MOVEMENTS OF THE EYEBALL. 133 



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 ex- 

 tent of the possible deviation of the visual line is correspond- 

 ingly 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. 1 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 being more easily accomplished by move- 

 ments of the head. 



Action of the Recti Muscles. The action of the recti, 

 particularly the internal and external, is quite simple. 



The internal and the external recti rotate the globe upon 

 a vertical axis, which is perpendicular to the axis of the eye. 

 The isolated action of these muscles, particularly the external 

 rectus, is often illustrated in certain forms of paralysis, which 

 have been alluded to in connection with the history of the 

 cranial nerves. 2 



The superior and the inferior recti rotate the globe upon 

 an horizontal axis, which is not at right angles with the axis 

 of the eye, but is inclined from the nasal side slightly back- 

 ward. The line which serves as the axis of rotation for these 

 muscles forms an angle of about seventy degrees with the 

 axis of the globe ; * and, as a consequence of this arrange- 

 ment, their action is not so simple as that of the internal and 

 external recti. The insertion of the superior rectus is such, 



1 HELMHOLTZ, Optique physiologique, Paris, 1867, p. 698. 



2 For an account of the effects of paralysis of one or more of the nerves of 

 the eyeball, the reader is referred to vol. iv., Nervous System, p. 128, et seq. 



8 HELMHOLTZ, op. dt., p. 40. 



