BINOCULAR VISION 873 



extra effort, they are usually supplemented by movements of the head 

 as a whole. This furnishes a much simpler means of bringing the 

 obj ect into direct opposition with the yellow spots. Furthermore, it will 

 be noted that the convergence of the eyes necessitates a symmetrical 

 innervation of the internal recti muscles, while a symmetrical in- 

 nervation of the external recti is quite superfluous, because we do 

 not diverge the visual axes during normal vision. In fact, a move- 

 ment of this kind would give rise to the condition of diplopia or double 

 vision, for the obvious reason that the rays of light would then be 

 made to fall upon areas of the retina which are not psychically cor- 

 related. A condition of diplopia, however, may be established without 

 much difficulty by exerting a gentle pressure upon one eyeball, so that 

 it is momentarily forced out of its normal position. Since the retinae 

 of the two eyes are then activated in dissimilar areas, a double im- 

 pression in consciousness is the natural consequence. 



Diplopia is a common symptom of certain disorders of the nervous 

 system, leading to disturbances in the coordinated action of the 

 different orbital muscles. It is true, however, that slight divergencies 

 are generally compensated for volitionally by simply causing the 

 weaker muscle to contract more forcibly than it would otherwise, but 

 naturally, a point will eventually be reached when these extra efforts 

 cease to produce the desired effect. A condition characterized by a 

 partial loss of balance of the eye muscles, is designated as heterophoria, 

 and one characterized by a more complete loss, as strabismus or squint. 

 In the latter case, the person is quite unable to direct the visual axes 

 jointly upon the object, but double vision need not result even then, 

 unless the strabismus is very pronounced or has arisen very suddenly in 

 consequence of some injury. Most generally, the patient learns by 

 experience to base his visual associations upon the impressions derived 

 from the more normal eye, and ignores or suppresses the image from 

 the non-corresponding area of the opposite retina. Heterophoria, 

 as well as strabismus, may be mitigated or remedied altogether by the 

 use of prisms. 



This discussion shows that single vision with the two eyes is due to 

 a fusion of the visual impressions in consciousness, and is largely the 

 result of experience. Thus, we speak of "corresponding points" 

 upon the retina, although it must be evident that a certain cone in one 

 retina cannot act in unison with a cone occupying the same position 

 in the opposite retina. The aforesaid term, therefore, is not indicative 

 of a histological identity, but of an identity in function. Consequently, 

 while certain areas in the two retinas may be correlated functionally, 

 they are not symmetrically placed. This fusion of the visual impres- 

 sions in consciousness may be illustrated in the following ways: 



(a) If the right eye is made to receive a certain impression of red and the left 

 eye, an identical impression of blue, the result is either a fusion of the two colored 

 fields (purple) or a struggle of the two fields for supremacy. In the latter case, a 

 sensation of red alternates with a sensation of blue. 



