402 



PHYSIOLOGY 



CHAP. 



the two eyes is that the visual axes shall constantly intersect at 

 the fixation point, i.e. they must always remain in the same plane, 

 and converge, in fixing near objects, or remain parallel, in fixing 

 distant objects. When this does not occur there is squint or 

 strabismus. 



Since binocular vision occurs not only when both eyes are in 

 symmetrical positions, that is, when we focus objects lying in 

 the same plane, but also when they are asymmetrical, we must 

 conclude with Hering that both the vertical and the lateral 

 movements of the two eyes are equal, it being an indispensable 



FIG. 188. Uinocular field of vision. (Sulzer.) F. Common fixation point for botli uniocular fields ; 

 /,, Mind sjiot of IP ft visual tield ; ft, blind spot of right visual field. The continuous line 

 limits the ritrht, the broken line the left visual field ; the area of the binocular visual field is 



shaded 



condition that the two visual axes shall hie in the same plane 

 and constantly converge on the common point in space. This 

 perfect co-ordination of the two eyes is brought about, as we 

 have seen, by special organisation of the cerebral centres; on 

 this depends the directive adaptation, i.e. the perfect compensation 

 and correlation of the motor innervation of the homonymous and 

 antagonist muscles. 



It is also necessary for binocular vision that the point of con- 

 vergence of the two visual axes shall fall within the binocular 

 field of vision. This results from incomplete superposition of the 

 two uniocular visual fields, for (as shown by Fig. 188) the outer 

 part of the visual field of the right eye, corresponding with the 



