442 STRUCTURE OF VERTEBRATA. 



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 / ventricles. 



Vesicle. j (2) Optic thalami, thalamencephalon, or tween- 



brain. Note — (a) optic, (6) pineal, [c) 

 pituitary outgrowths, and the third ven- 

 tricle. 



Median Embryonic / * 3 ' 0ptic IobeS ' mesence P halon . or mid-brain. 

 Ves'rl \ Note crura cerebri, and the aqueduct of 



j Sylvius. 



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

 Note pons Varolii. 

 Third Embryonic J (5) Medulla oblongata, myelencephalon, or 

 Vesicle. \ 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 irregular- 

 ities of the brain so carefully as does the pia mater. Thirdly, a firm 

 membrane — the dura mater — lines the brain-case, and is continued down 

 the spinal canal. In lower Vertebrates the dura mater is double 

 throughout ; in higher Vertebrates it is double only in the region of the 

 spinal cord, where the outer part lines the bony tunnel, while the inner 

 ensheaths the cord itself. In Fishes the brain-case is much larger than 

 the brain, and a large lymph space lies between the dura and the pia 

 mater. 

 An understanding of the relations of the different regions will be 



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