BINOCULAR VISION. J%7 



both visual planes (visual axes) lie in the same plane. In front both visual axes 

 can diverge only to a trifling extent, while they can converge considerably. If 

 individual ocular muscles are paralysed, the position of the visual axis in the same 

 place is disturbed, and squinting results, so that the patient no longer can direct 

 both visual axes simultaneously to the same point, but he directs the one eye after 

 the other. Even nystagmus (p. 721) occurs in both eyes simultaneously, and in 

 the same direction. The innate simultaneous movement of both eyes is spoken of 

 as an associated movement (Joh. Mailer). E. Hering showed that in all ocular 

 movements there is a uniformity of the innervation as well. Even during such 

 movements, in which one eye apparently is at rest, there is a movement, due to the 

 action of two antagonistic forces, the movements resulting in a slight to and fro 

 motion of the eyeball. 



The motor nerves of the ocular muscles are the oculomotorius ( 345), the trochlearis ( 346), 

 and the abducens ( 348). The centre lies in the corpora quadrigemina, and below it ( 379), 

 and partly in the medulla oblongata (379). 



400. BINOCULAR VISION. Advantages. Vision with both eyes affords 

 the following advantages : (1) The field of vision of both eyes is considerably 

 larger than that of one eye. (2) The perception of depth is rendered easier, as the 

 retinal images are obtained from two different points. (3) A more exact estimate 

 of the distance and size of an object can be formed, in consequence of the perception 

 of the degree of convergence of both eyes. (4) The correction of certain errors in 

 the one eye is rendered possible by the other. 



When the position of the head is fixed, we can easily form a conception as to the form of the 

 entire field of vision if we close one eye and direct the open eye inwards. We observe that it is 

 pear-shaped, broad above and smaller below, the silhouette, or profde of the nose, causes the 

 depression between the upper and lower part of the field. 



401. IDENTICAL POINTS HOROPTER. Identical Points. If we 



imagine the retime of both eyes to be a pair of hollow saucers placed one within the 

 other, so that the yellow spots of both eyes coincide, and also the similar quadrants of 

 the retinae, then all those points of both retinae which coincide or cover each other are 

 called " identical " or " corresponding points " of the retina. The two meridians 

 which separate the quadrants coinciding with each other are called the " lines of 

 separation." Physiologically, the identical points are characterised by the fact 

 that, when they are both simultaneously excited by light, the excitement proceeding 

 from them is, by a psychical act, referred to one and the same point of the field of 

 vision, lying, of course, in a direction through the nodal point of each eye. Stimu- 

 lation of both identical points causes only one image in the field of vision. Hence 

 all those objects of the external world, whose rays of light pass through the nodal 

 points to fall upon identical points of the retina, are seen singly, because their 

 images from both eyes are referred to the same point of the field of vision, so that 

 they cover each other. All other objects whose images do not fall upon identical 

 points of the retina cause double vision, or diplopia. 



Proofs. If we look at a linear object with the points 1, 2, 3, then the corresponding retinal 

 images are 1, 2, 3 and 1, 2, 3, which are obviously identical points of the retinse (fig. 567). If, 

 while looking at this line, there be a point, A, nearer the eyes, or B, further from them, then, 

 on focussing for 1, 2, 3, neither the rays (A, a, A, a) coming from A, nor those (B, b, B, b) from 

 B, fall upon identical points ; hence A and B appear double. 



Make a point {e.g., 2) with ink on paper ; of course the image will fall upon both fovea? cen- 

 trales of the retinae (2, 2), which of course are identical points. Now press laterally upon one 

 eye, so as to displace it slightly, then two points at once appear, because the image of the point 

 no longer falls upon the fovea centralis of the displaced eye, but on an adjoining non-identical 

 part of the retina. When we squint voluntarily all objects appear double. 



The vertical surfaces of separation of the retina do not exactly coincide with the vertical 

 meridians. There is a certain amount of divergence (0'5 o -3), less above, which varies in 

 different individuals, and it may be in the same individual at different times (Hering). The 

 horizontal lines of separation, however, coincide. Images which fall upon the vertical lines of 



