of the parietal. Behind and above it is continuous with part of the parietal lobe. Behind, it is separated from the occipital lobe by the lateral occipital sulcus. It is somewhat pyramidal in shape, having three surfaces, viz., lateral, superior and inferior, the superior concealed within the fissure of Sylvius. The under surface adjoins the limbic lobe, being separated from the hippocampal gyrus of that lobe by the collateral fissure. The rounded apex of the pyramid is free, is directed forwards, and lies underneath the orbital surface of the frontal lobe, from which it is separated by the broad commencement of the Sylvian fissure.

GYRI AND SULCI OF THE UPPER SURFACE.-The upper surface of this lobe, directed towards the insula and fronto-parietal operculum, is marked by two or three transverse temporal gyri (Heschl). On the nearly smooth part of the surface in front of these are three short gyri (m, fig. 108) separated by shallow sulci, which are directed from the gyrus longus insulæ towards the tip of the temporal lobe.

SULCI AND GYRI OF THE LATERAL AND UNDER SURFACES.-On these surfaces there are four sulci with an antero-posterior direction. The first or superior temporal sulcus, also termed from its relation to the Sylvian fissure the parallel fissure (fig. 101, t), is an important furrow, appearing in the sixth month, and being constant in the Primates. In many monkeys it is more extensive than in man, nearly reaching the parieto-occipital fissure at the margin of the hemisphere. The angular gyrus curves round its extremity. Both this and the second temporal turn upwards as they pass back. The upturned extremity of the second sulcus may appear as a continuation of the parallel, as in the brain here figured (fig. 101). The fissure is sometimes interrupted by a small gyrus connecting the convolutions above and below it. The second temporal sulcus (t2) runs parallel to and below the last, but is less constant in extent and direction. It is often interrupted by one or more vertical connecting gyri. The postparietal gyrus curves round its extremity. The third temporal sulcus (1) is seen on the under surface of the lobe, extending behind nearly to the occipital pole. It is often interrupted near its anterior and posterior ends, and sometimes also about the middle by annectent gyri. In front it generally fails to reach the extremity of the lobe, which forms a smooth "pole" from which most of the sulci diverge. A fourth temporal sulcus (t) is formed by the collateral fissure, which has already been noticed (p. 145) (figs. 101, 103).

On the lateral surface of the lobe three convolutions can usually be distinguished. The first or superior temporal gyrus (fig. 101, F) bounds the posterior limb of the Sylvian fissure below, and is continuous behind with the supramarginal and to a less extent with the angular convolution of the parietal lobe. The second temporal gyrus (T) is continuous with the angular and postparietal gyri. The third temporal gyrus (T,) is continuous with the occipital lobe by the annectent gyrus which passes between the calcarine and lateral occipital sulci. On the under surface of the lobe is the fourth temporal gyrus, lying between the third temporal sulcus and the collateral fissure. The second and third gyri are convolutions of some thickness, and are a good deal interrupted by transverse and oblique secondary sulci. The fourth, which was formerly known as the first occipito-temporal, is narrower, and more sharply marked off by its bounding sulci. Its posterior part, which has been termed the fusiform lobule, is usually described as belonging to the occipital lobe (see previous page). Lastly, between the posterior part of the collateral and the calcarine fissure is a fifth temporal gyrus, which was termed by Huschke the lingual lobule, but it is better termed infracalcarine gyrus (subcalcarine gyrus of Wilder). This is continuous in front with the hippocampal gyrus of the limbic lobe (see below). It is joined by two deep annectent gyri, which pass across the posterior calcarine fissure, connecting this gyrus with the cuneus (fig. 102, 2 and 3).

The temporal lobe lies in the middle fossa of the skull, and its posterior part overlies the tentorium.

THE CENTRAL LOBE OR ISLAND OF REIL.-This lobe is entirely concealed in the adult human brain within the fissure of Sylvius, the margins of which form opercula (see p. 142) which overlap the lobe. The central lobe corresponds with the corpus striatum on the interior of the hemisphere (fig. 88), and probably in consequence of its close connection with this mass of grey matter, which appears early as a thickening of the lateral wall of the vesicle of the cerebral hemisphere (vide Embryology, Vol. I., p. 69), this particular part of the hemisphere does not keep pace with the general expansion which the hemisphere-vesicles undergo, and hence in consequence of that expansion it becomes overlapped and concealed by the rest of the hemisphere. On account of this early fixation and the close topographical relation between it and the largest of the basal ganglia, the central lobe (along with the basal ganglia and the continuation of the peduncles between them) is frequently spoken of as the stem (German, Stammtheil) of the hemisphere, the remainder of the cerebrum, which covers it in, being known collectively as the mantle.

To see the island it is necessary to cut away the opercula (fig. 108). It then appears as a triangular surface somewhat bulged outwards, the base of the triangle

(Eberstaller.)

8.C., sulcus centralis insulæ ; 1, 2, 3, gyri breves; 4, 5, gyrus longus; s. R.a., s. R.s., s. R.p., anterior, superior, and posterior limiting sulci; 7, limen insulae ; F, orbital part of third frontal gyrus; T, Ta, first and second temporal gyri; .y, upper transverse temporal gyri; g.tr.i.. gyrus transversus insulæ, passing at a, into the orbital part of the third frontal; b, connection of gyrus longus insula with apex of temporal lobe; m, short gyri on the upper surface of the temporal pole.

being directed upwards and the apex being at the vallecula Sylvii. This surface is marked out by shallow sulci, which have a fan-like arrangement converging from the base towards the apex of the triangle, into several straight gyri having a similar convergent course. One of these sulci, which is deeper and appears earlier than the rest, and is also more constant in lower Primates, has been termed the sulcus centralis insula (Guldberg) (fig. 108, s.c.), and this serves to subdivide the lobe into two parts, a precentral and postcentral lobule. Since the line of direction of this sulcus nearly corresponds with that of the fissure of Rolando in the mantle, the pre- and post-central parts of the island similarly correspond to the frontal and parietotemporal lobes of the mantle; and they are in fact in continuity with the parts of those lobes which form the opercula. The island is, however, separated from these adjacent parts by a sulcus (sulcus limitans insula) which almost entirely surrounds it, and which is itself formed of an anterior, a superior, and a posterior part (fig. 108). The anterior and posterior parts of the limiting sulcus are, however, deficient near the apex of the lobe, so that there here occurs a direct continuity (by a small convolution, the gyrus transversus insula of Eberstaller) between the orbital part of the third frontal convolution as it dips round the margin of the Sylvian fossa, and the precentral lobule on the one hand, and between the limbic lobe and the extremity of the postcentral lobule on the other hand.

The precentral lobule of the insula is formed of a few short convolutions (gyri breves) (fig. 108, 1, 2, 3) converging from the base of the triangle but falling short of the apex of the lobule, which is smooth and forms a slight prominence (pole of the island, p). These convergent gyri are usually three in number (anterior, middle, and posterior), being subdivided in this manner by secondary and somewhat shallow sulci, having the same direction. The most marked of these sulci lies between the middle and posterior gyri breves, and has been named sulcus precentralis insula. A fourth small gyrus is described by Eberstaller as lying deeply underneath the orbital operculum. This he has termed the gyrus brevis accessorius. This accessory gyrus, and usually also the anterior of the three gyri breves, look forwards, lying upon an anterior surface of the insula, which is sometimes nearly at a right angle with the lateral or general surface, in other brains hardly marked off from it. Its grey matter is continued over the anterior limiting sulcus into that of the posterior orbital gyrus. The grey matter of the rest of the precentral lobule is continuous superiorly with that of the frontal lobe, inferiorly with that of the anterior perforated space. The inferior transitional part has been termed the threshold of the island (limen insula, Schwalbe).

The postcentral lobule (gyrus longus insula of Giacomini (fig. 108, 4, 5)) is also often subdivided at its upper end by a longitudinal furrow (sulcus postcentralis insula) into two parts, anterior and posterior. Its grey matter is continuous below with the tip of the hippocampal gyrus, laterally with that of the first temporal gyrus, above and posteriorly over the limiting sulci with the grey matter of the parietal and temporal opercula.

The three principal furrows of the insula (sulcus centralis, s. precentralis, and s. postcentralis insula), which radiate from the vallecula Sylvii, have been compared with the three similarly radiating fissures of the mantle, viz., the fissure of Rolando, the precentral fissure, and the intraparietal fissure. They are not, however, directly continuous with those, being arrested at the sulcus limitans insulæ. Sometimes the fissure of Rolando is continued by a small sulcus (transverse precentral) almost into the s. centralis insulæ. Below and anteriorly the sulcus centralis insulæ comes into close relationship with the extremity of the callosomarginal sulcus.

LIMBIC LOBE.-The callosal gyrus, with its continuation the hippocampal gyrus, were separated by Broca, on morphological grounds, from the other parts of the hemisphere, as a distinct lobe, intimately united in front and below with the olfactory lobe; and the name of grande lobe limbique was given by him to these combined lobes. To this lobe of Broca, Schwalbe added the other central parts of the mesial wall of the hemisphere, viz., the lamina septi lucidi and the dentate convolution, as well as the fornix; looking upon these as representing an inner encircling convolution concentric with the gyrus fornicatus and uncinatus, and naming the whole lobe thus reconstituted, the "falciform lobe," the olfactory lobe being, however, excluded.

To the parts included by Schwalbe there must now be added a rudimentary supracallosal gyrus (represented in man principally by the longitudinal striæ of the corpus callosum), which is continuous with the dentate convolution posteriorly, and with the rudimentary gyrus geniculi (see p. 158) anteriorly. The lobe must further include the peduncles of the corpus callosum (see p. 127), which appear to represent a rudimentary gyrus (gyrus subcallosus, Zuckerkandl), and also another rudimentary gyrus underneath the corpus callosum, partly imbedded in and intimately connected with the fornix, to which the name of gyrus infracallosus or gyrus fornicis may be applied.

As thus constituted the limbic lobe is bounded circumferentially mainly by two important fissures, the calloso-marginal above, and the anterior part of the collateral below, while the less constant postlimbic sulcus separates it behind from the parietal lobule, and the anterior end of the calcarine cuts into it just below the splenium.

The circumferential boundary is least distinct opposite the quadrate lobule, from which it is only separated by the small postlimbic sulcus, which in man is only occasionally continued into the calloso-marginal. But in many animals the fissure which forms the external boundary is uninterrupted or nearly so. and forms the limbic fissure of Broca. This might perhaps more appropriately be termed the outer limbic fissure, since the callosal sulcus and the hippocampal fissure form together a second crescentic fissure within the limbic lobe, which may be distinguished as the inner limbic fissure. The internal limit of the lobe is formed by the fissura choroidea (Schwalbe), through which the choroid plexus is invaginated into the

lateral ventricle. The lobe is pierced by the great commissure of the hemispheres (the corpus callosum) and also by the anterior commissure. It is connected by annectent gyri with the 1.ngual gyrus of the temporal lobe (see p. 153), with the cuneus of the occipital lobe (by a

gyrus hidden within the internal parieto-occipital fissure), with the quadrate lobule, with the orbital part of the frontal lobe, with the anterior part of the temporal lobe, and with the postcentral lobule of the insula.

Each end of this crescentic lobe is connected with one of the roots of the olfactory tract, viz., the commencement of the gyrus fornicatus with the mesial root and the termination of the hippocampal gyrus with the lateral root.

CONVOLUTIONS AND FISSURES IN THE LIMBIC LOBE (fig. 102).-Gyrus fornicatus, gyrus cinguli, callosal gyrus. This is an extensive convolution which lies between the marginal gyrus of the frontal lobe and the quadrate lobule of the parietal lobe above and the corpus callosum below. The convolution sweeps round the corpus callosum, beginning below its rostrum, where it abuts on the subcallosal gyrus, and ending a little below the level of the splenium by becoming continuous with the posterior end of the hippocampal gyrus. Where it passes into that gyrus it is much narrowed (isthmus). It is separated from the corpus callosum by a fissure which is termed the callosal sulcus.

The hippocampal gyrus (subiculum cornu ammonis), also known, together with the infracalcarine, as the uncinate gyrus, is continued from the isthmus of the callosal convolution, and passes forwards above the anterior part of the collateral fissure towards the apex of the temporal lobe (figs. 102, 103). It ends before reaching this, however, becoming considerably thickened, and forming a recurved projection which looks backwards and inwards, and is known as the uncus. The hippocampal gyrus is covered superficially by a well-marked reticular layer of white fibres (substantia reticularis alba, Arnold (fig. 103, s.r.a.)).

The dentate gyrus (fascia dentata Tarini (fig. 102, fig. 110, f.d.)) lies above the hippocampal gyrus, from which it is separated by the hippocampal or dentate fissure (f.h).

1 By some authors the term gyrus fornicatus is made to include the hippocampal gyrus.

It is a narrow convolution having a peculiarly toothed or notched appearance at its free border, hence the name dentate. Above and overlapping it is the fimbria, from which it is separated superficially by a shallow sulcus (fimbrio-dentate, f.f.-d.), narrow in front

Fig. 110.-DIAGRAM OF A SECTION THROUGH THE LEFT HIPPOCAMPAL REGION (E. A. S.) b.c., basis cerebri; tr., tractus opticus; n. c., nucleus caudatus ; f. ch., fissura choroidea; f., fimbria; f.f.-d., fissura fimbrio-dentata; f. d., fascia dentata, f. h., fissura hippocampi; h, hippocampus major; a., alveus; e. coll., eminentia collateralis; c. i., cornu inferius ventriculi; H, gyrus hippocampi; m., its medullary centre; s.7.a., substantia reticularis alba; f. coll., fissura collateralis.

but broadening out behind, where it forms a triangular depression below the splenium. of the callosum. The gyrus dentatus begins posteriorly just behind and above the splenium by a fine curved lamina (fasciola cinerea), which is continuous with the longitudinal striæ (both lateral and mesial) of the corpus callosum (p. 127). From here it bends downwards, lying along the isthmus of the gyrus fornicatus and then along the upper border of the hippocampal gyrus; here the posterior pillar of the

fornix passes to its upper border in continuity with the fimbria (see p. 130). Anteriorly it is continued into the cleft between the hippocampal gyrus and its uncus, where it becomes lost to view. But if this cleft be opened out (fig. 111) the dentate gyrus is seen to exhibit a sharp curve within it, and in continuity with the curved end a greyish band emerges from the cleft and passes transversely over the uncus to disappear on the ventricular surface of that gyrus (Luschka, Giacomini).

The part of the gyrus dentatus which lies below the splenium is sometimes much more developed than usual, and shows strongly marked folds or dentations which recall the broad