CEKEBRAL HEMISPHERES. 



621 



E *" 



the vicinity of the parts which lie subjacent to the parietal tuberosities of the 

 cranium. The massive rou-nded character of the anterior or frontal end of each 

 cerebral hemisphere constitutes a leading human characteristic ; but the posterior or 

 occipital end is narrow and pointed, and is directed somewhat downwards. The 

 two cerebral hemispheres are separated from each other by a deep median cleft, 

 termed the longitudinal fissure. 



The cerebral hemisphere is formed from a small area of the extreme anterior 

 end iof the alar lamina, in 

 the angle between the fore- 

 most part of the roof and 

 the upper end of the lamina 

 terminalis (Fig. 548, L), 

 which becomes continuous 

 with the roof (at the point 

 marked X). 



The rapid expansion of 

 this area leads to the 

 development of a lateral 

 bulging containing a diver- 

 ticulum of the third ven- 

 tricle, which is known as 

 the ventriculus lateralis. 

 This at first communicates 

 with the third ventricle by 

 means of a wide opening 

 (F.M.), the foramen inter- 

 ventriculare [Monroi], cor- 

 responding in size to the 

 extent of the area of the 

 side wall that was bulged 

 outwards to form the hemi- 

 sphere vesicle. The thin 

 epithelial roof of the telen- 

 cephalon takes no share in 

 the formation of the two 

 cerebral hemispheres, but 

 serves with the lamina ter- ^ 548> _ Two DRAWINQS OP THE - EMBRYONIC BRAIN (by His). 



A. Reconstruction of the fore-brain and mid-brain of His's embryo KO; 

 profile view. B, Same brain as A, divided along the median plane 

 and viewed upon its inner aspect. 



two fnlrU hp M ' Mamillary eminence ; Tc, Tuber cinereum ; Hp, Hypophysis 

 JTT" (hypophyseal diverticulum from buccal cavity) ; Opt, Optic stalk ; 



Come invagmated trom this TH> Thalamus ; Tg, Tegmental part of mesencephalon ; Ps, Pars 



epithelial roof in the whole hypothalamica ; Cs, Corpus striatum ; FM, Foramen interventricu- 



ov f OT1 f rt f fVo TKvnaoi lare ; L, Lamina terminalis ; RO, Recessus optic us ; Ri, Recessus 



infundibuli, Met, Metathalamus. 



cephalon, both its telen- 



cephalic and diencephalic parts. In the greater part of their length these 

 folds project into the third ventricle, and form its chorioid plexus (Fig. 549); 

 but the anterior parts of the two chorioidal folds, namely, those parts formed 

 from the roof of the inter ventricular foramina (F.M.), become greatly enlarged 

 and project each into the corresponding lateral ventricle. The furrow cor- 

 responding to this invagination of the roof is called the fissura chorioidea. 

 When the hemisphere vesicle first begins to expand, the thinner part of the 

 hemisphere wall, which is called the pallium, is freely continuous around the 

 vertical caudal margin of the foramen interventriculare (Fig. 548, Y) with the 

 thalamus (TH). 



But as development proceeds the wall of the prosencephalon becomes attenuated 

 along the line of this pallio-thalamic junction, and eventually the edge of the 

 pallium fringing this attachment to the thalamus becomes reduced to a thin layer 

 of epithelium which is continuous at its superior end with the lamina chorioidea of 



Ri. 



union between them. At 

 a somewhat later Stage in 



