CII. LTX.] 



THE CEREBRAL HEMISPHERES 



863 



TABLE OF PARTS DEVELOPED FROM FUNDAMENTAL PARTS OF BRAIX. 



I. Anterior 

 Primary 

 Vesicle. 



First Secondary Vesicle 

 Telencephalon, or Fore-J 

 brain. 



Anterior end of third ventricle, 

 foramen of Monro, lateral ven- 

 tricles, cerebral hemispheres, 

 corpora, striata corpus callosum, 

 fornix, lateral ventricles, olfac- 

 l v tory bulb. 



c T o , XT i r Thalami optici, pineal gland, part 

 ^Sete^^ ^HSr U. "bird vV 



brain> I infundibulum. 



TTT 



~ ie * 



T Fourth Secondary Vesicle,) Fourth ven-/ Cerebellum and 

 | or Metencephalon. / tricle. ^ Pons. 



I Fifth Secondary Vesicle,) Fourth ven-/ Medulla oblon- 

 V or Myelencephalon. j tricle. ( gata. 



The cerebral hemispheres formed by bifurcation of the telence- 

 phalon grow rapidly upwards and backwards, while from their 

 inferior surfaces the olfactory bulbs are budded off. The middle 

 cerebral vesicle (mesencephalon) for some time is the most pro- 

 minent part of the foetal brain, and in fishes, amphibia, and reptiles, 



it remains uncovered through life 

 as the optic lobes. But in birds 

 the growth of the cerebral hemi- 

 spheres thrusts the optic lobes 

 down laterally, and in mammalia 

 completely overlaps them. 



In the lower mammalia the 

 backward growth of the hemi- 

 spheres ceases, 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 cerebellum 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. 661). 



The next to appear is the parieto-occipital 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 Eolando appears : from this 

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



FIG. 661. Side view of foetal brain at six months, 

 showing commencement of formation of the 

 principal fissures and convolutions. F, Frontal 

 lobe; P, parietal; 0. occipital; T, temporal; 

 a a a, commencing frontal convolutions; 

 s, Sylvian fissure ; s', its anterior division ; 

 c, within it the central lobe or island of Reil ; 

 r, fissure of Rolando; p, parieto-occipital fis- 

 sure. (R. Wagner.) 



