REPT! LES. - this, class the optic nerses„ are derived from optic lobes very similar to those iri fish ; they are! two in nuniber and interposed between the cerebral hemispheres and the cerebellum : their size is proportional to the developement of thei, optic nerves, and they are best seen at the upper, or dorsal surface of the brain (fig. 411, c).
Iiirtos.—In birds the optic nerves originate, chiefly in two lobes situated at the inferior midi lateral aspect of the brain, and called in thil; class also " optic lobes." 'Ile size of ths$ lobes is in proportion to that of the optic nem* and organs of vision, and they are thereft immense in birds of prey, and much smaller in other birds not equally remarkable for perfec tion of sight (fig. 412). • what parts of the mammal's brain are analogOus to the optic lobes of the lower claskes.
The tubercula-quadrigemina in man and the mammalia are iden tical with the optic lobes of the lower vertebrata ; they occur as four small tubercles arranged in pairs of which the anterior are called the nates, and the posterior the testes. In some of the class, as for example, Ruminantia, Soli peda, and Rodentia, the nates are of larger dimensions than the testes; in others, as for instance, Carnivora, the testes predominate in size over the nates, and in Man and Quadrumana the two pairs are nearly equal.
The situation of these bodies in the brain of the bird, so different from their position in reptiles and fish, created at one period some doubts as to their true analogies ; but Serres has shown that during the early stages of deve lopement the optic lobes occupy precisely the same position in the encephalon of the chick I which they hold permanently in the brain of I the reptile and fish, and he has thereby divested 1 this subject of much of its obscurity. Thus 1 before the tenth day of incubation the optic 1 lobes of the chick are placed between the cere I bellum and the cerebral hernispheres, and are ,3 then best seen at the dorsal aspect of the brain : I but after this epoch the hemispheres and cere . bellum approach each other at the expense of i the optic lobes—the hemispheres extending I backwards, and the cerebellum inclining for ' wards. By this double movement the optic lobes are soon overlapped behind, separated ii I from each other, and at length pushed- down i wards and outwards to their permanent si i. 1 tuation (fig. 413). .
I I In Man the optic nerves derive some roots i .from the tabercula guadrigemina.
In birds, reptiles, and fish, the optic lobes I constitute the principal sources of the optic nerves, and therefore in any attempt to ascer tain the true origin of the second pair in man, a necessary preliminary will be to determine The tubercles in question have but little apparent similarity to the optic lobes of tlie lower Vertebrata : they occur asibur eminences, while the optic lobes of birds, reptiles, and fish, are but two in number : they are of dimi nutive size; the optic lobes of birds, reptiles, and fish are of large dimensions in proportion to the brain : they are solid; the optic lobes of birds, reptiles, and fish are hollow: and in Man and most Alammalia they are covered upon the upper surface by the cerebral hemi spheres, while the optic lobes in reptiles and fish are not so covered. Such obvious dissimi larity tended materially to obscure the real nature of the tubercula quadrigemina, but a careful study of the developenient of these bodies in the foetal brain led anatomists at length to discover their true analogies ; and the researches of Tiedemann and Serres have chiefly contributed to establish the following particulars.
" In the earlier stages of uterine life the tuber cula quadrigemina of Alan and Alamnialia occur in the form of two masses; they persist as such during tv;o-thirds of fcetal existence; they are hollow at first and not covered by the cerebral hemispheres, and their size. is im mense in proportion to the bulk of the ence phalon. As developement advances, a trans verse groove appears on the surface of the future tubercula quadrigemina ; this divides them into jirur eminences, which are now for the first time really entitled to be called " qua drigemina :" nervous matter is gradually depo sited from within on their walls, in conse quence of which they henceforth become solid; their growth in size is arrested, and the cere bral hemispheres having grown backwards, overlap and conceal them from view. This overlapping occurs in all except a few of the lowest families of the mammalia, in which the tubercula quadrigemina remain permanently uncovered.