THE BRAIN 



327 



consists in paired lateral outgrowths, the anlages of the cerebral hemispheres and 

 rhinencephalon . 



The cephalic flexure forms a very acute angle and, as a result, the long axis 

 of the fore-brain is nearly parallel to that of the hind-brain. The oculomotor 

 nerve takes its origin from the ventral wall of the mesencephalon. Dorsally 

 there is a constriction, the isthmus, between the mesencephalon and meten- 

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

 The dorsal wall of the myelencephalon is an exceedingly thin ependymal layer, 



Cerebral peduncle^ 



Hypatkala - 



EpUhalamus 

 Thalamus 

 >iencephalan 

 (Inter-brain) 



Cerebral aqueduct 



Mesencephalon 

 (Mid-brain) 



Rhombencephalic 

 isthmus 



Rhinencephalon-''terminalis strial 

 (Ollaclory-brain) 



FIG. 314. Brain of a 13.6 mm. human embryo in median sagittal section (after His from Sobotta's 

 Atlas of Anatomy), i, optic recess; 2, ridge formed by optic chiasma, 3; 4, infundibular recess. 



the tela chorioidea. The ventro-lateral walls of this same region on the other hand 

 are very thick. 



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

 cervical, pontine and cephalic flexures well marked (Fig. 314). The thin 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 (see Fig. 324). 

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

 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 (second) 



