340 THE SPECIAL SENSES. 



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, 

 deviation outward; hyperphoria, deviation up or down. The 

 condition may be obviated by prismatic glasses so placed as to 

 aid the weaker muscle. When the lack of balance between the 

 opposing muscles is so great that the visual axes can not be brought 

 to bear upon the same points we have the condition of squint or 

 strabismus. Such a condition may result from a deficiency in 

 strength or in actual paralysis of one or more of the muscles, or from 

 an overaction in some of the muscles as contrasted with their 

 antagonists. 



The Binocular Field of Vision. When the two eyes are fixed 

 upon a given point, placed, let us say, in front of us in the median 

 plane, each eye has. its own visual field that may be charted 

 by means of the perimeter. But the two fields overlap for a 

 portion of their extent, and this overlapping area constitutes 

 the field of binocular vision (see Fig. 148). Every point in the bin- 

 ocular field forms an image upon the two retinas. The most 

 interesting fact about the binocular field is that some of the objects 

 contained in it are seen single in spite of the fact that there are two 

 retinal images, while others are seen or may be seen double when 

 one's attention is directed to the fact. Whether any given object 

 is seen single or double depends upon whether its image does or does 

 not fall upon corresponding points in the two retinas. 



Corresponding or Identical Points. By definition corre- 

 sponding or identical points in the two retinas are those which when 

 simultaneously stimulated by the same luminous object give us a 



