784 THE CENTRAL AXIS OF TEE NERVOUS SYSTEM. 



On the superior aspect of the organ it is very deep, and when the two hemispheres 

 are separated to discover its extent, we see that it reaches to the upper face of the 

 great commissure — the corpus callosum. Behind, it curves between the posterior 

 lobes of the hemispheres, but without corresponding directly with the posterior, 

 thick, rounded margin of the coi'pus callosum, above which there is a feeble 

 adhesion between the two halves of the cerebrum, forming a kind of bridge. But 

 in front it passes to the anterior margin of this commissure, and is prolonged in 

 the interval between the anterior lobes of the hemispheres, to reach the inferior 

 face of the brain. 



Examined inferiorly, this interlobular fissure is well defined in front, where it 

 reaches the anterior border of the coi-pus callosum ; but behind, on leaving the 

 optic commissure — 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 spinal prolongation, at first crossing the optic nerve, 

 then turning round the crura cerebri and corpora bigemina, above which its 

 branches unite, and are confounded with the undivided part of the fissure that 

 separates the posterior lobes of the hemispheres (Fig. 424). 



There exists, then, around the point of entrance of the isthmus into the cere- 

 brum a well-marked line of demarcation, which constitutes — above and lateraUy — 

 a very deep fissure, in which is embedded the vascular expansion known as the 

 velum interpositum ; this space is designated the fissure of Bichat, or great {trans- 

 verse) cerebral fissure. 



The longitudinal 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. 



Preparation. — To see the fissures, sulci, and convolutions of the brain, the organ should be 

 macerated for some days in a solution of nitric acid (5 or 10 to 100 of water). On removing 

 it from this, the pia mater and vessels are carefully taken away and tlie brain gently dried, 

 when the grooves and fissures widen, and the lobes and lobules can be distinctly seen. 



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 — corresponds 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 



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