5(5 



THE ANATOMY OF VERTEBRATED ANIMALS. 



results from its embryonic division, by two constrictions, into 

 the three thin-walled vesicles — the anterior, middle, and pos- 

 terior cerebral vesicles — already mentioned. The cavities of 

 these vesicles — the primitive ventricles of the brain — freely 

 communicate at first, but become gradually diminished by the 

 thickening of their sides and floors. The cavity of the ante- 

 rior vesicle is, in tlie adult human brain, represented by the 

 so-called third ventride ; that of the middle vesicle, by the 

 iter (I. tertio ad qunrtura ventriculum ; that of the posterior 

 vesicle, hy the /'(n/rth ■oe/ttz-icle. 



The floor and sides of the posterior vesicle, in fact, thicken 

 and become the medulla oblongata/ together with the p07is 

 varolii, in those animals which possess the latter structure. 



Fir. 



C9 



RTh 



Fli. 19.— Dlnfframmatic bonzonfcil section of a Vertebrate brain. The followin? lettera 

 serve for botb this flgiire .ind Fig. 20: Ml\ Miil-biain. Wliat lies in fiont of this Is the 

 fore-brain, and what lies behind, the hind-brain. Z. i.. the lamina terminalis; Olf. the 

 olfactory lolies ; 7/m/), the hemispheres ; rii.B,\hc tlialainenceplialon; Pn, the nineal 

 pland: Pij. the pituitary bod_v; Fit, the for.amen of Mimm: IS. the corpus striatum; 

 7Vt, the optic thalamus ; i.'Q the corpora quadrii:emiua; (.7 '.the crura cerebri; (76,^10 

 cerebellum; /* F, the pons varolii; J/^/, the medulla oblong-atn; /, olfactorli; //,opticr 

 III, point of exit from the braio of the motores oeulorum \ I\\ of the itathetiei ;' VJ. ot 

 the abducentes; V-AVf, orieins of the other ceretiral nerves. 1, oltiictory ventricle* 

 2, lateral ventricle ; 3, third ventricle ; 4, fourth ventricle ; -f , iter a tertio ad a-uartuix 

 venti-iculum. 



