TEE CEREBRUM. 691 



where it attains the anterior border of the corpus callosum ; but behind, on 

 leaving the chiasma of the optic nerves, and which marks the anterior limit 

 of the isthmus, this fissure appears suddenly to stop. This is because it 

 becomes considerably enlarged, and is changed into a vast notch which 

 admits the anterior extremity of the isthmus : or rather, it bifurcates to pass 

 on each side between the hemisphere and the anterior extremity of the 

 medullary prolongation, at first crossing the optic nerve, then turning round 

 the cerebral peduncles and corpora bigemina, above which its branches 

 unite, and are confounded with the undivided part of the fissure, which 

 separates the posterior lobes of the hemispheres. 



There exists, then, around the point of immergence of the isthmus in the 

 cerebrum a well-marked line of demarcation, which constitutes, above and 

 laterally, a very deep fissure in which is imbedded the vascular expansion 

 known as the velum interpositum ; this aperture is designated the fissure of 

 Biclidt, or great (transverse) cerebral fissure. 



The interlobular fissure receives the longitudinal septum of the dura 

 mater or falx cerebri. It also lodges arteries and veins, among which it 

 is necessary to distinguish the great vena Galeni, which ascends from the 

 bottom of the fissure, after passing round the posterior border of the corpus 

 callosum. 



2. The Cerebral Hemispheres. 



Each hemisphere or lateral moiety of the cerebrum, represents an ovoid 

 segment, in which we may consider four faces and two extremities. 



The superior face is convex, and is covered by the roof of the cranium, 

 which is formed by the frontal and parietal bones. 



The external, equally convex and insensibly confounded with the 

 adjacent faces, responds to the lateral walls of this cavity : that is, with the 

 squamous portion of the temporal bone, the parietal and frontal bones, and 

 the ala of the sphenoid. 



The inferior, irregularly mammillated, rests on the sphenoid bone. The 

 internal is plane, and for the greater part of its extent is related to the other 

 hemisphere through the medium of the falx cerebri ; it is in its central and 

 inferior portion that the union of the two halves of the cerebrum takes place, 

 by means of the great cerebral commissure and the anterior extremity of the 

 isthmus. 



The posterior extremity of the hemisphere corresponds to the cerebellum, 

 which slightly depresses it, and from which it is separated by the transverse 

 septum of the dura mater (tentorium). 



The anterior extremity or lobe is lodged in the fossa formed on each side 

 of the crista galli by the frontal and sphenoid bones. 



The anatomical peculiarities to be found in these different regions of the 

 external surface of the hemisphere are : 1, On the inferior face, and from 

 before to behind, a detached appendage constituting the olfactory orethmoidal 

 lobule, a transverse groove named the fissure of Sylvius, and an elongated 

 eminence called the mastoid lobule ; 2, Everywhere else, the cerebral convolu- 

 tions depressed elevations curved about in a thousand ways, and separated 

 by sulci of varying depth. 



We will study these peculiarities in an inverse order to that of their 

 enumeration. 



1. CEREBRAL CONVOLUTIONS (Figs. 321, 322, 327). The cerebral convo- 

 lutions are constituted by the folding of the external surface of the brain, 

 apparently with the intention of considerably augmenting the extent of that; 



