930 GENERAL STRUCTURE. [BOOK in. 



and optic nerve, gives rise in front of itself to a pair of vesicles 

 placed side by side, or rather to a single vesicle with a deep 

 median furrow, the vesicle of the cerebrum, containing a cavity 

 divided by a median partition into two cavities, lying side by 

 side, which open into the cavity of the original fore-brain 

 by a Y-shaped opening. This embryonic chain of vesicles is 

 developed into the adult brain by unequal growth of the walls 

 and unequal expansion of the cavities, certain features being also 

 impressed upon it by the bend on the longitudinal axis, which 

 takes place in the region of the mid-brain and is known as the 

 cranial flexure. 



601. In the hind part of the hinder vesicle or hind-brain, 

 the ventral, basal portion or floor is thickened to form the bulb, 

 while the greater part of the dorsal portion or roof does not thicken 

 at all, is not transformed into nervous elements, but remains as 

 a single layer of epithelium, adherent to the pia mater overlying 

 it, and so forms a thin covering to the lozenge-shaped cavity of 

 the vesicle, now known as the fourth ventricle. 



In the front part of the same hind-brain, on the contrary, the 

 roof and sides are enormously developed into the conspicuous 

 cerebellum overhanging the front part of the fourth ventricle, 

 while the floor is also thickened into the pons Varolii. 



This thickening of the pons is largely made up on the one 

 hand of horizontal nerve fibres, which run transversely from each 

 side of the cerebellum into the pons or from one side of the 

 cerebellum to the other, and on the other hand of longitudinal 

 fibres, which run forwards from the bulb and are wrapped 

 round by and interlaced with the others. At the front margin of 

 the pons these longitudinal fibres, augmented in number, appear as 

 two thick strands, the crura cerebri, forming the floor of the 

 mid-.brain, the roof of which is thickened into the corpora quadri- 

 gemina, and the cavity of which is reduced to a narrow tubular 

 passage, the aqueduct of Sylvius, or iter a tertio ad quartum 

 ventricidum. 



At the level of the fore-brain the crura cerebri, diverging 

 rapidly from each other as they pass forwards, leave the median 

 portion of the floor of the vesicle now known as the third ventricle 

 very thin, but form, especially behind and ventrally, thick lateral 

 walls, which are further increased in thickness by the development 

 on each side of a mass largely composed of grey matter, known as 

 the optic thalamus. The roof of the third ventricle, like that of 

 the fourth ventricle, is not developed into nervous elements but 

 remains extremely thin, and consists of nothing more ' than a 

 single layer of epithelium. 



602. In front of the third ventricle each diverging crus 

 cerebri spreads out in a radial fashion into the corresponding 

 half of the paired vesicle of the cerebrum now developed into the 

 preponderant cerebral hemispheres, the two cavities of which are 



