BINOCULAR VISION. 349 



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 moved 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. If the eye is perfectly normal the 

 contractions of the muscles for objects in a symmetrical position are 

 equal in the two eyes; if, however, one or more of the muscles in the 

 eyes are weaker than normal, then to adjust the eyes properly 

 requires a greater contraction of these muscles to overcome the op- 

 posing action of their stronger antagonists. If the disproportion 

 in strength is not great, then by a stronger innervation, made under 

 the desire to prevent double vision, the visual axes may be properly, 

 adjusted; but the strain that results from this continual overcon- 

 traction may be injurious. A condition of lack of balance of this 

 kind in the muscles is spoken of as heterophoria, and, according to 

 the direction in which the visual axis tends to deviate the condition 

 is described specifically as esophoria, deviation inward; exophoria, 



