THE BRAIN 



329 



cephalon, and here the fibers of the trochlear nerve take their superficial origin. 

 The dorsal wall of the myelencephalon is an exceedingly thin ependjonal layer 

 which becomes the tela chorioidea. The ventro-lateral walls of this same region, 

 on the other hand, are very thick. 



A mechan sagittal section of a brain at a somewhat later stage shows the 

 cervical, pontine, and cephahc flexures well marked (Fig. 332). The tliin dorso- 

 lateral roof of the myelencephalon has been removed. The telencephalon is a 

 paired structure. In the figure its right half projects cranial to the primitive 

 median wall of the fore-brain which persists as the lamina terminalis (cf. Fig. 342). 

 The floor of the telencephalon is greatly thickened caudally as the anlage of the 



Cerebral peduncle 

 Hypollhilamus^ 

 Epithalamus ; 

 Thalamii 

 Diencephalon- ; 



Pallium -^t 



Telencephalon-- ' ^ a\ 



Cerebral aqueduct 



Mescneeplialon 



£;„. JiliumbcHccplialic isthmus 

 '/ 

 I \ 



Cerebellum 

 Melcnccphalon 

 Rhomboid fossa 



I I 123 4 V-A— 

 RlinitiUi pliaUvi I Lot pus sirialum Pons 

 Lamina terminalis 



Myelencephalon 



Spinal cord 



Fig. 332. — Brain of a 13.6 mm. human embryo in median sagittal section (after His in Sobotta). 

 1, Optic recess; 2, ridge formed by optic chiasma, 3; 4, infundibular recess. 



corpus striatum. A slight evagination of the ventral wall of the telencephalon 

 just cranial to the corpus striatum marks the anlage of the rhinencephalon. The 

 remaining portion of the telencephalon forms the pallium or cortex of the cerebral 

 hemispheres. The paired cavities of the telencephalon are the lateral (first and 

 second) ventricles, and these communicate through the interventricular foramina 

 (Monroi) with the cavity of the diencephalon, the third ventricle. The cavities 

 of the oMactory lobes communicate during fetal life with the lateral ventricles 

 and were formerly called the first ventricles. 



The crossing of a portion of the optic nerve fibers in the floor of the brain 

 forms the optic chiasma, and this, with the transverse ridge produced by it inter- 

 nally, is taken as the ventral boundary Hne between the telencephalon and dien- 



