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HAND-BOOK OF PHYSIOLOGY. 



II. Middle 



) 



Primary v 3. Mesencephalon. 

 Vesicle. ) 



III. Posterior 4 ' Epencephalon. 

 ' M etencephalon. 



Corpora quadrigemina, crura 

 cerebri, aqueduct of Sylvius, 

 optic nerve (secondarily). 



Cerebellum, pons Varolii, ante.- 

 rior part of fourth ventricle. 



Medulla oblongata, fourth ven- 

 tricle, auditory nerve. 



( Quain's A natomy. ) 



The cerebral hemispheres grow rapidly upward and backward, while 

 from their inferior surface the olfactory bulbs are budded off, and the 

 thalamencephalon, from which they spring, remains to form the third ven- 

 tricle and optic thalami. The middle cerebral vesicle (mesencephalon) 

 for some time is the most prominent part of the foetal brain, and in 



FIG. 447. Side view of foetal brain at six months, showing commencement of formation of the 

 principal fissures and convolutions. F, frontal lobe; P, parietal; O, occipital; T, temporal; a a a, com- 

 mencing frontal convolutions; s, Sylvian fissure; s', its anterior division; c, within it the central lobe 

 or island of Reil; r, fissure of Rolando; p, perpendicular fissure. (R. Wagner.) 



Fishes, Amphibia, and Reptiles, it remains uncovered through life as the 

 optic lobes. But in Birds the 'growth of the cerebral hemispheres thrusts 

 the optic lobes down laterally, and in Mammalia completely overlaps 

 them. 



In the lower Mammalia the backward growth of the hemispheres 

 ceases as it were, but in the higher groups, such as the monkeys and 

 man, they grow still further back, until they completely cover in the 

 cerebellum, so that on looking down on the brain from above, the cere- 

 bellum is quite concealed from view. The surface of the hemispheres is 

 at first quite smooth, but as early as the third month the great Sylvian 

 fissure begins to be formed (Fig. 446, ^. 



The next to appear is the parieto-occipital or perpendicular fissure; 

 these two great fissures, unlike the rest of the sulci, are formed by a 

 curving round of the whole cerebral mass. 



In the sixth month the fissure of Rolando appears: from this time till 

 the end of foetal life the brain grows rapidly in size, and the convolutions 

 appear in quick succession; first the great primary ones are sketched out, 

 then the secondary, and lastly the tertiary ones in the sides of the fissures. 

 The commissures of the brain (anterior, middle, and posterior), and the 



