590 NERVOUS SYSTEM. 



fasciculus, these fibres consisting of some derived from the same, and others 

 from the opposite lateral tract of the medulla. More deeply, are a layer of 

 finer fibres, mixed with gray matter, derived from the cerebellum, blended with 

 the former. The cerebral surface of the crus cerebri is formed of these fibres, 

 and is named the tegrnentum. 



The locus niger is a mass of gray matter, situated between the superficial and 

 deep layer of fibres above described. It is placed nearer the inner than the 

 outer side of the crus. 



The posterior lobes of* the cerebrum are concealed from view by the upper 

 surface of the cerebellum, and pons Varolii. When these parts are removed, 

 the two hemispheres are seen to be separated by the great longitudinal fissure, 

 this fissure being interrupted, in front, by the posterior rounded border of the 

 corpus callosum. 



GENERAL ARRANGEMENT OF THE PARTS COMPOSING THE CEREBRUM. 



As the peduncles of the cerebrum enter the hemispheres, they diverge from 

 one another, so as to leave an interval between them, the interpeduncular space. 

 As they ascend, the component fibres of each pass through two large masses 

 of gray matter, the ganglia of the brain, called the thalamus opticus and corpus 

 striatum, which project as rounded eminences from the upper and inner side of 

 each peduncle. The hemispheres are connected together, above these masses, 

 by the great transverse commissure, the corpus callosum, and the interval left 

 between its under surface, the upper surface of the ganglia, and the parts 

 closing the interpeduncular space, forms the general ventricular cavity. The 

 upper part of this cavity is subdivided into two, by a vertical septum, the 

 septum lucidum; and thus the two lateral ventricles are formed. The lower 

 part of the cavity forms the third ventricle, which communicates with the lateral 

 ventricles, above, and with the fourth ventricle, behind. The fifth ventricle is 

 the interval left between the two layers composing the septum lucidum. 



INTERIOR OF THE CEREBRUM. 



If the upper part of either hemisphere is removed with a scalpel, 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 

 (puncta vasculosa), produced by the escape of blood from divided bloodvessels. 

 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 sepa- 

 rated from the other, a broad band of white substance will be observed con- 

 necting 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 labta cerebri. Each labium is part of the convolution of the 

 corpus callosum (gyrus fornicatus), already described; and the space between it 

 and the upper surface of the corpus callosum, has been termed the ventricle of 

 the corpus callosum. 



The hemispheres should now be sliced off, to a level with the corpus callosum, 

 when the white substance of that structure will be seen connecting together 

 both hemispheres. The large expanse of medullary matter now exposed, sur- 

 rounded by the convoluted margin of gray substance, is called the centrum ovale 

 majus of Vieussens. 



The corpus callosum is a thick stratum of transverse fibres, exposed at the 

 bottom of the longitudinal fissure. It connects the two hemispheres of the 

 brain, forming their great transverse commissure ; and forms the roof of a space 

 in the interior of each hemisphere, the lateral ventricle. It is about four inches 



