THE NERVOUS SYSTEM. 



585 



epithelium, but it is covered in by the cerebellum, the superior pedun- 

 cles of which, converging forward, form its anterior limits, and the 

 inferior peduncles form its posterior boundaries on either side. 



The lateral, third and fourth ventricles communicate, and through 

 the last with the central canal of the spinal cord. They are all lined 

 with columnar ciliated epithelium, beneath which is a development of 

 neuroglia. This lining so formed is called the ependyma of the ven- 

 tricles. Where the superior peduncles of the cerebellum are approach- 

 ing each other at the upper part of the fourth ventricle, the interval 

 between them is bridged over by a thin layer of gray matter called the 

 valve of Vieussens. 



The portions of the central nervous system are thus classified : 



(i.) Cerebral hemispheres with the corpora striata, developed from 

 the cerebral vesicles and enclosing the lateral ventricles. 



(ii.) Fore-brain, formed of the parts, including the optic thalami, 

 which inclose the third ventricle. 



(iii.) Mid-brain, consisting of the parts inclosing the aqueduct of 



Fig. 356. Plan in outline of the encephalon, as seen from the right side. ^. The parts are 

 represented as separated from one another somewhat more than natural, so as to show their 

 connections. A, Cerebrum; /, g, h, its anterior, middle, and posterior lobes; e, fissure of Syl- 

 vius; B, cerebellum; C, pons Varolii ; D, medulla oblongata; a, peduncles of the cerebrum; 

 6, c, d, superior, middle, and inferior peduncles of the cerebellum. (From Quain.) 



Sylvius, viz., the corpora quadrigemina, which form the roof, and the 

 crura cerebri which form the floor. 



(iv.) Hind-brain, the pons Varolii and the cerebellum form respec- 

 tively the floor and roof of the fore-part of the hind-brain, and the bulb 

 the floor of the back part of the hind-brain, the roof being practically 

 absent. 



