CALIFORN

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Fig. 101.-VIEW, FROM THE SIDE AND SLIGHTLY FROM ABOVE AND BEHIND, OF THE RIGHT HEMISPHERE OF A SIMPLY CONVOLUTED EUROPEAN BRAIN. (E. A. S.)

Fig. 102.-MESIAL ASPECT OF THE LEFT HEMISPHERE OF A SIMPLY CONVOLUTED EUROPEAN BRAIN. (E. A. S.

Fig. 103.-UNDER SURFACE OF A SIMPLY CONVOLUTED EUROPEAN BRAIN. (E. A. S.)

The above are depicted nearly of the natural size. They have been drawn from photographs, and are all taken from different specimens.

The sulci are marked with small italic letters; the gyri with italic capitals. The corresponding parts are marked similarly in the three figures.

Fig. 101. Sulci-Ro., Rolandic or central; g, its superior genu; Sy. a, anterior limb of Sylvian (a, ascending part, y, horizontal part); Sy. p, posterior limb of Sylvian; Sy. p. asc., ascending ramus of posterior limb; f1, superior frontal; ƒ,, inferior frontal; ƒ,, middle frontal; f, paramesial frontal; d, diagonal, placed in this instance rather low down, and communicating with the Sylvian; p.c. inf, inferior precentral; p. c.i. ant., its anterior ramus; p. c. sup., superior precentral; p.c.m, mesial precentral; p.c. tr., transverse precentral; rtc. tr., transverse retro-central; i.-p. inf, intra-parietal, pars inferior (inferior postcentral); i.-p. sup., intraparietal, pars superior (superior postcentral); i.-p. post. s. hor., intraparietal, pars posterior seu horizontalis; i.p. post., intraparietal, pars posterior (paroccipital of Wilder); i.-p. pr. asc., an ascending branch of the intraparietal; p.-0., parieto-occipital; occ. ant., anterior occipital; occ. lat., lateral occipital; calc., posterior end of calcarine; t1, first temporal or parallel; t, asc., its posterior ascending extremity, detached; t, second temporal; t, asc., its posterior ascending extremity joined to and apparently continuous with the first temporal.

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Gyri-F1, F F, first, second and third (superior, middle, and inferior) frontal; a, posterior part of third frontal; b, middle part (pars triangularis); c, orbital part; A.F., ascending frontal; A.P., ascending parietal; T1, T., T,, first, second, and third temporal.

Fig. 102.-Sulci-Ro., upper end of Rolandic; p.c.m., mesial precentral; f, mesial frontal; c. m., calloso-marginal; pr. l., prelimbic (anterior end of calloso-marginal); pr. l. asc., an ascending branch of the prelimbic; paracentr., paracentral (posterior end of calloso-marginal); p.l., postlimbic; ro, rostral; ro. inf., inferior rostral; p.-o., parieto-occipital; calc. ant., stem of calcarine; calc. post., posterior part of calcarine; 1, 2, 3, 4, places where annectent gyri occur in calcarine and parieto-occipital fissures; t,, third temporal; coll., collateral or fourth temporal; h (placed on the fascia dentata) has the hippocampal fissure just below it.

Gyri-F1, marginal part of first frontal; C, callosal (gyrus fornicatus); H, hippocampal; unc., its uncus;, dentate; T., fourth temporal (fusiform lobule); T, fifth temporal or infracalcarine (lingual lobule).

c.c, corpus callosum; spl., its splenium; g, its genu; r, its rostrum ; fo, fornix; fi., fimbria.

Fig. 103.-Sulci-orb., orbital (sagittal rami); o.tr., transverse orbital; ol., olfactory; t1, t1, t ̧, first, second, and third temporal; coll., collateral (fourth temporal); calc., calcarine.

Gyri-R, gyrus rectus; T,, T,, T, Ts, first, third, fourth and fifth temporal; H, hippocampal ; s.r.a., substantia reticularis alba; unc., uncus.

ch, chiasma; s.p.a., substantia perforata antica; t.c., tuber cinereum; m, corpora mamillaria. accidentally separated from one another in the preparation; or, crusta; tm, tegmentum; spl., splenium of callosum.

Fissure of Sylvius.-The most distinct fissure in the adult brain, and also the first one to make its appearance in the development of the embryo, is the fissure of Sylvius. This deep and conspicuous fissure is seen at the base of the brain, opening out as the vallecula Sylvii (Broca) on to the anterior perforated space. From this origin the fissure passes transversely outwards to the lateral surface of the hemisphere, where it presently gives off a short horizontal branch (about 2 centimeters long), and a rather longer ascending branch (about 3 centimeters), the fissure being then continued nearly horizontally as the posterior limb for a considerable distance (8 or 9 centimeters) obliquely backwards and upwards in the direction of the parietooccipital fissure, having a slightly curved course. It ends at about the junction of the middle and posterior thirds of the lateral surface (midway from superior to lateral border), usually by bifurcating, one branch passing obliquely upwards, the other backwards and somewhat downwards. If the lips of the Sylvian fissure are separated, the island of Reil is seen at the bottom of the sulcus. In section this fissure, together with the limiting sulcus of the island, forms a T-shaped figure (fig. 95). Its bounding convolutions are folded over the island of Reil, and the upper bounding convolutions which belong partly to the parietal, partly to the frontal lobe, form the operculum of Burdach.

The anterior horizontal and anterior ascending limbs of the fissure subdivide this operculum into three unequal parts, an upper or fronto-parietal (pars fronto-parietalis), a middle or frontal (pars triangularis, "cap" of Broca), and a lower or orbital (pars orbitalis) (fig. 101, a, b, c). All these are now often spoken of as opercula. The convolution bounding the Sylvian fissure below is also spoken of as the temporal operculum, so that four opercula are thus enumerated. They are formed in the embryo by the manner in which the mantle comes to overlap the central lobe. The temporal and fronto-parietal appear first, above and below the Sylvian fossa of the embryo: the frontal and orbital develope much later. The meeting of the four opercula determines the formation of the three limbs of the Sylvian fissure (fig. 104). The Sylvian fissure is usually longer in the left than in the right hemisphere; the difference being due to the greater development of the frontal operculum on the left side (fig. 106)

Fig. 104.-DIAGRAMS ILLUSTRATING THE MANNER IN WHICH THE INSULAR REGION IS COVERED IN BY THE VARIOUS OPERCULA. (Cunningham.)

The part printed black represents uncovered part of the Sylvian fossa of the embryo, the unshaded portions represent the different opercula.

F.P., fronto-parietal operculum; T., temporal operculum; F., frontal operculum (pars triangularis of third frontal gyrus); O, orbital operculum; p.h., posterior limb of fissure; a.a., anterior ascending limb; a.h., anterior horizontal limb.

In I, the fronto-parietal and temporal opercula are beginning to grow over the fossa; in II, the formation of all four opercula is advanced, the pars triangularis being well marked, and the anterior limb U-shaped. In III, the pars triangularis is only slightly developed, and the anterior limb is a Y; in IV the pars triangularis is absent and the anterior limb is simple (I-form).

(localisation of speech centre, Broca). The horizontal and ascending limbs of the anterior branch of the Sylvian fissure often arise from a common stem (Y-condition, fig. 101 and fig. 106, right side), the pars triangularis being then less developed than usual. This part may

even not form a distinct opercular projection, in which case the anterior limb of the Sylvian fissure is undivided (I-condition).

The anterior and ascending limbs arose by a common stem from the main fissure (Y form), in 32 p. c. of the hemispheres examined by Cunningham; independently (V and U form) in 375 p. c.; and as a single limb (1 form) in 30 p. c. There are sometimes two secondary extensions of the Sylvian fissure on to the orbital lobe (orbital limbs), but these are less deep and have not the same morphological importance as the others.

The posterior limb about corresponds in the adult to the level of the squamous suture, but in children younger than fifteen years it is rather above this level. The angle which the direction of the posterior limb forms with a line perpendicular to the superior border of the hemisphere (Sylvian angle), varies considerably, but is more acute on the whole in the child than in the adult (Cunningham).

In the brain of some monkeys, the posterior limb of the Sylvian fissure joins (superficially) the parallel fissure, which it gradually approaches, and the two are continued as an apparently