But further, wheq the optic axes are very divergent, as in some quadrupeds, any object whiclt can be depicted upon both retinm siinul taneously, will throw its images on the outer parts of the ttvo retinm, and in order to explain single vision under such circumstances, the outer parts of the two retinm should therefore be reciprocally identical ; but these parts are formed by the outer filaments of the optic nerves, which come respectively from the cor responding sides of the brain, so that here, reciprocally identical parts of the two retinw, instead of having a common origin at one and the same side of the brain (as. they ought in order to suit theoretical views), are derived seve rally from opposite sides of the organ, and con sequently have no identity of origin (A,. 423). These considerations are sufficient to falsify the explanation of single vision put forward above, and the writer is of opinion that hitherto the exact use of the chiasma has not been disco vered. (See Visios.) Some remarkable varieties of optic nerves.
Optic nerves in certain cephalopods.—The loligo or calamary exhibits in its optic nerves a singularly beautiful arrangement, which the physiologist cannot but contemplate with the greatest interest, as it presents the most perfect decussation of nervous filaments hitherto dis covered. The loligo, in common with some of the allied families of the cephalopoda, possesses an extremely perfect organ of vision ; so elabo rate is the mechanism of the eye that it has attracted a considemble share of attention from comparative anatomists ; and the developement of the optic nerve bears proportion to the per fection of the other parts of the visual apparatus The nervous system of the loligo confo to the cyclogangliate type, and from each la ral surface of the supraresophageal ganglio (fig. 4240 one of the optic nerves comes o After pursuing a short course outwards t nerve swells into a large ganglion (the optic this body is oval in shape, and of enormo dimensions ; it exceeds considerably the volu of the supracesophageal ganglion: One surfa of the optic ganglion is directed towards t eye, and emits an immense number of filamen which spring chiefly from its edges, and wide after making their way to the eye-ball, and perforating the sclerotic, terminate in the retina. These filaments are arranged at their origin in a double linear series, each series streaming forth from one of the opposite margins of the ganglion. After becoming free of the ganglion the filaments of each series approach those of the other series and a most perfect and regular interlacement Of the two sets ensues ; so com I pletely do they decussate that either half of the retina is formed by filaments which spring from the opposite edge of the ganglion ; those filaments which shoot out from the anterior margin of that body terminating in, and actu 1 ally forming, the posterior moiety of the retina, and vice versil.
The reciprocal interlacement of these nervous filaments reminds one of the manner in which the two series of threads in a weaver's loom (technically called the woof and the warp), cross each other; this illustration is exact, for the interlacement of the threads in the one case is not more perfect or regular than that of the nervous filaments in the other.
The writer is indebted to his colleague, Dr. Power, for a knowledge of this curious arrange ment, which was discovered by that gentleman while inspecting a preparation of the nervous system in the loligo made by the writer, and ' which still exists in the Museum of the Rich mond IIospital School, Dublin.
Subsequent dissections have proved satisfac torily that the decussation in question occurs, invariably in the long°, and that in the octopus, vvhich exhibits close affinity to the loligo, the same arrangement prevails. In the sepia officinalis the optic ganglion is reniform, and its filaments come off in a double series, as in the loligo ; the majority of the filaments decus sate after the same manner as that above de scribed in the other two species, but some of the extreme filaments of each series pass in a direct : course to the retina, without exhibiting any de c ussation .
1 . The peculiar disposition of the optic filaments m these cephalopods was unnoticed until the publication of Dr. Power's paper on the sub ject in the " D ublin Journal of INIedical Science," —an ornission the more surprising, as accurate ' descriptions of the eye in this class of animals •have appeared from some of the ablest ana tomists.
Swammerdam long ago described and de lineated the optic ganglion of the sepia offici alis, but without making any mention of the intercrossing of the filaments which emanate irom that body. Cuvier, in his memoir on the Mollusca, describes the eye of the poulpe (octopus) with great accuracy, but makes no allusion to the decussation in question • and in the most recent descriptions of the cephalopods the same omission occurs.
The above discovery seemed. at first likely to afford an explanation of the problem so difficult of solution, namely, the, well-known I fact, that objects appear erect to a spectator although their images nn the retina are inverted ; and for some highly ingenious observations tending to elucidate this obscure question, the reader is referred to the paper by Dr. Power, already quoted ; it may \be stated, however, that many difficulties remain-4.01)e removed before such an explanation can be redeived, for 1. The filaments of the optic nerve of the loligo have not as yet been traced back to the central masses of the nervous system, and there can therefore be, no certainty that they preserve in the first part of their course the same relative positions which they are known to maintain in the interval between the optic ganglion and point of decussation.