From the foregoing exposition it appears that during their developement the tubercula quadrigemina in man and maminalia assume .1i,r a time all the characters which the optic lobes of birds, reptiles, and fish exhibit in the permanent condition ; and hence it can scarcely be questioned that the nates and testes of the former class are identical with the optic lobes of the latter animals ; but since the optic nerves in the oviparous Vertebrata are trace able to the optic lobes and manifestly derive from them the greater proportion of their roots, there is so far prima facie evidence that the optic nerves in man have their origin in part from the tubercula quadrigemina. further confirmation of the same view it may be re marked that some of the roots of the optic nerves in certain orders of the mammalia are seen to spring from these bodies ; for example, in the horse a large proportion of the nerves can be traced distinctly to the nates. In Ro dentia and Carnivora numbers of the fibres of the nerves emanate obviously from the same pair of tubercles, and in the Ruminants a simi lar anatomical arrangement prevails.
As an additional proof Tiedemann asserts that although much difficulty is encountered in attempts to follow the optic nerves to the tuber cula quadrigemina in the adult human subject, he has succeeded in tracing them to the nates in fcetuses of the third month, and at the fourth and fifth months he has frequently repeated the same observation.
Human pathology would seem to furnish some corroborative facts : thus in every case of long-continued atrophy of the optic nerve, where the wasting had involved the tractus opticus, Gall and Spurzheim found the nates of the side corresponding to the diseased tract diminished in size; and the experiments insti tuted upon living animals with a view to deter mine the functions of the several constituents of the bmin by the successive removal of the different parts of the organ and careful obser vation of the disturbance thereby produced, lead also to the belief that the optic nerves have an origin in the tubercula quadragemina.. Of course great allowance must be made for inac curacy in the result of such mutilations, but Flourens, Magendie, Desmoulins, and Ilertwig, all agree that destruction or mutilation of the nates and testis of one side invariably produces blindness of the opposite eye.
The writer agrees fully with Cruveilhier in the belief that the optic nerves in the human subject can be rarely traced to the tuber cula quadrigemina satisfactorily ; but never theless with the above facts before them, anatomists can scarcely refuse to allow that the optic nerves in man derive a share of their roots from these eminences.
The tubercula yuadrigemina probably fu?fil other purposes besides that of affording origin to the optic nerves.
This may be inferred from the fact that the optic nerves are not invariably developed in direct proportion to the tubercles ; thus in certain mammals which are either devoid of optic nerves altogether, or in which they are so excessively diminutive as to be with difficulty discovered, the tubercula quadri gemina are as large and perfect as in other allied species possessed of well-marked o%ans of vision.
The common mole, for example, has eyes so diminutive and imperfect in structure, and its subterranean habits bespeak so little necessity for organs of vision, that many excellent anato mists believe it to have no optic nerves; never theless the tubercula quadrigemina in this animal are of immense size. ( Fig. 414.) Serres never could satisfy himself that the mole pos sesses optic nerves, although he examined thirty or forty specirnens for the express purpose, and if they do exist (as has been maintained by Carus and Treviranus) their minuteness must be almost microscopic. (See INSECT! VORA, VOI. ii.fig. 453.) Other examples confirmatory of the same views are afforded by the mammalia ; it is stated on the authority of Serres that in the rat-mole of the Cape, and the Zanni or blind rat-mole, there is no appearance whatever of proper optic nerves, (the rudimental eyes being supplied by the fifth pair,) and yet in these animals the tubercula quadrigemina exist in great perfection.
The human optic nerve probably derives roots from the optic thalamus.
The writer is of opinion that modern anato mists have fallen into error in supposing thl none of the roOts of the second pair are derivei from the optic thalamus, although the argu.li ments by which that supposition has beeit sustained are sufficiently imposing, viz.:— 1') " The size of the optic thalami is not in general in direct proportion either to.that of the optic nerves or the acuteness of vision in animals.
" In most fishes the optic nerves are of great size, and the perfection of vision is extreme, yet in this class the optic thalami are absent.
" In birds, some of which enjoy exquisite powers of sight, the optic thalami are small.
" In mammalia the optic nerves bear no fixed proportion to the optic thalami ; for instance, the horse, the ox, and the stag have larger optic nerves than the human subject, and yet the optic thalami in these animals are infinitely smaller than in man." The foregoing arguments are not conclusive, for if the want of direct proportion between the optic thalami and optic nerves were a proof that the optic nerves draw none of their roots from the optic thalami, the very same principle would deprive the tubercula quadrigemina likewise of all claim to be considered a source of the second pair ; since in the Mole and some other man:maim, already specified, the nerves in question and the tubercula quadrigemina ac tually occur in inverse proportion to each other.