Quain's elements of anatomy . n the early condition of the human embryo, are of great propor-tionate volume, in harmony with what is seen in the lower vertebrata ; but sub-sequently they do not grow so fast as the anterior parts of the encephalon, andare therefore soon overlaid by the cerebral hemispheres, which at the sixth month 830 THE NERVOUS SYSTEM. cover them completely. Moreover, they become gradually solid by the deposi-tion of matter within them ; and as, in the meantime, the cpreljral pecLuncles areincreasing rapidly in size in the floor of this middle cephalic vesicle, the cavityin
Quain's elements of anatomy . n the early condition of the human embryo, are of great propor-tionate volume, in harmony with what is seen in the lower vertebrata ; but sub-sequently they do not grow so fast as the anterior parts of the encephalon, andare therefore soon overlaid by the cerebral hemispheres, which at the sixth month 830 THE NERVOUS SYSTEM. cover them completely. Moreover, they become gradually solid by the deposi-tion of matter within them ; and as, in the meantime, the cpreljral pecLuncles areincreasing rapidly in size in the floor of this middle cephalic vesicle, the cavityin its interior is quickly filled up, with the exception of the narrow passagenamed the Sylvian aqueduct. The fillet is distinguishable in the fourth corpora quadrigemina of the two sides are not marked off from each otherby a vertical median groove until about the sixth month ; and the transversedepression separating the anterior and posterior pairs is fiist seen about theseventh month of intra-uterine life. Fig. c^ zn JOS Fig. 734.—Lateral view of the BRAIN OF AN EMBRYO CALF OF 5 cji. (From Balfour, after Mihal-kovics.) The outer wall of the left hemi-sphere is removed to show the in-terior of the lateral ventricle; hs,cut wall of the hemisphere ; st,corpus striatum ; am, hippocampusmajor ; d, choroid plexus of ventricle ;fni, foramen of Monro ; op, optictract; in, infundibuhim ; nib, mid-brain ; cb, cerebelhim; , roofof fourth ventricle ; ps, pons Varohi;with fifth nerve and Grasserianganglion. The internal geniculate bodies belong to this division of the brain. 4. The Thalamencephalon, Inter-brain.—It is from this part, constituting atfirst the whole and subsequently the hinder part of the anterior primary encephalicvesicle, that the optic vesicles are developed in the earliest period, and the forepartis that in connection with which the cerebral hemispheres and accompanying partsare formed. The thalamus opticus of each side is formed by a lateral thick
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