520 NERYOUS SYSTEM. 



the optic tract, which is adherent by its upper border to the peduncle. In its 

 interior is contained a mass of dark-gray matter, called the locus niger. The third 

 nerves may be seen emerging from the inner side of either crus ; and the fourth 

 nerve winding around its outer side from above. 



Each crus consists of a superficial and deep layer of longitudinal white fibres, 

 continued upwards from the pons, separated by a mass of gray matter, the locus 

 niger. 



The superficial longitudinal fibres are continued upwards, from the anterior 

 pyramids to the cerebrum. They consist of coarse fasciculi, which form the free 

 part of the crus, and have received the name of the fasciculated portion of the 

 peduncle or crus. 



The deep layer of longitudinal fibres is continued upwards, to the cerebrum, 

 from the lateral and posterior columns of the medulla, and from the olivary fasci- 

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

 opposite lateral tract of the medulla. More deeply, is 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 tegumentum. 



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 this body. 



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

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

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

 fissure being arrested, 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 thalaini optici and corpora striata, 

 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 this 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, the 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 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 



