542 STRUCTURE OF VERTEBRATA 



brain) in the adult brain. The floor 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 

 ventricles. 

 (2) Optic thalami, thalamencephalon, or tween- 

 brain. Note — {a) optic, ih) pineal, (r) 

 pituitary outgrowths, and the third ven- 

 tricle. 



Median Embryonic j ^^^ ^ptic lobes, mesencephalon or mid-brain 

 y • , -^ ■{ Note crura cerebri, and the aqueduct of 



[ Sylvius. 



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

 Note pons Varolii. 



(5) Medulla oblongata, m^'elencephalon, or 

 after - brain. Note rudimentary roof, 

 fourth ventricle, and origin of most of 

 the cranial nerves. 



First Embryonic 

 Vesicle. 



Third Embryonic 

 Vesicle. 



Enswathing the brain and spinal cord, and following its irregularities, 

 is a delicate membrane — the pia mater — rich in blood vessels, which 



