52 THE CEREBRUM. 



folds named convolutions, 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 con- 

 sequence of them, the gray substance is increased in bulk to 

 fifty-eight and one-half per cent, of the entire cerebrum (DeReg- 

 ibus). 



The name fissure is properly applied, first to those deep furrows 

 which represent clefts between embryonic vesicles, viz., the 

 median, verticle 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 poste- 

 rior surface of the medulla oblongata (Figs. 19 and 20); and, 

 second, the deep linear depressions in the cerebral hemisphere 

 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. 



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. 22, 26 and 27). 

 The basal surface comprises the orbital and tentorial areas, 

 separated by the stem of the fissura laterah's cerebri (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, 22 and 26). 



FISSURES AND SULCI OF CONVEX SURFACE. 



The convex surface of the cerebral hemisphere (fades convexa 

 cerebri) is related to two very extensive fissures, viz., the longit- 

 udinal and the transverse. The longitudinal fissure of the 

 cerebrum (fissura longitudinalis cerebri) is the vertical median 



