. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. Human foetal brain. (From nature.) Lateral view. About fourth month. a, a, optic nerves ; b, chiasma ; c, right tractus opticus ; e, right optic thalamus ; f, mass ot the tubercula quadrisremina; g, g, cerebellum; d, posterior extremity of right cerebral hemisphere, displaced to exhibit the origin of the optic nerve. Tubercula quadrigemina, together with portions of the optic thalami and tractus opt id of a horse. (From nature.) a, a, nates ; b, b, testes ; c, c, optic thalami j d, d, tractus optici, springing partly fro
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. Human foetal brain. (From nature.) Lateral view. About fourth month. a, a, optic nerves ; b, chiasma ; c, right tractus opticus ; e, right optic thalamus ; f, mass ot the tubercula quadrisremina; g, g, cerebellum; d, posterior extremity of right cerebral hemisphere, displaced to exhibit the origin of the optic nerve. Tubercula quadrigemina, together with portions of the optic thalami and tractus opt id of a horse. (From nature.) a, a, nates ; b, b, testes ; c, c, optic thalami j d, d, tractus optici, springing partly from the nates, but deriving a great portion ot their roots from the optic thaldini, c, c. in the sheep precisely the same arrangement exists: in the hare many filaments of the tractus opticus originate in the optic thalamus : and in carnivora and quadrumana a similar disposition prevails. Recent microscopic discoveries in ovoloay (if it be fair to argue from the developement of the chick to the evolution of the human foetus) tend to confirm the views here put forward. Baer states that on the fourth day of incubation the encephalon of the chick consists of several cells, one of which corresponds to the third ventricle, and another to the optic lobes, and that these two cells are distinct from each other. The first rudiment of the eye observable in the chick occurs in the form of a vesicle which shoots out from the parietes of the cell of the third ventricle, and which becomes gradually elongated and drawn out into a canal. On the fourth day the eye represents a spherical cavity communicating with the third ventricle by a canal; this canal is the rudimental optic nerve, which becomes gradually solid, its cavity disap- pearing after the sixth day. During the earlier periods of growth there is no connection what- ever between the optic nerves and the cell of the optic lobes, but the nerves just specified are from the very commencement in free com- munication with the cell of the thi
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