MOVEMENTS OF THE EYEBALL 



699 



the sclerotic a little below the insertion of the superior oblique. The 

 general arrangement of these muscles is shown in Fig. 179. 



The movements of the eyeball are easily understood from a study of 

 the associated movements of the muscles just enumerated, at least so 

 far as is necessary to the comprehension of the mechanism by which 

 the eyes are directed toward any particular object. The centre of 

 distinct vision is in the f ovea ; and it is evident that in order to see any 

 object distinctly, it is necessary to bring it within the axes of vision of 

 both eyes. As the globe is so balanced in the orbit as to be capable 

 of rotation, within certain limits, in every direction, it is necessary only to 



2 4 



Fig. 179. Muscles of the eyeball (Sappey). 



i, attachment of the tendon connected with the inferior rectus, internal rectus and external rectus; 

 2, external rectus, divided and turned downward, to expose the inferior rectus ; 3, internal rectus ; 

 4, inferior rectus ; 5, superior rectus ; 6, superior oblique ; 7, pulley and reflected portion of the superior 

 oblique ; 8, inferior oblique ; 9, levator palpebri superioris ; 10, 10, middle portion of the levator 

 palpebri superioris; n, optic nerve. 



note the exact mode of action of each of the muscles, in order to com- 

 prehend how the different movements are accomplished ; and it is 

 sufficient for practical purposes to admit that approximately there is a 

 common axis of rotation for each pair of muscles. 



Under ordinary conditions in the human subject, the action of the 

 six ocular muscles is confined to movements of rotation and torsion of 

 the globe. It is said that in the human subject there is no such thing 

 as protrusion of the eye from general relaxation of these muscles, and 

 that it is impossible, by a combined action of the four recti muscles, to 

 retract the globe in the orbit ; but those who have operated on the eyes 

 assert that this statement is erroneous, and that the globe is almost 



