Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/933

Rh BRAIN.] ANATOMY 875 The callosal convolution encloses the corpus callosum Within the concavity of its arch, and from its direction is appropriately called fornicatus (arch-shaped). The pos terior end of the callosal convolution curves downwards and then forwards, under the name of gyrus hippocampi, to the tip of the inner surface of the temporo-sphenoidal lobe. This gyrus is separated anteriorly by a narrow curved fissure called hippocampal fissure, from a white band, the tcenia hippocampi, which band possesses a free curved border, round which the pia mater and choroidal artery enter the lateral ventricle through the great transverse fissure of the cerebrum. The hippocampal fissure is con tinuous round the posterior end of the corpus callosum with the callosal fissure, and at the bottom of the hippo campal fissure the grey matter of the gyrus hippocampi terminates in a well-defined dentated border (fascia den- tata). The hippocampal fissure on this surface of the hemisphere marks the position of an eminence in the de scending coruu of the ventricle called hippocampus major. The gyrus hippocampi is separated posteriorly from the adjacent temporo-sphenoidal convolution by a fissure, named collateral, which marks the position on this surface of the hemisphere of the collateral eminence in the interior of the ventricle. From the lower end of the parieto-occipital fissure an offshoot, called the calcarine fissure, passes almost horizontally backwards in the occipital lobe, which fissure marks on this surface of the hemisphere the eminence named calcar avis, or hippocampus minor, in the posterior cornu of the ventricle. If a horizontal slice be removed from the upper, part of each hemisphere, the peripheral grey matter of the convo lutions will be seen to follow their various windings, whilst the core of each convolution consists of white matter con tinuous with a mass of white matter in the interior of the hemisphere. If a deeper slice be now made down to the plane of the corpus callosum, the white matter of that structure will be seen to be continuous with the white centre of each hemisphere. The corpus callosum does not equal the hemispheres in length, but approaches nearer to their anterior than their posterior ends (PL XVIII. fig. 3, B.) It terminates behind in a free rounded end, whilst in front it forms a knee-shaped bend, and passes downwards and backwards as far as the lamina cinerea. If the dissec tion be performed on a brain which has been hardened in spirit, the corpus callosum is seen to consist almost entirely of bundles of nerve fibres, passing transversely across the mesial plane between the two hemispheres ; these fibres may be traced into the white cores and grey matter of the convolutions, and apparently connect the corresponding convolutions in the opposite hemispheres. Hence the corpus callosum is a connecting or commissural structure, which brings the convolutions of the two hemispheres into anatomical and physiological relation with each other. On the surface of the corpus callosum a few fibres, the strica longitudinales, run in the antero-postericr or longitudinal FIG. 74. To show the right ventricle and the left half of the corpus callosnm. a, transverse fibres, and 6, longitudinal fibres of corpus callosum ; c, anterior, and d, posterior cornua of lateral ventricle; e, septum lucidum; /, corpus striatum ; &amp;lt;7, tsenia semicircularis ; h, optic thalamus; k, choroid plexus ; /, ta;nia hippocampi; m, hippocampus major; n, hippocampus minor; o, eminent in col- la teralis. direction. If the corpus callosum be now cut through on each side of its mesial line, the large cavity or lateral ventricle in each hemisphere will be opened into. The lateral ventricle is subdivided into a central space or body, and three bent prolongations or cornua; the anterior cornu extends forwards and outwards into the frontal lobe ; the posterior coi~nu curves backwards, outwards, and inwards into the occipital lobe ; the de scending cornu curves backwards, outwards, downwards, forwards, and inwards, behind and below the optic tha lamus into the temporo-sphenoidal lobe. On the floor of the central space may be seen from before backwards the grey upper surface of the pear-shaped corpus striatum, and to its inner and posterior part a small portion of the optic thalamus, whilst between the two is the curved flat band, the tcenia semicircularis. Resting on the upper surface of the thalamus is the vascular fringe of the velum interposi- turn, named choroid plexus, and immediately internal to this fringe is the free edge of the white posterior pillar of thefornix. The anterior cornu has the anterior end of the corpus striatum projecting into it. The posterior cornu has an elevation on its floor, the hippocampus minor, and between this cornu and the descending cornu is the eleva tion called eminentia collaterals. Extending down the descending cornu and following its curvature is the hippocampus major, which terminates below in a nodular end, the pes hippocampi ; on its inner border is the white tcenia hippocampi, continuous above with the posterior pillar of the fornix. If the tsenia be drawn on one side the hippocampal fissure is exposed, at the bottom of which the grey matter of the gyrus hippo campi may be seen to form a well-defined dentated border (the so-called fascia dentata). The choroid plexus of the pia mater turns round the gyrus hippocampi, and enters tho descending cornu through the great transverse fissure be tween the tsenia hippocampi and optic thalamus. The lateral ventricle is lined by a cylindrical endothelium,