude that in the primary position of the eyes, when 

 the visual axes are parallel in distant vision, the two external recti 

 are in a state of moderate contraction. 



Fia. 183. Diagram to show axes of rotation of the eyeball, and 

 lines of traction of the external ocular muscles. (Landois.) 



