932 GENERAL STRUCTURE. [BOOK in. 



vesicle is not uniform in all parts. At an early period there may 

 be observed in the ventral wall or floor of the vesicle a thickening, 

 which assuming a special, more or less semilunar, form arid pro- 

 jecting 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 crus cerebri 

 passing between the two, and also as we shall see dividing* the 

 corpus striatum into two bodies, called the nucleus caudatus and 

 nucleus lenticularis. 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 de- 

 velopment 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 the 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 ventricles. The band is not 

 flat but curved ventral wards; hence in a longitudinal vertical section 

 of the brain taken in the middle line it presents a curved form with 

 the concavity directed ventralwards. While this dorsal portion of 

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

 the ventral 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 forwards, 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 



