808 SPECIAL SENSES. 



of action of each of the muscles, in order to comprehend how the different movements 

 are accomplished ; and it is sufficient for our purposes to admit that, approximative^, 

 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 the 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 upon the eyes assert posi- 

 tively that this statement is erroneous, and that the globe is almost always suddenly and 

 powerfully drawn within the orbit when a painful impression is made upon the cornea. 

 This is stated as a matter of common observation by ophthalmic surgeons. 



FIG. 254. Muscles of the eyeball. (Sappey.) 



1, 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 ; 8, internal rectus ; 4, inferior rectus ; 5, superior 

 rectus; 6 superior oblique; 7, pulley and reflected portion of the superior oblique; 8, inferior oblique : 9, leva- 

 tor palpebn superioris; 10, 10, middle portion of the levator palpebri superioris ; 11, optic nerve. 



The extent to which the line of vision may be turned by a voluntary effort varies in 

 different individuals, even when the eyes are perfectly normal. In myopic eyes, the centre 

 of rotation is deeper in the orbit than normal, and the extent of the possible deviation of 

 the visual line is correspondingly diminished. Helmholtz states that, in his own person, 

 with the greatest effort that he is capable of making, he can move the line of vision in 

 the horizontal plane to the extent of about fifty degrees, and, in the vertical plane, about 

 forty-five degrees ; but he adds that these extreme rotations are very forced, and that 

 they cannot be sustained for any length of time. It is probable that we seldom move the 

 eyeball in any direction to an angle of forty-five degrees, the direction of the visual line 

 >emg more easily accomplished by movements of the head. 



Action of the Recti Muscles. The action of the recti, particularly of the internal and 

 external, is quite simple. 



The internal and the external recti rotate the globe upon a vertical axis, which is per- 

 icular to the axis of the eye. The isolated action of these muscles, particularly of the 

 rnal rectus, is often illustrated in certain forms of paralysis, which have been alluded 

 o in connection with the history of the cranial nerves. 



The superior and the inferior recti rotate the globe upon a horizontal axis, which is 



