438 STRUCTURE OF VERTEBRATA. 



and lateral walls form the thickened crura cerebri, the roof 

 becomes the two optic lobes, which are hollow in almost all 

 Vertebrates. In Mammals a transverse furrow divides each 

 optic lobe into two (corpora quadrigemina). The cavity of 

 the vesicle becomes much contracted, and forms the narrow 

 iter or aqueduct of Sylvius, a canal connecting the third 

 ventricle with the fourth. 



The third primary vesicle gives rise to the metencephalon, 

 or hind brain, or region of the cerebellum, and to the 

 myelencephalon, or after brain, or region of the -medulla 

 oblongata. 



In the metencephalon the roof develops greatly, and 

 gives rise to the cerebellum, which often has lateral lobes, 

 and overlaps the next region. In the higher forms the 

 floor forms a strong band of transverse fibres the Pons 

 Varolii. 



From the region of the medulla oblongata most of the 

 cranial nerves are given off. Here the roof, partly over- 

 lapped by the cerebellum, degenerates, becoming thin and 

 epithelial, the cavity called the fourth ventricle is con- 

 tinuous with the canal of the spinal cord. 



Summary. 



(i) Cerebral hemispheres, prosencephalon, or 

 fore brain. Note commissures, olfactory 

 lobes and nerves, and first and second 



First Embryonic 

 Vesicle. 



ventricles. 

 (2) Optic thalami, thalamencephalon, or tween- 



brain. Note (a) optic, (b] pineal, (c] 

 pituitary outgrowths, and the third ven- 

 tricle. 



Median Fmhrvonic f (3) ptic lobeS ' mesence P halon > or mid brain - 

 Median fcmbryon N cerebri, and the aqueduct of 



Veslcle " I Sylvius. 



1(4) Cerebellum, metencephalon, or hind brain. 

 Note Pons Varolii. 

 (5) Medulla oblongata, myelencephalon, or 

 after brain. Note rudimentary roof, 

 fourth ventricle, and origin of most of 

 the cranial nerves. 



Enswathing the brain, and following its irregularities is a delicate 

 membrane the pia mater rich in blood vessels which supply the ner- 

 vous system. Outside this, in higher Vertebrates, there is another 

 membrane the arachnoid which does not follow the minor irregulari- 



