INTERIOR OF THE CEREBRUM. 659 



of a darker color, while its convolutions are smaller and narrower, and arranged 

 like the leaves of a book, and hence called folia. 



General Arrangement of the Parts Composing the Cerebrum. Each hemisphere, 

 as already stated, consists of a central cavity, the lateral ventricle, surrounded by 

 thick and convoluted walls of nervous tissue. 



Interior of the Cerebrum. If the upper part of either hemisphere is removed 

 with a knife, about half an inch above the level of the corpus callosum, its internal 

 white matter will be exposed. It is an oval-shaped centre, of white substance, 

 surrounded on all sides by a narrow convoluted margin of gray matter, which 

 presents an equal thickness in nearly every part. This white central mass has 

 been called the centrum ovale minus. Its surface is studded with numerous 

 minute red dots (pnncta vasculosa], produced by the escape of blood from divided 

 blood-vessels. In inflammation or great congestion of the brain these are very 

 numerous and of a dark color. If the remaining portion of one hemisphere is 

 slightly separated from the other, a broad band of white substance will be observed, 

 connecting them at the bottom of the longitudinal fissure ; this is the corpus 

 callosum. The margins of the hemispheres which overlap this portion of the brain 

 are called the labii cerebri. Each labium is part of the callosal convolution 

 already described ; and the space between it and the upper surface of the corpus 

 callosum is termed the callosal fissure (Fig. 348). The hemispheres should now 

 be sliced off to a level with the upper surface of the corpus callosum, when the 

 white substance of that structure will be seen connecting the two hemispheres. 

 The large expanse of medullary matter now exposed, surrounded by the con- 

 voluted margin of gray substance, is called the centrum ovale majus of Vieussens 

 (Fig. 352). 



The Corpus Callosum. The corpus callosum is a thick stratum of transversely 

 directed nerve-fibres, by which probably almost every part of one hemisphere is 

 connected with the corresponding part of the other hemisphere. The fibres of 

 this body, when they pass from it into the hemispheres radiate in various direc- 

 tions, to terminate in the gray matter of the periphery. It thus connects the two 

 hemispheres of the brain, forming their great transverse commissure, and at the 

 same time roofs in the lateral ventricles. The best conception of its size and 

 form is obtained by making an anterior posterior vertical section through the 

 centre of the brain (Fig. 353). It is then seen to be a long, thick, irregularly 

 flattened arch ; in front taking a sharp bend, the genu, and dipping downward 

 and backward to the base of the brain by a reflected portion, the rostrum, which 

 is connected with the lamina cinerea ; behind it terminates by a rounded end, 

 which is folded over and is named the splenium. It is about four inches in length, 

 and extends to within an inch and a half of the anterior, and two inches and a 

 half of the posterior extremity of the cerebrum. It is somewhat broader behind 

 than in front, and is thicker at either end than in its central part, being thickest 

 behind. The reflected anterior portion of the corpus callosum is called the beak or 

 rostrum ; it becomes gradually thinner as it descends, and is attached by its lateral 

 margins to the frontal lobes. At its termination, in addition to joining the lamina 

 cinerea, the corpus callosum gives off two bands of white substance, the peduncles 

 of the corpus callosum, already described (page 656). 



Posteriorly, the corpus callosum forms a thick rounded fold, called the splenium 

 or pad, which is free for a little distance as it curves forward, and is then contin- 

 uous by its under surface with the fornix. The splenium overlaps the mesencepha- 

 lon, but is separated from it by the pia mater, which is prolonged forward to form 

 the velum interpositum. On its upper surface, the structure of the corpus callo- 

 sum is very apparent, being collected into coarse transverse bundles. Along the 

 middle line is a longitudinal depression, the so-called raphe, bounded laterally by 

 two or more slightly elevated longitudinal bands, called the strice longitudinales or 

 nerves of Lancisi ; and, still more externally, other longitudinal strire are seen, 

 beneath the callosal convolutions. These are the stria' longitudinales laterales, or 

 tcenia tectce. On each side of the middle line the under surface of the corpus cal- 



