372 THE SPECIAL SENSES. 



arrangement' of the optic nerve fibers. Those from the right halves 

 of the two retinas, which are corresponding halves, unite in the 

 right optic tract and are distributed to the right side of the brain, 

 while the fibers from the left halves go to the left side of the brain. 

 The basis of the single sensation from two visual images is to be 

 found probably in the fact that the cerebral terminations through 

 which the final psychical act is mediated lie close together or possibly 

 unite. 



The Horopter. — In every fixed position of the eyes there are 

 a certain number of points in the binocular field which fall 

 upon corresponding points in the two retinas and are therefore 

 seen single. The sum of these points is designated as the horopter 

 for that position of the eyes. It may be a straight or curved fine, 

 or a plane or curved surface. Helmholtz calls attention to the fact 

 that, when standing with our eyes in the primar}^ position, — that 

 is, directed toward the horizon, — the horopter is a plane coinciding 

 with the ground, and this fact may possibly be of service to us in 

 walking. 



Suppression of Visual Images. — It happens not infrequently 

 that when an image of an object falls upon non-corresponding 

 points in the two retinas the mind ignores or suppresses one of the 

 images. This peculiarity is exhibited especially in the case of per- 

 sons suffering from "squint" (strabismus). In this condition the 

 individual, for one reason or another, is unable to adjust the contrac- 

 tions of his eye muscles so as to unite his lines of sight upon the 

 object looked at. The image of the object falls upon non-corre- 

 sponding points and should give double vision, diplopia. This 

 would undoubtedly be the case if the condition came on suddenly ; 

 just as double vision results when we dislocate one eyeball by 

 pressing slightly upon it. But in cases of long standing one of the 

 images, that from the abnormal eye, is usually suppressed. The 

 act of suppression seems to be a case of a stronger stimulus prevail- 

 ing over a weaker one in consciousness, just as a painful sensation 

 from stimulation of one part of the skin may be suppressed by a 

 stronger pain from some other region. 



Struggle of the Visual Fields. — When the images of two dis- 

 similar objects are thrown, one on each retina, the mind is presented, 

 so to speak, simultaneously with two different sensations. Under 

 such circumstances what is known as the struggle of the visual 

 fields ensues. If the image on one eye consists of vertical lines 

 and on the other of horizontal lines we see only one field at a time, 

 first one then the other, or the field is broken, vertical lines in part 

 and horizontal lines in part; there is no genuine fusion into a con- 

 tinuous, constant picture. The struggle of the two fields is better 



