BINOCULAR VISION. 367 



oblique, and internal rectus ; movements downward and outward — 

 inferior rectus, superior oblique, and external 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 secondary 

 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 

 balanced or co-ordinated action of the opposing muscles is neces- 

 sary. The object of these movements is to bring the point looked 

 at in the fovea of each eye and thus prevent double vision, diplopia 

 (see following paragraphs). This object is attained when the eye- 

 balls are so m.oved that the lines of sight unite upon the object or 

 point looked at. In viewing an object or in reading we keep 

 readjusting the eyes continually to bring point after point at the 

 junction of the lines of sight. When we look before us at a 

 distant object the muscles in each eye should be so adjusted 

 that without any contraction the antagonistic muscles will 

 just balance each other — that is, when the eye muscles are 

 entirely relaxed, except for their normal tone, the visual axes 

 should be parallel. If this balance does not exist, we have a 

 condition designated as heterophoria. In this condition a 

 constant contraction of one or more muscles is required, even 

 in far vision, to prevent diplopia. When the eye at rest shows 

 a tendency to drift toward the temporal side, owing to the fact 

 that the pull of the external rectus overbalances that of the 

 internal rectus, the condition is known as exophoria. If, for the 

 opposite reason, there is a tendency to drift to the nasal side, the 



