MESENCEPHALON. 417 



like downward projecting folds from the inferior lamina of the 

 chorioid tela, which constitute the chorioid plexuses of the third 

 ventricle. 



The dorsal lamina of the diencephalon thickens greatly and 

 so encroaches upon the ventricle as to convert it into a narrow 

 median slit. Externally, the dorsal lamina fuses with the cere- 

 bral hemisphere, as a result of the formation of the internal cap- 

 sule; and the fusion at one point of the medial surfaces of the two 

 dorsal laminae gives rise to the massaintermedia (middle commissure). 

 The development of the thickened dorsal lamina of the diencepha- 

 lon produces the thalamus with its pulvinar and the geniculate 

 bodies (Fig. 124). 



The ventral lamina and floor-plate of the diencephalon con- 

 stitute the pars mammillaris hypothalami, which embraces some 

 gray and white matter beneath the thalamus, and the corpora 

 mammillaria (Fig. 118, VI). A single oval eminence situated 

 in the median line, represents the mammillary bodies up to the 

 third month; but, during the third month, that eminence is divided 

 into the two white bodies (corpora albicantia) of the adult. The 

 gray substance immediately in front of the corpora mammillaria 

 also belongs to this region, hence a part of the tuber cinereum is 

 included among the derivatives of the diencephalon. 



MESENCEPHALON. 



This is the mid-brain (Figs. 17, 118, IV, and 120). It is the 

 third of the secondary vesicles. The elbow of the mesencephalic 

 flexure of 180 degrees is formed by it; and that flexure almost 

 brings the diencephalon and metencephalon in contact with one 

 another beneath it. The mesencephalon remains small, but its 

 walls thicken greatly. As a result of the thickening, its cavity 

 is reduced to a slender canal, the cerebral aqueduct (Fig. 120). 



TABLE VIII. 



DERIVATIVES OF MESENCEPHALON. 



f Lamina quadrigemina 



Colliculi 



Dorsal Zones ~\ , . ,. .. 



Brachia (in part) 



[_ Red nuclei. 

 27 



