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Surface of

medial, basal, border and margo

SURFACE OF The exterior surface of the fore-brain is divided by distinct borders into three regions, namely, the convex surface, the medial surface, and the basal surface (Figs. 26, 31 and 33). The basal surface comprises the orbital and tentorial areas, separated by the stem of the fissura cerebri lateralis (Sylvii). The convex surface is separated from the medial surface by the supero-medial border (margo supero-medialis), from the tentorial area of the basal surface by the infero-lateral border (margo infero lateralis, or m. occipitalis lateralis), and from the orbital area of the basal surface by the superciliary border (margo superciliaris). The medial orbital border (margo orbitalis medialis) separates the orbital area of the basal surface from the medial surface, and the medial occipital border (margo occipitalis medialis) divides the medial surface from the tentorial area of the basal surface (Figs. 19, 26 and 31).

The surface of the fore-brain is composed of a thin sheet of gray matter varying in thickness from 1.5-5 mm. (little less than one-sixteenth to slightly more than one-fifth of an inch). That gray matter forms a bark-like covering for the underlying white substance and is, therefore, called the cortex (Figs. 42 and 46). It is thrown into irregular elongated folds named con4 volutions, or gyri, by deep linear depressions, which greatly increase the relative amount of cortical substance. The linear depressions are called fissures, or sulci; and, in consequence of them, the gray substance is,increased in bulk to 58;2 per cent.

of the entire cerebrum (DeRegibus).

The name fissure is properly applied, first to those deep furrows which represent clefts between embryonic vesicles, viz., the median, vertical cleft between the cerebral hemispheres; and the two arched clefts, one between the cerebellum and the cerebral hemispheres and the other between the cerebellum and the posterior surface of the medulla oblongata (Figs. 19 and 2o); and, second, the deep linear depressions in the cerebral hemi sphere which indent the entire ventricular wall and produce eminences on the interior surface are properly called fissures. All other furrows in the cerebral surface are called sulci.

A satisfactory explanation for the folding of the cortex into gyri has not been found. The folding is rendered necessary by the area of the cortex, since it has three times the extent of the free, exposed surface of the cerebrum; but this does not explain the permanence of pattern in sulci and gyri which results from such folding. The production of cerebral gyri must be a posi tive process based upon developmental factors not yet under stood. Through a long phylogeny the functional demands of different areas of cortex have gradually built up these factors of development and they have been rendered permanent by the persistence of the same functional demands.