These views apparently strengthen Newton's hypothesis, for it may be presumed that the " identical " parts of the two retinw are those which derive their origin from the same tractus opticus, and the " non-identical " on the con trary those which come from different tractus optici.
The comparative anatomy of the nerves in question furnishes some facts favourable to Newton's hypothesis ; thus many animals in which the eyes are directed laterally, in such manner that each embraces a totally different field of vision, have no chiasma, and their optic nerves cross each other, so that the right retina is in connection solely with the left side of the brain, and vice versa. This arrangement pre vails in the majority of osseous fish, and, for so far, affords negative proof of the hypothesis under consideration. The following theoretical explanation may be offered. " In these animals, owing to the position of their eyes, the same object can never be depicted on the two re tiree simultaneously ; consequently, in them no provision to ensure single vision of the same object by both eyes is required, and therefore no parts of the two retinae have a common con nection with one and the same side of the brain, the two optic nerves being derived respectively from opposite sides of the organ." Again, Mr. Solly has shovvn that in many fish, such as the skate, (in which the eyes arc so set that the respective fields of vision may comprise in a great measure the same objects,) a ehiasma exists : and the anatomy of the chiasma in birds is likewise on the whole fa vourable to the hypothesis; for in these animals the optic axes are in general very divergent, and consequently the respective fields of vision can have but little identity ; a fact which agrees theoretically with the ahnost perfect decussation observable in their laminated chiasma.
Although this explanation of single vision has been sanctioned by the authority of Newton and Wollaston, and supported by strong ana tomical facts and analogies, it will scarcely stand the test of critical examination ; its va lidity has therefore been much questioned and apparently with j uctice, for the following reasons : 1. " Identity of sensation" is not exclusively a special attribute of the second pair ; although it exists in them in great perfection, other nerves must also be admitted to possess the same pro-, perty ; thus notwithstanding that both ears are commonly employed siinultaneously for the ordi nary purposes of hearing, the sensation of a singk impression of sound is in general propagated to the sensorium; and although both nares are used in the appreciation of odours, the sensation of single impressions of scents is most usually produced : now since neither the olfactory nor the auditory nerves are provided with a chiasma, and nevertheless these nerves unde niably evince a unity of sensation, there is good reason for scepticism when the very same pro, perty in the optic nerves is attributed to the pre sence of a chiasma.
2. Many facts in pathology are obviously at variance with Newton's theory ; if it were the true explanation of single vision, morbid affec tions of one side of the brain (whenever pro ductive of amaurosis) ought to implicate more or less of one half of each retina, whereas cirri& rience proves that in the majority of sudi cases one or other retina is wholly paralysed, and not unfrequently vision continues perlect in one eye although exting-uished 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 be observes, " the three phenomena being alternative no doubt proceed from the same org,anic 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 retinw as laid down by Muller, 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 retinw must be considered as in cluded one within the other, so that all points of the two retinw 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 retinw are opposed to each other or different, just as any two points in the retina of the same eye.