Chap. XIX] 



THE NERVOUS SYSTEM 



391 



(4) Occipital lobe occupies the posterior extremity of the 

 cerebral hemisphere. When one examines the external surface of 

 the hemisphere, there is no marked separation of the occipital lobe 

 from the parietal and temporal lobes that lie to the front ; but 

 when the surface of the longitudinal cleft is examined, the parieto- 

 occipital fissure serves as a boundary anteriorly for the occipital 

 lobe. 



(5) Central lobe, or Island of Reil, is not seen when the sur- 

 face of the hemisphere is examined, for it lies within the fissure 



Fig. 187. — The Lobes of the Convex Surface of the Hemisphere, Left 



Side. (Gerrish.) 



of Sylvius, and the overlying convolutions of the parietal and 

 frontal lobes must be lifted up before the central lobe comes into 

 view. 



Ventricles of the brain. — In describing the spinal cord, ref- 

 erence was made to the central canal, being a minute canal running 

 through the centre of the cord throughout its entire length, thus 

 converting the cord into a tube with exceedingly thick walls but 

 very small internal calibre. In the brain proper this same central 

 channel persists, and just as the walls or solid portions of the brain 

 are directly continuous with the wall or solid portion of the spinal 

 cord, so is the internal hollow of the brain directly continuous 

 with the hollow or central canal of the cord. The cavity in the 

 brain presents some marked differences to that of the cord ; while 



