870 



THE SENSE OF SIGHT 



to the lacrimal groove. From here it passes outward, upward and 

 backward, and finally ends in a tendinous expansion which is inserted 

 in the sclera upon the outer part of the eyeball, near to but somewhat 

 behind the tendon of the superior oblique. 



The movements of the eyeball are similar to those of the head of a 

 long bone within its socket, an unrestricted motion being made im- 

 possible by several resistances, such as the insertion of the different 

 muscles, the capsular aponeurosis and the entrance of the optic nerve. 

 The recti muscles act antagonistic to one another, the range. of con- 



Ill 



II 



iv 



FIG. 470. DIAGRAM SHOWING THE LINES OF INSERTION OF THE OCULAR MUSCLES INTO THE 

 SCLEROTIC. (Merkel and Kallins.) 



I, Globe from above; II, from the nasal side; III, from below; IV, from the temporal 

 side, s, rectus superior; i, rectus inferior; m, rectus internus (s. mesialis) ; e, rectus 

 externus (s. lateralis); os, obliquus superior; oi, obliquus inferior. 



traction of one being restricted by the extension of the opposite one. 

 Their action, however, is unable to pull the eyeball backward owing 

 to the antagonistic action of the smooth musculature of Tenon's 

 capsule. In some animals, such as the reptilia and amphibia and 

 several mammals, a movement of this kind is effected by a special 

 muscle known as the retractor bulbi. 



The globe of the eye does not alter its position in rotating, but is 

 merely turned around its axes. Thus, if it stated tha.t the eye is moved 

 upward, reference is had merely to the relative position of its anterior 

 and posterior poles. While the cornea moves upward, the back of the 

 eyeball moves downward, and vice versa. Although the axes around 

 which the eye may be rotated are many, it is customary to recognize 



