CHAP. IL] THE BRAIN. 931 



now known as the lateral ventricles. The growth of the cerebral 

 hemispheres is not only much greater than that of the rest of the 

 brain, but also takes place in a special manner. At their first 

 appearance the cerebral hemispheres lie wholly in front of the 

 fore-brain or vesicle of the third ventricle, but in their subsequent 

 growth while expanding in nearly all directions they extend 

 especially backwards. Thus in the adult brain, on the dorsal 

 surface they not only completely cover up the third ventricle but 

 also overlap the mid-brain, reaching so far back as to cover the 

 front border of the cerebellum, while on the ventral surface, though 

 in the middle line they leave exposed the floor or ventral 'portions 

 of the walls of the third ventricle, at the sides they are seen to 

 reach as far backward as on the dorsal surface. The median furrow 

 on the dorsal surface which separates each hemisphere from its 

 fellow is at first shallow but rapidly deepens, so that as the hemi- 

 spheres grow they become separated from each other by a narrow 

 deep longitudinal fissure, into which as we shall see a fold of the 

 dura mater dips. This fissure is not only deep vertically, i.e. from 

 the dorsal surface ventrally, but at the front of the brain runs 

 backward in the middle line almost as far as the level of the third 

 ventricle, so as completely to separate from each other the anterior 

 parts of each hemisphere, known as the anterior lobes; at the 

 back of the brain also it similarly runs forward in the middle line 

 for a considerable distance, so as to separate from each other the 

 posterior lobes. Hence the two great masses of the cerebral 

 hemisphere are united with each other, not along their whole 

 length but for about a third of that length, the isthmus or bridge 

 thus connecting them lying at some depth below the dorsal 

 surface at the bottom of the longitudinal fissure, in about the 

 middle third of its length. 



At its first appearance each lateral ventricle is of a more or less 

 oval form, its walls are of uniform thickness, and it lies in front of 

 the third ventricle. During the growth of the hemispheres it 

 acquires a peculiar shape and becomes divided into an anterior 

 cornu or horn stretching into the anterior portion, a posterior 

 horn stretching into the posterior portion, and a descending horn, 

 which curves laterally and ventrally into the middle portion of the 

 hemisphere ; owing to the great backward extension of the hemi- 



rsres the lateral ventricles come to lie not only in front of but 

 at the side of, and indeed, to a certain extent, above or dorsal 

 to the third ventricle ; and during the growth of the parts the 

 originally wide Y-shaped opening which placed the hind ends of 

 the two lateral ventricles in communication with the front of the 

 third ventricle becomes narrowed into a slit-like passage of similar 

 form, the foramen of Monro, which still opening into the front of 

 the third ventricle, now leads on each side from a point rather in 

 front of the middle of the lateral ventricle. 



As the hemisphere enlarges the growth of the walls of the 



