56 



THE AXATOMY OF YERTEBRATED 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 

 thickenino^ of their sides and floors. The cavitv of the ante- 

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

 so-called third ventricle ^ that of the middle vesicle, by the 

 iter a tertlo ad quartuni ventriculum y that of the posterior 

 vesicle, by the fourth ventricle. 



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

 and become the medulla oblongata ^ together with the pons 

 varolii^ in those animals which possess the latter structure. 



FW. 



FitJ. 19. — Diajrrainmatic horizontal section of a Vertebrate brain. The tollowinfir letters 

 serve for both this figure and Fig. 20: 3Ib, Mid-brain. What lies in front of this is the 

 fore-brain, and what Ues behind, the hind-brain. L. ^., the lamina terminalis; Olf. the 

 olfactory lobes ; limp, the hemispheres ; Tli. E, the thalamencephalon ; Pn, the pineal 

 P'land; P.y, the pituitary body; FM, the foramen of Munro; Ci', the corpus striatum; 

 7"A, the optic thalamus; CQ. the corpora quadriiremina; CC the crura cerebri; C6. the 

 cerebellum; /T, the pons varolii; ^1/6*, the medulla oblongata ; /, olfactorii ; //, optici; 

 ///, point of exit from the brain of the motores oculorum ; /F, of the pathetici ; F7, of 

 the abducentes; F-X/7, origins of the other cerebral nerves. 1, olfactory ventricle; 

 2, lateral ventricle; 3, third ventricle; 4, fourth ventricle ; +, iter a tertio dd quavtum 

 r,e,ntHculum. 



