STRUCTURE OF THE BRAIN. 585 



wrapped round by and interlaced with the others. At the front margin of 

 the pons these longitudinal fibres, augmented in number, appear as two thick 

 strands, the crura cerebri, forming the floor of the mid-brain, the roof of 

 which is thickened into the corpora qiiadrigemina, and the cavity of which 

 is reduced to a narrow tubular passage, the aqueduct of Sylvius, or iter a tertio 

 ad quartum ventriculum. 



At the level of the fore-brain the crura cerebri, diverging rapidly from 

 each other as they pass forward, leave the median portion of the floor of the 

 vesicle now known as the third ventricle very thin, but form, especially behind 

 and ventrally, thick lateral walls, which are further increased in thickness 

 by the development on each side of a mass largely composed of gray matter, 

 known as the optic thalamus. The roof of the third ventricle, like that of 

 the fourth ventricle, is not developed into nervous elements, but remains 

 extremely thin, and consists of nothing more than a single layer of epi- 

 thelium. 



515. In front of the third ventricle each diverging crus cerebri spreads 

 out in a small radial fashion into the corresponding half of the paired vesicle 

 of the cerebellum now developed into the preponderant cerebral hemispheres, 

 the two cavities of which are 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 expand- 

 ing in nearly all directions, they extend especially backward. 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 ventricle 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 sur- 

 face which separates each hemisphere from its fellow is at first shallow, but 

 rapidly deepens, so that as the hemispheres 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 ver- 

 tically 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 sur- 

 face 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 ven- 

 tricle. 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 descend- 

 ing horn, which curves laterally and ventrally into the middle portion of the 

 hemisphere ; owing to the great backward extension of the hemispheres the 

 lateral ventricles come to lie not only in front of but also at the side of, and 

 indeed, to a certain extent, above or dorsal to the third ventricle ; and dur- 

 ing the growth of the parts the originally wide Y-shaped opening which 



