BINOCULAR VISION. 339 



eyes, that is, that position taken by the eyes when we look straight 

 before us toward the horizon, a position, therefore, in which the 

 plane of the horizontal axes is parallel to the ground; all other 

 positions of the eyes are spoken of as secondary. 



With regard to the movements of the eyes about its axes of 

 rotation the following general statements are made : Starting from 

 the primary position, rotations of the eyes about the vertical axis 

 that is, movements directly to right or left may be made by the 

 contraction of the internal or the external rectus as the case may be. 

 Rotations around the transverse axis that is, movements directly 

 up or down require in each case the co-operation of two muscles. 

 In movements upward the superior rectus, acting alone, would in 

 rotating the eyeball upward also give it a slight torsion so as to turn 

 the upper part of the vertical meridian inward. To obtain a move- 

 ment directly upward (rotation around the horizontal axis) the su- 

 perior rectus and inferior oblique must act together. For a similar 

 reason rotation directly downward requires the combined action of 

 the inferior rectus and superior oblique. Rotation of the eyeballs 

 around oblique axes require the co-operation of three of the muscles: 

 movements upward and outward the superior rectus, inferior 

 oblique, and external rectus; movements upward and inward 

 superior rectus, inferior oblique, and internal rectus; movements 

 downward and outward inferior rectus, superior oblique, and ex- 

 ternal rectus; movements downward and inward inferior rectus, 

 superior oblique, and internal rectus. Most of the movements of 

 the eyes are of the latter kind, namely, rotations around an oblique 

 axis, and the position of the axis for each definite movement of 

 this character may be determined by Listing's law, which may be 

 stated as follows: When the eye passes from a primary to a sec- 

 ondary position it may be considered as having rotated around an 

 axis perpendicular to the lines of sight in the two positions. It will 

 be noted readily from observations upon the movements of one's 

 own eyes that they ordinarily make only such movements as will 

 keep the lines of sight of the two eyes parallel or will converge them 

 upon a common point. In movements of convergence the internal 

 recti of the two eyes are associated, while in symmetrical lateral 

 movements the internal rectus of one eye acts with the external 

 rectus of the other. Under normal conditions it is impossible for 

 us to diverge the visual axes, that is, to associate the action of the 

 external recti. A movement of this kind would produce useless 

 double vision (diplopia), and it is therefore a kind of movement 

 which all of our experience has trained us to avoid. 



The Co-ordination of the Eye Muscles Muscular Insuf- 

 ficiency Strabismus. In order that the eyeballs may move 

 with the minute accuracy necessary in binocular vision, a beautifully 



