114 ANTHROPOLOGY. 



The posterior segment, as occupying tlie posterior fossa of tlie cranium, is 

 on a level considerably below that of the middle segment. A part worthy 

 of especial notice is the pons Varolii^ which is situated immediately behind 

 the interpeduncular space, the crura cerebri appearing to emerge just above 

 its anterior border. From its posterior edge the medulla oblongata extends 

 downwards and a little backwards, occupying a notch or depression between 

 the lobes of the cerebellum. The fibres of the pons are seen passing out- 

 wards and backwards into each hemisphere of the cerebellum, forming the 

 inferior layer of each crus cerebelli. 



2. Dissection of the Encephalon. If we make a horizontal secti on 

 of one hemisphere, nearly on a level with the corpus callosum, we shall per- 

 ceive what is known as the centrum minus ovale, a mass of white fibrous 

 substance surrounded by an irregularly undulating line of grey ; a second 

 horizontal section passing through both hemispheres on a level with the 

 corpus callosum presents the centrum magnum ovale, a line of grey sub- 

 stances surrounding the central mass of white. The coipus callosum, or 

 superior cerebral commissure, will now be seen in the middle line of the 

 cerebrum, between three and four inches long, marked by two or three 

 raised longitudinal lines nearly parallel to each other (the raphe); from 

 these, several transverse lines pass to either side. The posterior end of the 

 corpus callosum is bent downwards above the transverse fissure, and is con- 

 tinuous with the fornix and hippocampi ; anteriorly it is continued into the 

 anterior lobes : in the middle it joins the tuber cinereum and the optic com- 

 missure. It connects the white fibrous substance of both hemispheres, and 

 covers like an arch the lateral ventricles, the septum lucidum, and the 

 fornix. 



On dividing the corpus callosum at a little distance from either side of 

 the raphe, the lateral ventricles or cavities of the brain will be opened, one 

 on each side, and separated by the septum lucidum. This consists of four 

 laminee on each side: 1. The lining membrane of the ventricle; 2. A 

 thin grey layer; 3. A compact white fibrous layer; 4. A delicate mem- 

 brane which lines the small cavity existing in it, and termed the fifth 

 ventricle. 



The lateral ventricles extend from the middle of the brain into the 

 anterior and posterior lobes, also into the inferior part of the middle lobe ; 

 each has, therefore, three cornua. The anterior cornu is bounded superiorly 

 and laterally by the corpus callosum, and inferiorly by the large extremity 

 of the corpus striatum. The middle or body of each ventricle is bounded 

 superiorly and externally by the corpus callosum ; internally by the septum 

 lucidum, and inferiorly by the posterior extremity of the corpus striatum, 

 the lamina cornea, the taenia semicircularis, the optic thalamus, the choroid 

 plexus, and the fornix. The posterior cornu is bounded superiorly and 

 laterall}^ by white substance, and inferiorly by the hippocampus minor. 

 The inferior cornu is bounded superiorly by the optic thalamus and corpus 

 striatum ; inferiorly by the hippocampus major and corpus fimbriatum or 

 taenia hippocampi, over which the choroid plexus is folded. 



In making a brief individual reference to the different bodies observed in 

 820 



