VaE ENCEFUALON. 6^ 



The posterior part of the roof is not converted into nervoua 

 matter, liui remains thin and attenuated ; the ependyma, or 

 lining of the cerebral cavity, and the arachnoid, or serous 

 membrane which covers the brain externally, coming nearly 

 into contact, and forming, to all appearance, a single thin 

 membrane, which tears with great readiness, and lays open 

 the cavity of the fourth ventricle. Anteriorly, on the other 

 hand, the roof becomes converted into nervous matter, and 

 may enlarge into a complex mass, which overhangs the 

 posterior division, and is called the cerebellum. The pons 

 varolii, when it exists, is the expression of commissural fibres, 

 which are developed in the sides and floor of the anterior part 

 of the posterior cerebral vesicle, and connect one half of the 

 cerebellum with the other. 



Thus, the hind-brain differs from the posterior cerebral 

 vesicle in being differentiated into the medulla oblongata (or 

 myelencephalon) behind, and the cerebellum with the pons 

 varolii (which together constitute the metencephalon) in 

 front. 



The floor of the middle cerebral vesicle thickens and 

 becomes converted into two great bundles of longitudinal 

 fibres, the crura cerebri. Its roof, divided into two, or four, 

 convexities by a single longitudinal, or a crucial, depression, 

 is converted into the " optic lobes," corpora bigernina or 

 quadrigemina. And these parts, the optic lobes, the crura 

 cerebri, and the interposed cavity, which either retains the 

 form of a ventricle, or is reduced to a mere canal (the iter a 



M.t. 



Pig. 20. A longitudinal and vertical section of a Vertebrate brain. — Tbe letters as befDra 



The lamina Urminalls la represented by the strong black line between FMajii 3. 



tertio ad quartum ventriculum), are the components of the 

 tnid-brain or mesencephalon. 



The anterior cerebral vesicle undergoes much greatci 



