. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 766 OPTIC NERVES. occur in the form of two masses; they persist as such during two-thirds of fetal 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 /'our eminences, which are now for the first time really entitled to be called " qua- drigemina :" nervous matter is gradually de
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 766 OPTIC NERVES. occur in the form of two masses; they persist as such during two-thirds of fetal 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 /'our 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. From the foregoing exposition it appears that during their developement the tubercula quadrigemina in man and mammalia assume for 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. In 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 obviousl
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