CH. XLIII.] THE BRAIN. 619 



An outline diagram of its parts, such as is presented in the 

 next figure, will indicate the various parts of the brain which we 

 shall have to take into consideration. 



At the lowest part of the brain, continuing the spinal cord 

 upwards, is the medulla oblongata or bulb (D). Next comes the 

 pom Varolii (C), very appropriately called the bridge, because in 

 it are the connections between the bulb and the upper regions of 

 the brain, and between the cerebellum or small brain (B) and the 

 rest of the nervous system. 



Fig. 459. Plan in outline of the brain, as seen from the right side. j. The parts 

 are represented an separated from one another somewhat more than natural, so as to 

 show their connections. A, cerebrum ;/, g, A, ita anterior, middle, and posterior lobes ; 

 e, fissure of Sylvius ; B, cerebellum ; C, pons Varolii ; D, medulla oblonpata ; a, 

 peduncles of the cerebrum ; b, c, d, supenor, middle, and inferior peduncles of the 

 cerebellum. (From Quain.) 



The mid-brain comes next (a, b), and this leads into the peduncles 

 or crura of the cerebrum (A), the largest section of the brain. 



Through the brain runs a cavity filled with cerebro-spinal fluid, 

 and lined by ciliated epithelium ; this is continuous with the 

 central canal of the spinal cord. In the brain, however, it does 

 not remain a simple canal, but is enlarged at intervals into what 

 are called the ventricles. There is one ventricle in each half or 

 hemisphere of the cerebrum; these are called the lateral ventricler, 

 they open into the third ventricle, which is in the middle line ; 

 and then a narrow canal (aqueduct of Sylvius) leads from this to 

 the fourth ventricle, which is placed on the back of the bulb and 

 pons, which form its floor ; its roof is formed partly by the over- 



