CH. XLIII.] THE BKAIN 623 



At the lowest part of the brain (fig. 454), continuing the spinal 

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

 pons 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. 



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 

 (see p. 178), and lined by ciliated epithelium ; this is continuous with 

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



FIG. 454. Plan in outline of the brain, 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 Sylvius ; B, cerebellum ; 

 C, poiis Varolii ; D, medulla oblongata ; a, peduncles of the cerebrum ; b, c, d, superior, middle, and 

 inferior peduncles of the cerebellum. (From Quain). 



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 ventricles, 

 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 overhanging cere- 

 bellum, partly by pia mater. This piece of pia mater is pierced by a 

 hole (Foramen of Magendie), and so the cerebro-spinal fluid in the 

 interior of the cerebro-spinal cavity is continuous with that which 

 bathes the external surface of brain and cord in the sub-arachnoid 

 space. The fourth ventricle leads into the central canal of the spinal 



