586 THE BRAIN. 



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 sim- 

 ilar 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 vesicle is not 

 uniform in all parts. At an early period there may be observed in the ven- 

 tral wall or floor of the vesicle a thickening, which assuming a special, more 

 or less semilunar, form and projecting into the cavity becomes the body 

 known as the corpus striatum. As development proceeds the corpus striatum 

 on each side becomes attached to the optic thalamus, lying behind and to the 

 median side of itself, the radiating fibres of the cms cerebri passing between 

 the two, and also as we shall see dividing the corpus striatum into two bodies, 

 called the nucleus caudatas and nucleus lenticular is. A notable result of this 

 growth and change of position of the hemispheres and of the coalescence of 

 the corpus striatum with the optic thalamus is that the latter body, though 

 really belonging to the third ventricle, comes to project somewhat into the 

 lateral ventricle ; a strip of the upper surface of the optic thalamus, along 

 its outer, lateral edge, forms a portion of the floor of the lateral ventricle in 

 the median region on each side of the third ventricle. Besides this special 

 development of the corpus striatum, the walls of each vesicle, with the 

 exception of the median part by which the two vesicles coalesce with each 

 other, become (we are now speaking of the higher mammals) thickened 

 much in the same way all over, the surface being folded so as to give rise to 

 convolutions or gyri separated by furrows or sulci; and the thickening 

 taking place in such a way as to give the ventricle its peculiar shape. The 

 median coalesced part undergoes a different and peculiar change. This part, 

 which at first lies in front of the third ventricle, through the changes 

 brought about by the growth of the hemispheres so shifts its position as to 

 lie immediately over, dorsal to third ventricle, very much as if this part of 

 the cerebral vesicles had been folded back over the fore-brain. In the 

 junction itself we may distinguish a dorsal and a ventral portion. The 

 dorsal portion is developed into a system of transverse commissural fibres 

 passing across from one hemisphere to the other. In the median region these 

 fibres form a thick compact band, called the corpus callosum, which may be 

 exposed to view at the bottom of the longitudinal fissure, while on each side 

 they spread away in all directions to nearly all parts of the surface of the 

 hemispheres, passing over and helping to form the roof of the lateral ven- 

 tricles. The band is not flat but curved ventralward ; hence in a longitudi- 

 nal vertical section of the brain taken in the middle line it presents a curved 

 form with the concavity directed ventralward. While this dorsal portion of 

 the junction is developed at the sides as well as in the middle line, the ven- 

 tral portion is developed in the median region only, and that in a special 

 way, so that it forms below, ventral to, the corpus callosum an arched plate, 

 in the shape of a triangle with the apex directed forward, called the fornix, 

 which lies immediately above the thin epithelial roof of the third ventricle. 

 In front, the narrower apical portion of the fornix lies at some little distance 

 below, ventral to, the corpus callosum, and here the junction between the 

 two vesicles is reduced to a thin sheet, the septum lucidum; but behind, the 

 broader basal portion of the fornix is arched up so as to lie immediately 

 under and touch the corpus callosum. Hence the septum lucidum has the 

 form of a more or less triangular vertical sheet, broad in front and narrow- 

 ing behind, separating the two lateral ventricles. The sheet may be con- 

 ceived of as being double and formed by the apposition of two layers, one 

 belonging to each ventricle ; between these two layers is developed a narrow 



