OPTIC NERVES. 



773 



rience proves that in the majority of such cases 

 one or other retina is wholly paralysed, and not 

 unfrequently vision continues perfect in one 

 eye although extinguished in the other. More- 

 over, although the theory in question affords an 

 ingenious explanation of the defect in vision 

 noticed by Wollaston, such explanation can 

 scarcely be the true one, for Mayo has known 

 " this visus dimidiatus to alternate in the same 

 individual with temporary insensibility of the 

 centre and circumference of the retina," and, 

 as he observes, " the three phenomena being 

 alternative no doubt proceed from the same 

 organic source, but as the hypothesis will not 

 explain two of them, it is probably not the 

 right explanation of the third." 



3. The structure of the human chiasma does 

 not afford satisfactory explanation of the so 

 called " identical " and " non-identical " parts 

 of the two retinae as laid down by Mu'ller, and 

 this has been so clearly shown by himself that 

 his own words are quoted : 



" With reference to their identity of sensa- 

 tion, the two retinae must be considered as in- 

 cluded one within the other, so that all points 

 of the two retinas which lie within the same 

 degrees of latitude and longitude (the eyes being 

 regarded as globes) are identical in their sensa- 

 tions ; all other points in the two retinae are 

 opposed to each other or different, just as any 

 two points in the retina of the same eye. 



" If the image fall on identical points in 

 both eyes it will be seen single, and if the 

 image does not fall on such identical points it 

 will appear double. 



" The two globes of the eyes are most mi- 

 nutely divided into degrees, minutes, and 

 seconds of latitude and longitude ; at all cor- 

 responding points they are identical, at all dif- 

 ferent points non-identical. The outer lateral 

 portion of one eye is identical with the inner 

 portion of the other eye ; the upper part of one 

 retina is identical with the upper part of the 

 other, and the lower parts of the two eyes are 

 identical with each other. 



" The left half of the retina A, from 1 to 5, 

 however, (Jig. 421,) is not, as a whole, identical 

 with the left half of the retina B, from 1 to 5, 

 but certain points only of the left halves of both 

 retinae are identical, viz. those which in the two 



Fig. 421. 



A / 



Diagram to represent the supposed identical part* of 



the two retinae. {After Midler.) 

 1, 2, 3, &c., the identical parts of the two retinae ; 

 c, c, the optic axes. 



retinae occupy the same degrees of latitude and 

 longitude: 1 is identical with 1, 2 with 2, 

 and so on ; but 1 in the one eye is not identical 

 with 5 in the other eye. 



" To explain the single vision, therefore, it 

 is necessary that not merely each root of the 

 optic nerve, but each primitive fibre of each 

 root should in the chiasma divide into two 

 branches for the two optic nerves, so that the 

 identical fibres of the two nerves might com- 

 municate with the brain at one point only, viz. 

 by one radical fibre, as in tiie annexed wood- 

 cut (fg. 422). But such a division of the fibres 

 in the chiasma does not exist : Treviranus and 

 Volkmann were unable to detect any division 

 of fibres in the chiasma, and I also was unsuc- 

 cessful in my search for such dividing fibres. 

 (Fig. 422. 



Fig. 422. 



Diagram to represent an ideal division of the fibres in 

 the chiasma suitable to this theory. (After Miiller.) 

 a, a, optic nerves ; h, b, tractus optici ; c, c, sup- 

 posed division of each radical fibre in the chiasma 

 into two branches, one for each optic nerve. 



4. If single vision in man be explained on 

 the assumption that certain parts of the two 

 retinae are reciprocally identical, and that such 

 identity depends upon a partial decussation in 

 the chiasma, single vision in animals should of 

 course admit of explanation upon the same 

 principles; and if this be granted, the relative 

 directions of the optic axes in the vertebrate 

 classes ought to afford a good criterion of the 

 extent to which the retinae are reciprocally 

 identical ; for when the optic axes have a 

 strictly lateral direction (as in many osseous 

 fish), the same object can never be depicted on 

 both retinae simultaneously, and consequently 

 it may be inferred that, in such cases, no parts 

 of the two retinae are reciprocally identical. 

 Again, when the optic axes are very divergent, 

 as in many quadrupeds, the respective fields of 

 vision must comprise in great measure different 

 objects, and under such circumstances it may 

 be presumed that the two retinae have but little 

 identity. And when the eyes are so set that the 

 optic axes are parallel, or capable of becoming 

 parallel, or convergent (as in man), .the same 

 objects, or nearly the same, will almost con- 

 stantly occupy the two fields of vision; and in 

 such case the greatest amount of reciprocal 

 identity may be assumed to occur in the two 

 retinae. 



Now, if the relative directions of the optic 

 axes in animals bear relation to the amount of 

 reciprocal identity in their retinae, and if this 

 reciprocal identity depend upon the decussa- 

 tion in the chiasma, as has been assumed, the 

 structure of the chiasma in animals generally 

 should vary as the relative directions of their 

 optic axes. 



Such variation in the structure of the chiasma 

 has not, however, been proved to occur gene- 



