470 



STRUCTURE OF VERTEBRATA. 



epithelial, the cavity called the fourth ventricle is con- 

 tinuous with the canal of the spinal cord. 



Summary. 



First Embryonic 

 Vesicle. 



Median Embryonic 

 Vesicle. 



Third Embryonic 

 Vesicle. 



(1) 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. 



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 

 facilitated by a study of the following table, which Dr. Gadow gives in 

 his great work on Birds in Bronn's Thierreich : 



TABLE. 



