THE BRAIN 



4*7 



The second primary vesicle of the brain forms the third 

 region, that of the optic lobes (mesencephalon or mid-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. 



First Embryonic 

 Vesicle. 



Median Embryonic 

 Vesicle. 



Third Embryonic 

 Vesicle. 



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, (b) pineal, (c) 

 pituitary outgrowths, and the third ven- 

 tricle. 



(3) Optic lobes, mesencephalon, or mid-brain. 



Note crura cerebri, and the aqueduct of 

 Sylvius. 



(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. 



