SHELTB THIS Ifm Im TO WOSIzi I (OOS) COLLECTION. J TOPOGRAPHICAL ANATOMY OF THE / BY V J. C. DALTON, M.D., PROFESSOR EMERITUS OF PHYSIOLOGY IN THE COLLEGE OF PHYSICIANS AND SURGEONS, NEW YORK ; AND PRESIDENT OF THE COLLEGE. PHILADELPHIA: LEA BROTHERS & CO. 1885. Entered according to Act of Congress, in the year 1885, by LEA BROTHERS & CO., In the Office of the Librarian of Congress. All rights reserved. DORNAN, PRINTER. CONTENTS. SERIES B.-HORIZONTAL SECTIONS. PAGE Plate I. Horizontal section, thirty millimetres below the upper surface of the hemispheres, and twelve millimetres above the corpus cal- losum, ........... 59 Plate II. Horizontal section, seven millimetres above the corpus cal- losum, ........... 63 Plate HI. Horizontal section, two millimetres above the corpus cal- losum, ........... 65 Plate IV. Horizontal section through the corpus callosum and upper part of the corpus striatum, . . . . . . . 67 Plate V. Horizontal section through the fornix and operculum, . . 69 Plate VI. Horizontal section through the anterior bend and splenium of the corpus callosum, ........ 75 Plate VII. Horizontal section through the knee of the corpus callosum, the septum lucidum, and the middle commissure, . . . 79 Plate VIII. Horizontal section through the knee of the corpus cal- losum, and the middle and posterior commissures, ... 83 Plate IX. Horizontal section through the anterior commissure, . . 87 IV CONTENTS. PAGE Plate X. Horizontal section beneath the anterior commissure, cerebral ganglia, and tubercula quadrigemina, . . . . . . 95 Plate XL Horizontal section through the corpora mammillaria, crura cerebri, and upper end of the fourth ventricle, .... 99 Plate XII. Horizontal section through the optic chiasma and tuber annulare, ........... 103 Plate XIII. Horizontal section through the lowermost part of the frontal lobes and the pons Varolii, . . . . . .105 Plate XIV. Horizontal section through the lowermost part of the temporal lobes and the pons Varolii, . . . . . .109 Plate XV (Extra-serial). Horizontal section through the anterior, middle, and posterior commissures, . . . . . .111 TOPOGRAPHICAL ANATOMY OF THE BRAIN. SERIES B. HORIZONTAL SECTIONS. SERIES B. THIS series presents a succession of horizontal sections of the brain, five millimetres distant from each other, begin- ning about twelve millimetres above the corpus callosum, and terminating just below the inferior surface of the frontal lobes. The plates show, in each case, the upper surface of the lower segment, o PLATE I. HOR1ZONTAL SECTION, THIRTY MILLIMETRES BELOW THE UPPER SURFACE OF THE HEMI- SPHERES, AND TWELVE MILLIMETRES ABOVE THE CORPUS CALLOSUM. In this section the two hemispheres are completely separated from each other by the great longitudinal fissure. The cortical layer, at this level, is very abundant in comparison with the white substance; since the fissures and convolutions penetrate 60 TOPOGRAPHICAL ANATOMY OF THE BRAIN. from above as well as from the sides. Owing to the obliquity of the upper and lateral surface of the hemispheres, some of the secondary fissures appear deeper than those which are really more important; and the position of the principal fissures on the section surface is not always that which would be expected from an external view. A striking feature in all sections of this kind is the appa- rently variable thickness of the cortical layer. In many places, as at y, y, y, it is hardly two millimetres; in others, as at z, z, z, z, it reaches ten millimetres or over. This is due to the fact that whenever a convolution is cut obliquely, the apparent thickness of the gray substance is greater than its real thickness. In some instances the section may pass for a short distance nearly lengthwise through the gray substance; or it may cut across the folded end of a convolution, so as to expose a broad zone of its gray matter, without reaching its central white substance. As the fissures and convolutions penetrate the brain in all possible directions, with many different turns and undulations, they may present a great variety of appear- ances dependent on these conditions. Another appearance, visible at x, x, is that of isolated portions of .brain substance, completely surrounded by fissures. They are not, however, really isolated, but become continuous with other parts at a higher or lower level. They are con- volutions reached by the section where they make a sudden turn, in the re-entering angle between two fissures; appearing consequently, on the section surface, as if separated from the rest of the brain. An analogous appearance is that of a fissure in the brain substance, as at P, not connected with the exterior. In reality, HORIZONTAL SECTIONS. 61 it has penetrated the hemisphere from above, a portion of it being deep enough to be visible at this level, though not reaching the lateral border of the section. In this view the first frontal convolution (I, I) is easily recognized on each side the median line. The second frontal convolution (II, II) shows itself on the left side connected with the central white substance; on the right, it has the form of one or two isolated folds, cut off by the section. The third frontal convolution is not visible. The anterior and posterior central convolutions (III, IV) are in their normal position with regard to the fissure of Rolando. In the posterior half of the brain is the beginning of the upper parietal convolution (V); behind it the cut surface of the supramarginal convolution (VI); and still farther back the angular convolution (VII). The deep- seated fissure, P, is the lower part of the parietal fissure, which extends from the upper surface of the hemisphere to this level. The fissura calloso-marginalis shows itself at C M; and at the extreme posterior part of the section is the upper end of the fissura parieto-occipitalis (P O), running transversely out- ward from the median surface. The region marked VIII, between the fissura calloso-marginalis and the fissura parieto- occipitalis, is the lobulus quadratus. It extends for a consid- erable distance along the median line, because in a horizontal section at this level the lobule is cut almost diagonally, from its lower corner in front to its upper corner behind. This is evident from its position in the median sections, Series A, Plates VII and VIII. Fissure of Rolando Fissure of Rol ando Posterior End of Parietal Fissure Posterior End of Parietal Fissure I, First frontal convolution. Il, Second frontal convolution. IH, Anterior central convolution. IV, Posterior central convolution. V, Anterior part of upper parietal convolution. VI, Supra-marginal convolution. VII, Angular convolution. VH1, Lobulus quadratus. CM, Ascending branch of calloso-marginal fissure. PO, Parieto-occipital fissure. P, Parietal fissure, penetrating from above. x, x, Angular turns of small convolutions, cut off by the section, y, y, y, y, Cortical layer cut vertically. z, z, z, z, z, Cortical layer cut obliquely. Series B. Plate I HORIZONTAL SECTION. Twelve millimetres above the corpus callosum. PLATE II. HORIZONTAL SECTION, SEVEN MILLIMETRES ABOVE THE CORPUS CALLOSUM. In Plate II the appearance of the brain is but little modified from that in the preceding section. The second frontal con- volution (II) is now connected on both sides with the interior white substance. The upper parietal convolution has disap- peared, and the supramarginal convolution (V) occupies a large part of the lateral border of the section. The remains of the parietal fissure (P) are still visible, but much less extensive than in Plate I. The posterior part of the fissura calloso- marginalis (C M) is also present, but is interrupted on the right side, about its middle, by a transverse band of brain substance. The whole section surface is somewhat larger than the last, owing to its being situated farther down on the sloping sides of the hemispheres. Fissure of Rolando Fissure of Rolahdo Parietal Fissure'' ''Parietal Fissure I, First frontal convolution. H, Second frontal convolution. IH, Anterior central convolution IV, Posterior central convolution. V, Supra-marginal convolution. VI, Angular convolution. CM, Calloso-marginal fissure. PO, Parieto-occipital fissure. P, Parietal fissure and adjacent convolutions, penetrating from above. Series B. Plate II HORIZONTAL SECTION. Seven millimetres above the corpus callosum. PLATE III. HORIZONTAL SECTION, TWO MILLIMETRES ABOVE THE CORPUS CALLOSUM. This view presents, in both hemispheres, the largest con- tinuous extent of white substance, as most of the fissures penetrate only to a moderate depth, and the lateral ventricles are not yet opened. Near the median line the section has passed, on each side, just below the calloso-marginal fissure and above the corpus callosum; following almost exactly the longitudinal axis of the gyrus fornicatus (G F). This con- volution accordingly appears as a nearly straight band of gray substance, bordering the median line and extending backward, through one of its folds of communication, into the region of the lobulus quadratus, in front of the fissura parieto-occipitalis (P O). Anteriorly it shows a broader zone of gray substance, owing to its commencing downward curvature round the knee of the corpus callosum, and its consequent obliquity, at this point, to the plane of the section. In front of its anterior border the fissura calloso-marginalis (C M) comes into view; and still farther forward are the median folds of the first 66 TOPOGRAPHICAL ANATOMY OF THE BRAIN. frontal convolution. On the left side, the second frontal con- volution sends off, forward and inward, an undulating prolonga- tion (x), which is apparently disconnected from the brain at its farther extremity, but which unites, at a lower level, with the first frontal convolution. This junction appears in the following section, Plate IV, five millimetres farther down. The fissura parieto-occipitalis (P O) is still a prominent feature in this view; and the upper end of the fissure of Sylvius appears on the lateral border of the section. In all the horizontal sections thus far, the ovoidal form of the section-surface is very marked; its anterior part being narrow and pointed, its posterior large and rounded. Below this level the form of the section-surface changes; the anterior part of the brain becoming more rectangular in shape, and the extremity of the occipital lobes more pointed. Fissure of Sylvius Fissure ' of I Sylvius I, First frontal convolution. II, Second frontal convolution. GF, Gyrus fornicatus. CM, Calloso-marginal fissure. PO, Fissura parieto-occipitalis. x, Fold of communication between first and second frontal convolutions. Series B. Plate III. HORIZONTAL SECTION. Two millimetres above the corpus callosum. PLATE IV. HORIZONTAL SECTION THROUGH THE CORPUS CALLOSUM AND UPPER PART OF THE CORPUS / STRIATUM. In this section the two hemispheres are united for a dis- tance of about four centimetres from front to rear, the arched portion of the corpus callosum having been traversed by the section to that extent. But the upper surface of the 'corpus callosum seldom or never forms a continuous curve. It usually presents in its posterior half a noticeable depression, as seen in Series A, Plates VI, VII, and VIII. In the present view this depressed portion remains untouched; showing its free upper surface in the form of an irregularly oval spot (z), sur- rounded by the edges of the section. In the middle of this free surface are the " striae longitudinales mediales; " and in its lateral parts the transverse striations indicating the direction of the fibrous bundles of the corpus callosum. Farther outward from the median line, the lateral ven- tricles (v, v) are laid open, and the gray substance of the corpus striatum (C S) is also exposed. This almost necessarily 68 TOPOGRAPHICAL ANATOMY OF THE BRAIN. happens in such a section, since the lateral borders of the corpus callosum join the white substance of the hemispheres at a higher level than that of its median parts. This con- formation is shown in the vertical sections, Series C, Plates VI, VII, VIII, and IX, where it is evident that a horizontal section through the corpus callosum would pass, on each side, below the highest point of the lateral ventricle. As the two corpora striata are not always of equal height, it is also plain how such a section may expose a considerable portion of gray matter in one of these bodies, and comparatively little in the other. In front of the corpus callosum is the descending portion of the gyrus fornicatus (G F), bordered anteriorly by the fissura calloso-marginalis (C M). The first and second frontal convo- lutions occupy their usual position; and on the left side the fold of communication between them (x), partly visible in the preceding section, is seen with both its connections complete. The fissure of Sylvius now shows itself as a rather deep cleft in the lateral border of the hemisphere. A little in front of its inner extremity on each side are two apparently isolated spots of gray substance (y, y). These are the upper ends of the interior convolutions of the operculum, which rise to a high level in this region, and which are fully exposed in the following section, Plate V. In the posterior part of the brain is the fissura parieto- occipitalis (P O), presenting much the same appearance as heretofore. Immediately behind it, the cortical layer on each side the median line is marked along its middle by a white striation, which shows it to be a convolution adjoining the fissura calcarina. This fissure is not yet visible, but it comes into view, for a large part of its extent, in the two following sections. Fissure oj5 S y Ivius fissure । Sylvius I, First frontal convolution. IT, Second frontal convolution. GF, Gyrus fornicatus. CM, Calloso-marginal fissure. CS, Upper part of corpus striatum, v'.v; Lateral ventricles. PO, Fissura parieto-occipitalis. X, Fold of communication between first and second frontal convolutions. y, Interior convolutions of the operculum. Z, Upper free surface of corpus callosum, not reached by the section Series B. Plate IV. HORIZONTAL SECTION. Through corpus callosum and upper part of corpus striatum. PLATE V. HORIZONTAL SECTION THROUGH THE FORNIX AND OPERCULUM. In Plate V the section surface of the brain is longer and narrower than in the preceding. A large part of the corpus callosum has been cut away, leaving a portion of it (C C) divided obliquely in front and behind. Between these separated portions its whole thickness has been removed, exposing to view the fornix, with its anterior pillars (i, i) coming up from the anterior part of the lateral ventricles, and its posterior pillars (2, 2) curving outward and downward into the descending horns. The highest part of the corpus striatum has also been cut away, leaving in front its rounded head (C S) projecting into the anterior horn of the lateral ventricle, and farther back its slender, tail-like prolongation, or surcingle (s), descending into the inferior horn. The whole of this body, formerly called the " caudate nucleus of the corpus striatum," is now generally known simply as the " corpus striatum." Its slender prolonga- tion, the "surcingle," does not terminate posteriorly, as some- 70 TOPOGRAPHICAL ANATOMY OF THE BRAIN. times described, by a pointed extremity, but continues in a curvilinear course, running forward, in the roof of the inferior horn of the ventricle, to reach the amygdala at the base of the brain. It thus embraces, in its loop-like curvature, the whole of the brain substance between the upper and lower portions of the lateral ventricle, returning finally to a point nearly opposite the head of the corpus striatum. It is visible, where approaching the amygdala, in the horizontal sections, Series B, Plates IX and X; and in the vertical sections, Series C, Plates IX, X, XI, and XII. The whole extent of the corpus striatum, with its loop-like prolongation, may often be seen by opening the lateral ven- Longitudinal Section of Right Hemisphere, with the lateral ventricle opened from its inner side. A, Amygdala. C, Corpus striatum. S, Surcingle. V, Cavity of ventricle. 1, Fissura parieto-occipitalis. 2, Fissura calcarina. tricle completely from its inner side, as in Figure 9. The head of the corpus striatum then forms a marked prominence in the anterior part of the cavity, and its slender continuation is visible throughout, from its pinkish-gray color showing HORIZONTAL SECTIONS. 71 through the transparent lining of the ventricle. Its down- ward curvature corresponds in position to the posterior end of the optic thalamus ; and, as it everywhere closely embraces the intervening brain substance, it is situated in the floor of the lateral ventricle above, and in the roof of its inferior horn below. It is for this reason that the caudate portion of the corpus striatum has been sometimes described as terminating pos- teriorly, near the end of the optic thalamus. In a horizontal section of the brain at this level, or immediately below it, the head of the corpus striatum in front and its surcingle behind are separated from each other by some distance, as in Plate V, because their intervening parts have been cut away. In vertical sections through the optic thalamus, as in Series C, Plates IX, X, XI, and XII, the surcingle is cut twice, once in the upper and once in the lower part of its curvature. But its two section surfaces, though apparently disconnected, are really parts of the same body; and they both disappear when the sections pass the posterior end of the optic thalamus, as in Series C, Plate XV. Sometimes a section will include part of its down- ward curvature, which then appears as a more or less elongated strip of gray matter, following an obliquely vertical direction on the inner edge of the white substance of the hemisphere. This has happened, in Series C, on the left side in Plate XIII, and on the right side in Plate XIV7. It is not easy to make a single longitudinal section through the whole loop-like curve of the surcingle, because many of its parts are so slender and so widely separated from each other. But it may be shown by following with the edge of a knife the track of its gray substance from point to point, and then cutting away the adjacent white substance to the same level. 72 TOPOGRAPHICAL ANATOMY OF THE BRAIN. Such a section, represented in Figure io, exhibits the white surface and radiated structure of the internal capsule (3) ; near its centre a portion of the lenticular nucleus (1, 2) ; and en- circling its border the gray tract of the corpus striatum and surcingle. In Plate V, between the head of the corpus striatum and the surcingle, are a number of small, isolated spots of gray substance (x, x, x), arranged in linear series with a slight Fig. io. Longitudinal Section of Right Hemisphere through the corpus striatum and surcingle. C, Corpus striatum. C C, Part of corpus callosum. F, Frontal lobe. O, Occipital lobe. T, Temporal lobe. S, Surcingle. I, 2, Inner and middle zones of the lenticular nucleus. 3, Internal capsule. outward curvature. These belong to the lenticular nucleus, the upper edge of which is barely reached by a horizontal section at this level. As the lenticular nucleus is somewhat serrated along its upper border, the section shows its projecting points of gray matter, separated by intervening portions of white substance. HORIZONTAL SECTIONS. 73 Nearer the median line, and in close proximity to the edge of the fornix, is a partial section of the optic thalamus (O T). This is the upper part of the thalamus, situated near the outer border of the lateral ventricle. It is covered, in the natural condition, with a thin layer of white substance; but this has been cut away by the section, exposing the gray matter within. In the frontal lobe, the gyrus fornicatus (G F) continues to follow a sloping direction, and the fissura calloso-marginalis (C M) is farther forward than heretofore. In the lateral region of the hemispheres, the interior convolutions of the operculum are now fully apparent, intercalated with those of the insula. Behind the last convolution of the operculum, the fissure of Sylvius still appears as a cleft of moderate depth, penetrating the section surface nearly at right angles from its lateral border. In the occipital lobe, the convolutions bordering the median line behind the fissura parieto-occipitalis all belong to the fissura calcarina, and exhibit throughout their characteristic white striation. At this level the fissura calcarina is partly horizontal, and at F C it has been opened for a short distance nearly in its own plane. i Fissure ... \ Sylvius Fissure oj --- Sylvius CM, Calloso-marginal fissure. GF, Gyrus fornicatus. CC, Corpus callosum. CS, Corpus striatum OT, Optic thalamus. PO, Fissura parieto-occipitalis. FC, Fissura calcarina. s, Surcingle. x, x, x, Upper border of lenticular nucleus. I, I, Anterior pillars of the fornix. 2, 2, Posterior pillars of the fornix, Series B. Plate V HORIZONTAL SECTION. Through the fornix and operculum. PLATE VI. HORIZONTAL SECTION THROUGH THE ANTERIOR BEND AND SPLENIUM OF THE CORPUS CALLOSUM. The section here represented exhibits both the anterior and posterior borders (C C and C C), or the knee and splenium, of the corpus callosum. The anterior horns of the lateral ventricles, which are widely opened, are separated from each other by the septum lucidum (x). The septum consists of a thin double lamina of white substance, extending from the knee of the corpus callosum in front to the pillars of the fornix (p, i) behind; but just in front of the pillars of the fornix it is covered on each side with a perceptible layer of gray substance. The section of the optic thalamus (O T) is much larger than before. At its front part, just behind and on each side of the anterior pillars of the fornix, is an oval portion of its gray matter, about seven millimetres long by four millimetres wide, more or less completely isolated by white substance. 1 his is the "anterior tubercle" of the thalamus, which gives origin 76 TOPOGRAPHICAL ANATOMY OF THE BRAIN. to the descending radicle of the fornix, and consequently to the whole length of the fornix and the fimbria. It is also visible in the vertical section, Series C, Plate IX. The head of the corpus striatum (C S) appears of about the same size as in the preceding section, the surcingle (s) being somewhat smaller. But the lenticular nucleus (L N) is much larger than before, and begins to exhibit its lens-like figure. It is separated from the corpus striatum and optic thalamus, both of which lie nearer the median line, by a broad tract of white substance, the " internal capsule." This capsule is composed of bundles of nerve fibres from the crura cerebri, passing upward and outward to the white substance of the hemispheres. In a horizontal section these bundles are divided obliquely, and their partial separation gives to the internal capsule a distinctly striated appearance. The tract of white substance immediately outside the lenticular nucleus is the "external capsule;" and the adjacent undulating lamina of gray substance (y) is the "claustrum." The splenium of the corpus callosum (Cz C') is divided, in this section, very near its under surface. Posteriorly it sends off, on each side, a band of white substance (4), which turns backward into the occipital lobe. This is the " posterior forceps of the corpus callosum." Its backward prolongation forms the inner wall of the posterior horn of the lateral ventricle, and supplies commissural fibres from the corpus callosum to the median convolutions of the occipital lobe. On the anterior border of the splenium, at some distance from the median line, are the cut surfaces of the posterior dillars of the fornix (2, 2) descending into the inferior horns of the lateral ventricles, and already assuming on each side HORIZONTAL SECTIONS. 77 the form and position of the fimbria, with which they are continuous. Directly behind the fimbria, and following a similar course in the descending horn of the ventricle, is the upper portion of the hippocampus (3). The surface of the hippocampus is invested with a covering of white substance, but internally it contains a mass of gray matter, from the lateral extension of the cortical layer of the gyrus fornicatus. As this gyrus passes forward beneath the splenium, to become the gyrus hippocampi (see Series A, Plate VIII), its cortical layer is rolled outward in such a way as to cause a lateral prominence in the inferior horn of the ventricle. This configuration is distinctly shown in the following section, Plate VII, where the splenium is entirely removed, exhibiting the unbroken continuity between the cortical layer of the gyrus hippocampi next the median line and the gray matter of the hippocampus in the lateral ventricle. In the frontal lobe the gyrus fornicatus is cut transversely while passing round the knee of the corpus callosum. On the left side it is single, but on the right side it is divided by a shallow furrow in its longitudinal axis. On the sides of the brain the Sylvian fissure is extensively opened, showing the space surrounded by the adjacent con- volutions. The anterior half of the operculum still remains, but the convolutions of its posterior half are represented only by their lower ends and edges. The convolution marked z is the extreme posterior border of the operculum, and is appa- rently detached from the remainder, showing only a little white substance at its median extremity. Since the posterior branch of the Sylvian fissure rises to a higher level than its anterior branch, the posterior convolutions of the operculum may be 78 TOPOGRAPHICAL ANATOMY OF THE BRAIN. detached by a horizontal section which leaves its anterior por- tion still connected with the hemisphere; and, in successive sections from above downward, its anterior convolutions are the last to disappear. In the following section, Plate VII, the operculum is much farther diminished; in Plate VIII it is reduced to a few remnants; and in Plate IX it is no longer visible. The Sylvian fissure, in Plate VI, is bordered posteriorly by the first temporal convolution, which presents a somewhat extensive section surface, owing to its oblique position in regard to the horizontal plane. A short distance behind the splenium is the fissura parieto- occipitalis (P O). The position of this fissure has been gradually shifted, in the foregoing sections, from behind for- ward, until it has now nearly reached the posterior border of the corpus callosum. As the fissura parieto-occipitalis runs downward and forward on the median surface of the hemi- sphere (see Series A, Plates VII and VIII), the lower the section the farther forward will be the place of the fissure. The fissura calcarina, on the other hand, which is more nearly horizontal, is reached and passed by the sections at a certain level, though the convolutions immediately below it still show the peculiar striation of their cortical layer. In the occipital lobe, on the left side, a part of the pos- terior horn of the lateral ventricle (v) is exposed by the sec- tion. On the right side it is hardly perceptible. Fissure of Sylvius Fissure oj Sylvius CM, Calloso-marginal fissure. GF, Gyrus fornicatus. CC, Anterior bend of corpus callosum, C'C', Splenium of corpus callosum. CS, Corpus striatum. LN, Lenticular nucleus. PO, Fissura parieto-occipitalis. x, Septum lucidum. y, Claustrum. z, Posterior convolution of the operculum s, Surcingle, v, Posterior horn of lateral ventricle. 1,1, Anterior pillars of the fornix. 2,2, Posterior pillars of the fornix. 3, Upper part of hippocampus. 4, Posterior forceps of corpus callosum. Series B. Plate VI. HORIZONTAL SECTION. Through anterior bend and splenium of the corpus callosum. PLATE VII. HORIZONTAL SECTION THROUGH KNEE OF CORPUS CALLOSUM, SEPTUM LUCIDUM, AND MIDDLE COMMISSURE. In this section the occipital lobes have been drawn back- ward, separating the hippocampal convolutions (G H) from the posterior borders of the thalami, and giving to the whole brain a somewhat elongated appearance. The knee of the corpus callosum (C C) continues to form the front wall of the lateral ventricles. The septum lucidum (x) holds its former position, and shows, more distinctly than before, the median cleft of the fifth ventricle, enlarging ante- riorly into a triangular cavity, from the separation of its two laminae. At its posterior border the pillars of the fornix (i) are now apparently consolidated into a single cord; behind which, running from side to side, is a continuous canal, the "foramen of Monro," forming a communication between the third ventricle on the median line and the anterior horns of the two lateral ventricles. The corpus striatum (C S) appears somewhat diminished in 80 TOPOGRAPHICAL ANATOMY OF THE BRAIN. size; but the lenticular nucleus (L N) is much increased, showing a larger external and a smaller internal zone of gray matter, with a narrow intervening tract of white substance. The optic thalamus (O T) forms on each side a wide expanse of gray substance, somewhat striated with white anteriorly, and connected on the median line by the middle commissure (z). Three or four millimetres farther back are two projecting elbows of white substance (3, 3), distinctly contrasted with the general gray tint of the optic thalami. These are the "pe- duncles of the conarium," which run at a higher level along the median surface of the thalami, and are cut across at this point, where they curve downward to reach the conarium (4), indistinctly visible below the edges of the section. The rela- tion of the peduncles to the thalami and the conarium is shown in the median sections Series A, Plates VII and VIII. In this view the splenium of the corpus callosum is no longer visible, the section having passed below it. The gyrus fornicatus is consequently exposed where it passes forward, beneath the level of the splenium, to form the gyrus hippo- campi (G H). Its cortical layer at this situation is rolled outward; and its lateral projection in the descending horn of the ventricle, covered with a layer of white substance, forms the hippocampus. In front it shows a thicker cord-like band of white substance, diminishing on its external border to a ribbon-shaped edge. This band is the "fimbria." It repre- sents the downward continuation of the posterior pillar of the fornix, which, after joining the hippocampus, runs forward through the whole length of the inferior horn of the ventricle. In the outer wall of the ventricle, opposite the situation of the fimbria, is the cut surface of the surcingle (s). This body HORIZONTAL SECTIONS. 81 presents, throughout most of its course, alternate constrictions and enlargements; so that it varies in size in different sections, and is seldom of the same size on both sides. In this plate it is larger on the left side than on the right; in Plate VIII it is larger on the right side than on the left; and in Plate IX its proportions are again reversed. In the frontal lobe the gyrus fornicatus, somewhat narrower than in the preceding section, retains its position between the corpus callosum and the fissura calloso-marginalis. On the sides of the brain the convolutions of the insula are exposed for nearly their whole extent. The middle convolu- tions of the operculum are seen (at y, y) completely detached on the right side and nearly so on the left. Its posterior convolutions are no longer visible, this part of the insula being covered by the temporal lobe. The inner portion of the occipital lobe is now formed by the lobulus lingualis, which is deeply indented by furrows run- ning outward from its median surface, as well as by others penetrating from below. CM, Calloso-marginal fissure. GF, Gyrus fornicatus. CC, Knee of corpus callosum. CS, Corpus striatum. LN, Lenticular nucleus. GH, Gyrus hippocampi. OT, Optic thalamus. PO, Fissura parieto-occipitalis. x, Septum lucidum. y, y, y, y, Lower ends of middle convolutions of the operculum. S, Surcingle. z, Middle commissure. I, Anterior pillars of fornix. 2, Fimbria. 3, 3, Peduncles of the conarium. 4, Conarium. Series B. Plate VII. HORIZONTAL SECTION. Through knee of corpus callosum, septum lucidum, and middle commissure. PLATE VIII. HORIZONTAL SECTION THROUGH THE KNEE OF THE CORPUS CALLOSUM, AND THE MIDDLE AND POSTERIOR COMMISSURES. In Plate VIII the knee of the corpus callosum (C C) has been divided near its under surface. Behind it, is a remnant of the septum lucidum, enclosing (at x) the inferior angle of the fifth ventricle. Its two laminae are now visibly reinforced by narrow folds of gray substance which come from below, running parallel with each other, in close proximity to the median line, along the lower border of the septum, and also for a short distance on the under surface of the corpus cal- losum. From this circumstance they are sometimes called the "peduncles of the septum," and sometimes the "peduncles of the corpus callosum." The anterior pillars of the fornix (i, i), which were consolidated in the preceding section, are now again nearly separate from each other, uniting on each side with the wall of the third ventricle. At this level, accordingly, the communication between the third and the lateral ventricles no longer exists. 84 TOPOGRAPHICAL ANATOMY OF THE BRAIN. The form and size of the corpus striatum (C S) are nearly as in the last section. The lenticular nucleus, on the other hand, presents in this view its largest area, with three con- centric zones of gray matter, increasing in size from within outward. The narrow intervening tracts of white substance are the "laminae medullares;" and from their outer borders bundles of white fibres radiate into the adjacent zones. Behind the pillars of the fornix the third ventricle is divided into two parts, anterior and posterior, by the middle commis- sure (z). Between the optic thalamus and the lenticular nucleus, the internal capsule forms a broad tract of white substance, its fibrous bundles directed mainly upward and outward, and separated from each other by thin longitudinal partitions. The anterior part of the capsule, between the corpus striatum and lenticular nucleus, is comparatively narrow. From its inner border it sends off diverging bundles of fibres into the corpus striatum; but it is mainly composed of fibres running longi- tudinally. Where this portion of the internal capsule meets the anterior extremity of the optic thalamus, the gray matter of the thalamus is striated by fibres penetrating it from the capsule. This connection is described by Meynert and Hu- guenin as the "anterior root of the optic thalamus," the fibres of which come from the frontal convolutions, passing backward between the corpus striatum and lenticular nucleus, and reach- ing in this way the anterior extremity of the thalamus. They accordingly form tracts of communication between the gray substance of the thalamus and the cortical layer of the frontal lobe. At the hinder extremity of the third ventricle is a transverse band of white substance, the " posterior commissure," beneath HORIZONTAL SECTIONS. 85 which is the passage from the third ventricle into the aque- duct of Sylvius. The posterior commissure is formed, as shown in Series A, Plates VI and VII, by the backward folding of a lamina of white substance at the upper and anterior border of the tubercula quadrigemina. The present section has passed just below the level of the conarium, exposing the transverse furrow behind the edge of the commissure, and bringing into view the anterior tubercula quadrigemina (4, 4). From these bodies a narrow band of white substance, the " brachium con- junctivum anterius," passes out on each side to the posterior part of the optic thalamus. The everted cortical layer of the gyrus hippocampi (G H) now presents on its inner aspect a thin superficial investment of white substance. Its lateral projection in the descending horn of the ventricle, forming the hippocampus (3), as well as the fimbria (2) with which it is connected, and the surcingle (s) in the roof of the ventricle, occupy the same relative posi- tions as before. The gyrus fornicatus (G F) is cut somewhat obliquely, as it is beginning to turn backward under the knee of the corpus callosum. On the lateral border of the frontal lobe (at y, y) is the third frontal convolution, covering the anterior portion of the insula. The first temporal convolution is considerably reduced in size at this level, and the second forms a much larger portion of the lobe. In the median part of the occip- ital lobe the lobulus lingualis is still visible, sending forward (at 5) the fold of communication' which unites with the gyrus hippocampi. Between the occipital lobes is a small part of the superior vermiform process of the cerebellum, projecting from below. I, II, Hl, Internal, middle, and external zones of lenticular nucleus. CM, Fissura calloso-marginalis. GF, Gyrus fornicatus. CC, Knee of corpus callosum. CS, Corpus striatum. OT, Optic, thalamus. GH, Gyrus hippocampi, x, Fifth ventricle, in lower part of septum lucidum. y, y, Third frontal convolution, forming anterior part of operculum, z, Middle commissure. p, Posterior commissure, s, Surcingle. 1,1, Anterior pillars of fornix. 2, Fimbria. 3, Hippocampus. 4, 4, Anterior pair of tubercula quadrigemina. 5, Junction of lobulus lingualis with gyrus hippocampi. Series B. Plate VIII HORIZONTAL SECTION. Through knee of corpus callosum, and middle and posterior commissures. PLATE IX HORIZONTAL SECTION THROUGH THE ANTERIOR COMMISSURE. In this section the corpus callosum has entirely disappeared, and beneath the level of its anterior bend is the gyrus forni- catus (G F) following a nearly straight longitudinal direction. The anterior commissure (x, x, x, x) is exposed for a consid- erable part of its course; namely, on the median line, where it forms the anterior boundary of the third ventricle; at some distance outward, where it passes beneath the corpus striatum and middle zone of the lenticular nucleus; and beyond the outer border of the nucleus, where it descends into the tem- poral lobe. The median portion of the commissure in this brain is not perfectly symmetrical. On the left side it is exposed continuously for a distance of two centimetres from the median line; but on the right side a portion of it crosses the corpus striatum a little above or below the horizontal plane, and consequently does not appear in the section. The whole extent of the anterior commissure, owing to its 88 TOPOGRAPHICAL ANATOMY OF THE BRAIN. various curves and undulations, can never be shown by a sec- tion following a single plane; and it often appears, in such a section, alternately enlarged or diminished, from its changes of level in different parts. From the median line, as a general rule, it runs outward and a little upward for about 5 milli- metres ; then outward and slightly downward for 3 or 4 millimetres; and then outward for about 10 millimetres, making at the same time a pronounced curvature in the horizontal plane, with its convexity forward. This entire portion, extend- ing for 18 or 20 millimetres on each side the median line, is the nearly horizontal transverse portion of the commissure, and may Te sometimes exposed, by a fortunate section at the proper level, as in the Extra-serial Plate XV. Beyond this region it turns backward and downward, until it passes the outer and posterior angle of the lenticular nucleus, when it turns sharply downward, in a sort of cascade, into the tem- poral lobe, and is finally distributed to the convolutions of this region. It is consequently a transverse commissure between the right and left temporal lobes; the remainder of the two hemispheres being connected with each other by the commis- sural fibres of the corpus callosum. The point where the anterior commissure turns suddenly downward to the temporal lobe is about ten millimetres farther back than where it crosses the median line. Its descending part is visible, beneath the outer border of the lenticular nucleus, in the vertical section, Series C, Plate IX. The anterior commissure may be exposed throughout, as in Figure 11, by following it with an undulating section and removing the greater part of the occipital lobe. It presents nearly the classical form of a Cupid's bow; curving forward 89 HORIZONTAL SECTIONS on each side from the median line, and then turning backward to its outer extremity. Immediately behind the anterior commissure, in Plate IX, are the anterior pillars of the fornix (i, i), now distinctly Fig. 11. Horizontal and Oblique Section of the Brain ; showing the course of the anterior commissure, from the median line to its entrance into the temporal lobe, x, x, x, x, Anterior commissure. C, Corpus striatum. L N, Lenticular nucleus. O T, Optic thalamus. I, i, Anterior pillars of the fornix. 2, 2, Internal capsule. 3, 3, Tubercula quadrigemina. divergent from each other and consolidated with the walls of the third ventricle. In this region they are known as the 90 TOPOGRAPHICAL ANATOMY OF THE BRAIN. "ascending radicles of the fornix;" a name justified by the course of their fibres from the corpora mammillaria to the level of the anterior commissure. These fibres originate as a distinct bundle in the optic thalamus, immediately beneath its anterior tubercle. Thence they pass downward almost verti- cally to the corpus mammillare, forming in this part of their course the "descending radicle of the fornix," visible on each side at 2. Arrived at the .corpus mammillare, they wind round its median and inferior surface, and then follow an upward direction, as the "ascending radicle," until they emerge from the wall of the ventricle and become the anterior pillar of the fornix. In Plate IX the descending and ascending radicles of the fornix are situated at a distance from each other of five or Fig. 12 Diagram of the Optic Thalamus, Gyrus Hippocampi, and Fornix, from the median side. I, Anterior tubercle of the thalamus. 2, Descending radicle of the fornix. 3, Corpus mammillare. 4, Ascending radicle of the fornix. 5, Anterior commissure. 6, Fornix. 7, Posterior pillar of the fornix. 8, Fimbria. 9, Fascia dentata. six millimetres. A little lower down, in Plate X, they are nearly in contact; the ascending radicle (i) being a little to HORIZONTAL SECTIONS. 91 the outside, the descending radicle (2) nearer the median line. In Plate XI they have both disappeared, and their place is occupied by the corpora mammillaria (c, c). Figure 12 gives a diagrammatic view of these fibres in their entire course. The fornix, therefore, is mainly a circuitous tract of nerve fibres, connecting the optic thalamus with the hippocampus. It originates from the anterior tubercle of the thalamus, passes downward as the "descending radicle" to the corpus mammil- lare, thence upward as the "ascending radicle" to the level of the anterior commissure. Here it becomes the "anterior pillar of the fornix," which curves backward, gradually diverging from the median line along the floor of the lateral ventricle to the commencement of the inferior horn, into which it descends as the "posterior pillar of the fornix." It there joins the everted portion of the gyrus hippocampi and becomes the " fimbria," which runs forward along the upper border of the hippocampus to its anterior extremity, distributing fibres to its cortical layer. The ascending and descending radicles of the fornix, the fornix itself, the fimbria, and the white covering of the hippocampus, are all consequently parts of one continuous tract. Above the level of the anterior commissure, the corpus stri- atum and the lenticular nucleus are separated from each other by the anterior branch of the internal capsule. In Plate VI this separation is complete. In Plates VII and VIII it is imperfect, especially on the right side, where the two masses of gray matter are partially united; and in Plate IX their fusion is very marked. This is due to the fact that the fibrous bundles of the internal capsule, as they pass upward and for- ward, reach the white substance of the frontal lobe after 92 TOPOGRAPHICAL ANATOMY OF THE BRAIN. traversing the upper half of the corpus striatum and lenticular nucleus, which consequently appear separated in horizontal sections at that level. But lower down the two ganglia unite with each other round the front border of the capsule; and near the base of the brain they are also connected with the gray matter of the cortex. The union of the corpus striatum with the lenticular nucleus is shown in the vertical sections, Series C, Plates V and VI; that of the lenticular nucleus with the gray matter of the cortex in Series C, Plate VII. In the present section the walls of the third ventricle are formed by a layer of gray substance,- two or three millimetres in thickness. This is a continuation of the gray matter sur- rounding the central canal of the spinal cord, thence extending upward along the floor of the fourth ventricle and around the aqueduct of Sylvius, and so forward into the third ventricle. From its proximity throughout to this longitudinal cavity, it is called the " gray matter of the medullary canaland it gives origin to the root-fibres of all the motor and sensitive cerebro- spinal nerves. It terminates at the anterior extremity of the third ventricle. In this view the former situation of the internal capsule is occupied on each side by a section of the crus cerebri, in which the oblique outward and forward direction of its fibrous bundles is distinctly shown ; and farther backward and inward the optic thalami are replaced by the "tegmentum." In the anterior part of the tegmentum, on each side, near the median line is a circular gray spot (z) about six millimetres in diam- eter, surrounded by a zone of white substance, from which one or two narrow bands penetrate its interior This is the " nucleus tegmenti," sometimes called the " red nucleus of the HORIZONTAL SECTIONS. 93 tegmentum," from its rosy or yellowish-gray color. Its white substance consists of fibres from the superior peduncle of the cerebellum, which are here arranged in a nearly cylindrical form. Its reddish-gray substance contains pigmented nerve cells between the bundles of nerve fibres. Farther back, on the median line, is the aqueduct of Sylvius (3) passing obliquely downward; and behind it are the pos- terior tubercula quadrigemina (4, 4), the anterior pair having been removed by the section. The lateral white band passing outward and forward from each of the posterior tubercula quadrigemina is the " brachium conjunctivum posterius." In this section the internal and external corpora geniculata (5, 6) are exposed on each side in a nearly symmetrical position. The corpus geniculatum internum is of a uniform pale gray color; the corpus geniculatum externum is marked with alter- nate gray and white striations. The cortical layer of the gyrus hippocampi (G H) is here shown in its extreme lateral extension, as well as in its reverse folding toward the median line, to form the convexity of the hippocampus (7). Both its outer and inner surfaces are now covered with a distinct layer of white substance. In the occipital lobes the lobulus lingualis has nearly disap- peared, the section having reached the convolutions of the lobulus fusiformis farther downward and outward. The median portion of the cerebellum is more freely exposed than in the preceding section. GF, Gyrus fornicatus. LN, Lenticular nucleus. CS, Corpus striatum. GH, Gyrus hippocampi, x, x, x, x, Anterior commissure, v, Third ventricle, z, Nucleus tegmenti. s, Surcingle. 1, 1, Ascending radicle of fornix. 2, 2, Descending radicle of fornix. 3, Aqueduct of Sylvius. 4, 4, Posterior pair of tubercula quadrigemina. 5, Corpus geniculatum internum. 6, Corpus geniculatum externum. 7, Hippocampus. Series B, Plate IX HORIZONTAL SECTION. Through the anterior commissure. PLATE X. HORIZONTAL SECTION BENEATH THE ANTERIOR COMMISSURE, CEREBRAL GANGLIA, AND TUBERCULA QUADRIGEMINA. The section represented in Plate X is at so low a level that the frontal lobes are nearly separated from the rest of the brain. At their back part, near the median line, the gray substance of their inferior convolutions is visible (at o, o), just above the posterior extremity of the fissura olfactoria. A little outside this point is the locus perforatus anterior (y), through which branches of the Sylvian artery pass upward into the lenticular nucleus. On the right side, at the anterior and outer border of the locus perforatus, is a distinct white spot about one millimetre in diameter. This is a section of the "external root of the olfactory nerve," which has been divided in its passage downward and inward, toward the "tuber olfac- torium" beneath the convolutions at (o). The fibres of this root are described by Henle as coming from the insula; by Hu- guenin as coming from the anterior portion of the gyrus hip- 96 TOPOGRAPHICAL ANATOMY OF THE BRAIN. pocampi. In our own observations, the gyrus hippocampi has always seemed the place of origin of these fibres. The third ventricle is now closed in front by a thin trans- verse sheet of gray matter, the "lamina cinerea terminalis" (x). This lamina, which is still thinner in its lower portion, is seen in vertical section in Series A, Plates VI and VII, The ascending and descending radicles of the fornix (i, 2) are in close proximity to each other; the descending radicle, owing to its vertical position, being still seen in transverse section, while the ascending radicle is cut obliquely as it passes forward and upward toward the anterior commissure. The crura cerebri have now nearly lost their attachment to the hemispheres, exposing, immediately outside their cut edges, a portion of the optic tracts (4, 4). Between the crura cerebri and the tegmentum the substantia nigra (S N) appears in very oblique section ; and inside and behind it, the nucleus tegmenti (z) presents a smaller surface and more triangular form than in Plate IX. The dark triangular space on the median line, immediately behind the descending radicles of the fornix, is not a part of the third ventricle, but lies outside and beneath the cerebral substance. At this point on the base of the brain there is a strongly depressed locality, the "locus perforatus posterior," between the corpora mammillaria in front and the diverging halves of the tegmentum behind. Series A, Plates VI and VII, show how this space would be opened by a horizontal section a little above the level of the corpora mammillaria. In the present section there is seen, on each side, a slender white band (8), approaching the median line in front of the optic tract, then turning suddenly backward, to reach the inner 97 HORIZONTAL SECTIONS. and anterior part of the tegmentum. This is the beginning of the " posterior longitudinal fasciculus of the tegmentum," already described (page 48) and shown in Series A, Plates VI and VII. It takes its rise from a layer of white and gray matter, called by Reil the " substantia innominata," between the upper surface of the optic tract and the lower surface of the crus cerebri and lenticular nucleus. It thence passes inward and backward, as here shown, and becomes a part of the tegmentum. It appears in transverse section, after making its downward turn in front of the aqueduct of Sylvius, in the two following Plates, XI and XII, and also in the vertical sec- tion, Series C, Plate XIII. The aqueduct of Sylvius (3) is nearly triangular in form, and surrounded by gray substance. Behind, where it is covered by the valve of Vieussens (v, v), the gray layer is very thin; on the sides and in front it is nearly or quite two millimetres in thickness. This deposit is the nucleus of origin for the oculomotorius and patheticus nerves. The root-fibres of the oculomotorius pass almost directly downward and forward, through the tegmentum, to their point of emergence at the inner border of the crus cerebri, and are not visible in this section; those of the patheticus turn backward and upward, cross the median line in the valve of Vieussens, and emerge from it on the opposite side. In this plate the origin of the patheticus nerve from the valve of Vieussens is visible on the right side; on the left it is concealed beneath a projecting remnant of the tubercula quadrigemina. It is seen on both sides in the vertical section, Series C, Plate XIV. The white layer on the surface of the gyrus hippocampi (G H) and that covering the convexity of the hippocampus 98 TOPOGRAPHICAL ANATOMY OF THE BRAIN. (H) are both distinctly shown in this section; and at 7 is the "fascia dentata," interposed between the first of these layers and the fimbria (5). At 6 is the posterior end of the "hook," or recurved extremity of the gyrus hippocampi; and on the right side the attachment of the thin edge of the fimbria to this part is brought into view. The surcingle (s) retains its position in the roof of the ventricle, but is closely approaching the gray matter of its anterior wall. The first frontal convolution runs in a nearly straight direc- tion, next the median line, quite to the anterior border of the lobe. A portion of its cortical layer appears very wide, prob- ably from the section having passed for some distance along the fissure between this convolution and the gyrus fornicatus above. The second frontal convolution is visible on the outer border of the lobe; and the third, already somewhat folded upon itself, forms the anterior and inner boundary of the Syl- vian fissure. The insula (I) is reduced to a single conical projection, representing its apex. The temporal lobe is consti- tuted at this level mainly by its third or inferior convolution; a part only of the lobulus fusiformis being still recognizable on the right side. The cerebellum has been traversed by the section as far back as its great horizontal fissure. H, Hippocampus. GH, Gyrus hippocampi. I, Apex of the insula. SN, Substantia nigra. o, o, Gray matter at base of frontal lobes, s, Surcingle, v, Third ventricle, v v, Valve of Vieussens. x, Lamina cinerea terminalis. y, Locus perforatus anterior, z, Nucleus tegmenti. 1,1, Ascending radicle of fornix. 2,2, Descending radicle of fornix. 3, Aqueduct of Sylvius. 4, 4, Optic tracts. 5, Fimbria. 6, Hook of gyrus hippocampi. 7, Fascia dentata. 8, Origin of posterior longitudinal fasciculus of the tegmentum. Series B. Plate X. HORIZONTAL SECTION. Beneath the anterior commissure, cerebral ganglia, and tubercula quadrigemina. PLATE XI. HORIZONTAL SECTION THROUGH THE CORPORA MAMMILLARIA, CRURA CEREBRI, AND UPPER END OF THE FOURTH VENTRICLE. The section at this level has reached the lower part of the fissure of Sylvius, completely isolating the frontal lobes. At the inner and back part of these lobes is the fissura olfactoria (F O), the posterior extremity of which is closed on the left side, and partially so on the right, by a fold of the cortical layer projecting into it from behind. This fold forms the so- called "tuber olfactorium," which receives the olfactory nerve roots (n, n) in their passage from the gyrus hippocampi for- ward and inward. It is the gray substance of the tuber olfac- torium which extends forward into the prismatic body of the olfactory nerve, justifying the opinion, now generally held, that this nerve is in reality a cerebral convolution. It is covered on its lower surface with a white tract of longitudinal fibres, running forward to the olfactory bulb. The optic tracts (O T) are already connected by a narrow transverse band of white substance, representing the upper 100 TOPOGRAPHICAL ANATOMY OF THE BRAIN. edge of the chiasma. In front of it is a sort of pocket or terminal cul-de-sac of the third ventricle, bounded anteriorly by the thinnest part of the lamina cinerea terminalis (x). Be- hind the white band is the infundibulum (i), surrounded by the gray substance of the tuber cinereum; and a little farther back are the sections of the corpora mammillaria (c, c). One of these bodies has been divided at a lower level than the other, making their sections of unequal size, probably from a slight want of symmetry between them, either in form or position. The crura cerebri in this section are completely detached from the hemispheres. Between their median borders is a deep cleft, running backward to their place of separation on the median line. This is the subarachnoid space at the base of the brain, immediately beneath the locus perforatus pos- terior, named by Magendie the "inferior confluence" of the subarachnoid spaces. It is filled, during life, with the sub- arachnoid serosity. A remnant of the substantia nigra is still perceptible between the crura cerebri and the tegmentum. Behind the crura cerebri and substantia nigra, a variety of new configurations are visible. The section has here passed just below the inferior orifice of the aqueduct of Sylvius, through the extreme upper part of the fourth ventricle, the cavity of which presents a triangular form, with its apex in front and its base behind. The dark colored spot a little in front of its outer angle is the "locus coeruleus," a region on the floor of the fourth ventricle at its upper and outer borders, characterized by an abundant deposit of nerve cells with brownish pigment, which assumes a bluish tinge when seen through its semi-transparent covering of nervous matter. The HORIZONTAL SECTIONS. 101 gray substance in front of the ventricular cavity is a continua- tion of that adjoining the aqueduct of Sylvius in the preceding plate; and a little farther forward, next the median line (at i, i), are the sections of the posterior longitudinal fasciculus of the tegmentum, following its downward course into the medulla oblongata. Immediately behind the ventricle, the valve of Vieussens is now covered by the "lingula" (5), a thin extension from the gray substance of the cerebellum. On each side is a section of the " superior peduncle of the cerebellum" (3), which has at this level the form of a white ribbon-like tract, thicker behind and thinner in front, embracing the central parts of the tegmentum. Externally it is over- lapped by the "lemniscus" (4), a tract of fibres coming ob- liquely downward from the tubercula quadrigemina, and con- tinuing their course into the anterior part of the tegmentum. According to Huguenin, the fibres of the lemniscus are finally mingled with those of the general mass of the tegmentum which go to the optic thalami. In the temporal lobe the inferior horn of the lateral ventricle (v) appears as an undulating transverse fissure, somewhat widened at its outer extremity. Directly in front of it, is the gray substance of the end of the gyrus hippocampi, forming the so-called amygdala (A). Behind it, is the inner end of the pes hippocampi (H), forming part of the "hook" of the hippo- campal convolution. The anatomical relation of the parts in question may be further shown by comparing this view with those in the pre- ceding and following plates (X and XII); with the vertical sections, Series C, Plates X and XI; and especially with the median section, Series A, Plate VIII. In the last-named view, 102 TOPOGRAPHICAL ANATOMY OF THE BRAIN. the gyrus hippocampi seems to turn backward in a free extremity, forming the hook or unciform process. But the horizontal furrow between this process and the body of the hippocampal convolution is not deep enough to make a com- plete separation between them. It extends outward only for a distance of twelve or thirteen millimetres; and the cavity of the ventricle, above it and in front, comes within three or four millimetres of the median surface. The "hook" of the gyrus hippocampi is really the inner end of the hippocampus itself, which at its extreme anterior part turns sharply round toward the median line, continuing the curvature until its extremity at last points backward. It is this terminal curved portion of the hippocampus, or the " pes hippocampi," which, from its supposed resemblance to the paw of a marine animal, has given its name to the whole convolution. The gray matter of the amygdala presents, at its anterior and outer part, an appearance of striation, owing to its being traversed by narrow bands of white substance. This is also seen in the vertical section, Series C, Plate VIII. In its posterior regions the present section has nearly reached the middle level of the cerebellum, showing a part of its central white substance and the extension of its medullary laminae outward and forward. The temporal lobes are some- what reduced in size, and are deeply penetrated by fissures from behind and below. Fissure oj Sylvius \, Fissure oj '* Sylvius A, Amygdala. FO, Fissura olfactoria. OT, Optic tracts. H, Reflected end of hippocampus. GH, Gyrus hippocampi, c, c, Corpora mammillaria. i, Infundibulum, n, n, Olfactory nerve roots, v, v, Inferior horn of lateral ventricle, x, Lamina cinerea terminalis. 1, 1, Posterior longitudinal fasciculus of the tegmentum. 2, 2, Locus caeruleus. 3, 3, Superior peduncles of the cerebellum. 4, 4, Lemniscus. 5, Lingula. Series B. Plate XI. HORIZONTAL SECTION. Through the corpora mammillaria, crura cerebri, and upper end of the fourth ventricle. PLATE XII. HORIZONTAL SECTION THROUGH THE OPTIC CHIASMA AND TUBER ANNULARE. In Plate XII the first frontal convolution appears, next the median fissure, as the "gyrus rectus," with a double cortical layer and a central tract of white substance. At the pos- terior extremity of the fissura olfactoria is the olfactory nerve (n, n), passing downward and forward between the adjacent borders of the fissure. The optic chiasma (Ch) has been divided by the section, but not symmetrically, its wider part being on the right side. Immediately behind it (at i) is the infundibulum. The crura cerebri are no longer visible, the section having passed below the level of their emergence from the tuber annulare. The oculomotorius nerve (x) is divided obliquely, a short distance below its origin from the inner border of the crus. The tuber annulare shows transverse bundles of fibres cross- ing at different levels in its interior, and between them the longitudinal bundles passing through it to the crura cerebri 104 TOPOGRAPHICAL ANATOMY OF THE BRAIN above. At its back part, which is still a continuation of the tegmentum, the posterior longitudinal fasciculi (i, i) are visible next the median line, with slender tapering extensions outward and backward. These extensions lie immediately in front of the locus coeruleus (2, 2), which is longer and narrower than in the preceding section. In the median plane of the tegmentum is a delicate white lamina, separating it into right and left halves. This structure is the " raphe." It is visible in all sections from this point downward, to just above the decussation of the pyramids. It contains nerve fibres running horizontally from front to rear, crossed by many commissural or decussating fibres from side to side. It is regarded as the place of decussation of many of the root fibres of the cranial nerves. The cavity of the fourth ventricle (4) has now increased in width, and on each side of it the superior peduncles of the cerebellum (3, 3) appear at the level of their greatest thickness. In the central white substance of the cerebellum, near the median line, is the "corpus dentatum" (5), a thin lamina of gray substance folded upon itself in many closely set indentations. It has the form of a wrinkled sac, closed on its outer and posterior aspects, but open in front and toward the median plane. It resembles in many respects the olivary nucleus in the medulla oblongata. In the temporal lobes the inferior horn of the lateral ven- tricle (v), the amygdala (A), and the pes hippocampi (H), retain their former relative positions. The undulations of the pes hippocampi extend through to its inferior surface, where they rest upon the upper border of the hippocampal convolu- tion. A, Amygdala. Ch, Chiasma. FO, Fissura olfactoria. H, End of pes hippocampi. GH, Gyrus hippocampi i, Infundibulum, n, n, Olfactory nerves, v, v, Inferior horn of lateral ventricle, x, Oculo-motorius nerves. 1, 1, Posterior longitudinal fasciculus of the tegmentum. 2, 2, Locus cseruleus. 3, 3, Superior peduncles of the cerebellum. 4, Fourth ventricle. 5, Corpus dentatum. Series B. Plate XII. HORIZONTAL SECTION. Through the optic chiasma and tuber annulare. PLATE XIII. HORIZONTAL SECTION THROUGH THE LOWERMOST PART OF THE FRONTAL LOBES AND THE PONS VAROLII. The frontal lobes are now reduced to a trifling remnant, showing hardly any of their white substance and only one or two of their convolutions. Near the median line these lobes reach a lower level than elsewhere, and they consequently remain visible at this region in sections from which their lat- eral parts have disappeared. The olfactory nerve (n) is ex- posed for a portion of its length in the fissura olfactoria, and on the left side a trace of the olfactory bulb is indistinctly perceptible below the anterior part of this fissure. The appa- rent difference in size of the olfactory nerves is due to their prismatic form and to a slight difference in their level on the two sides. On the left side the section has passed along the upper or deep-seated narrow border of the nerve. On the right side it has traversed its lower, superficial, and wider portion. The vertical section, Series C, Plate V, shows how this might happen in a horizontal section of the frontal lobes. 106 TOPOGRAPHICAL ANATOMY OF THE BRAIN. The optic nerves (i, i) in this plate are also unsymmetrical, owing to a difference in their position near the chiasma. The preceding section, Plate XII, passed through the chiasma in such a way as to show its thicker portion on the right side. In Plate XIII the section on the left side still includes a por- tion of the chiasma, making it the thicker of the two. On the right side it has traversed only a portion of the nerve, the remainder'being already engaged in the optic foramen. Irregularities of this kind are constantly perceptible in brain sections. They may be due either to imperfect symmetry of the brain, to slight obliquity of the section, or to both these causes combined. It is manifestly impossible to secure mathe- matical accuracy of level in the section-plane; and an exam- ination of all the plates of Series B will show that the sections of this series are generally, though not always, a little lower on the right side than on the left. There is a similar ine- quality between the two sides in vertical sections, as well as in views of the exterior of the brain; and there can be no doubt that the hemispheres are, as a rule, unsymmetrical in most of their parts. But the apparent irregularity of their section surfaces may be either increased or diminished by ob- liquity of the section plane. The temporal lobes, in Plate XIII, are still of considerable size, though they have lost their characteristic features. Between them is the infundibulum (2), passing downward to the pitui- tary body. Posteriorly, the section has passed through the pons Varolii, showing its curved commissural fibres. It is now below the level of the superior peduncles of the cerebellum, and has reached that of the middle peduncles (5, 5), by which the right HORIZONTAL SECTIONS. 107 and left halves of the cerebellum are connected with each other through the tuber annulare and the pons. A large por- tion of the fibres of these peduncles pass, in their well-known arched form, as the pons Varolii, round the superficial border of the tuber annulare. The remainder cross, in a more direct manner, as deep-seated transverse bands, separated from each other by the longitudinal bundles of the interior. The raphe shows itself, as a well-marked whitish lamina, in the median plane. The emergence of the trigeminus nerve (3, 3) from the side of the pons is shown just below the level of the section su face. On the floor of the fourth ventricle, next the median line, are two eminences, separated from each other by a well- marked median furrow. These projections, which are near the inferior and posterior ends of the eminentiae teretes, corre- spond in situation with the nucleus of origin of the abducens and facial nerves, and with a knee, or bend, of the facial root fibres as they approach this nucleus from the side of the medulla. The narrow white tracts, extending from this point horizontally outward, are probably those of the facial root fibres. In the central white substance of the cerebellum, which is here shown in its widest extent, is the corpus dentatum, more fully developed than in the preceding section. The inferior and median convolutions of the cerebellum also come into view, where they cover the fourth ventricle. A, Remnant of frontal lobes. FO, Fissura olfactoria. n, n, Olfactory nerves. I, I, Optic nerves. 2, Infundibulum. 3, 3, Trigeminus nerve. 4, Fourth ventricle. 5, 5, Middle peduncles of the cerebellum 6, 6, Corpus dentatum. Series B. Plate XIII. HORIZONTAL SECTION. Through lowermost part of frontal lobes and the pons Varolii. PLATE XIV. HORIZONTAL SECTION THROUGH THE LOWERMOST PART OF THE TEMPORAL LOBES, AND THE PONS VAROLII. In Plate XIV the frontal lobes have entirely disappeared, and only the lowermost portions of the temporal lobes remain. The tuber annulare still shows on its anterior and lateral bor- ders the arched bundles of the pons Varolii; but their con- nection with the cerebellum, through the middle peduncles, is no longer visible. On the other hand, the inferior peduncles of the cerebellum (i, i) have now come into view, as two rounded cords beneath the lateral parts of the fourth ventricle. They form the connection between the cerebellum and the spinal cord. The fourth ventricle is here opened at its widest part, and is covered posteriorly by the gray matter of the uvula and tonsils (3, 4), on the under surface of the cerebellum. On the front border of the cerebellum, next the pons Varolii, is the small and comparatively isolated lobule (2) termed the "floc- culus." The section surface of the entire cerebellum is smaller than in Plate XIII, and its white substance is now divided by the intervening convolutions of its middle portion. 1,1, Inferior peduncles of the cerebellum. 2, 2, Flocculus. 3, Uvula. 4, 4, Tonsils, v, Fourth ventricle. Series B. Plate XIV HORIZONTAL SECTION. Through lowermost part of temporal lobes, and the pons Varolii. 111 EXTRA-SERIAL PLATE XV. HORIZONTAL SECTION THROUGH THE ANTERIOR, MIDDLE, AND POSTERIOR COMMISSURES. This section does not belong to those of Series B, but is taken from a different brain. It is introduced as an additional illustration, because it happens to show the anterior commissure extending uninterruptedly for a considerable distance on each side, and in an unusually symmetrical form. Its apparent variations in thickness are mainly due to slight changes of level in its different parts, so that the section passes some- times through its middle and sometimes nearer its upper or lower margin. When carefully exposed by a section so made as to follow exactly its undulating course, it appears through- out of nearly uniform size. It crosses the median line immediately in front of the anterior pillars of the fornix, thence runs outward and forward, and then turns backward, making a curvature with an anterior convexity. It disappears about the middle of the lenticular nucleus, where it begins to follow a downward sloping direc- tion, toward the convolutions of the temporal lobe. CS, Corpus striatum. LN, Lenticular nucleus. OT, Optic thalamus, x, x, x, Anterior commissure S, Surcingle. H, Hippocampus. 1,1, Anterior pillars of the fornix. 2, Middle commissure. 3, Posterior commissure and conarium. Extra-serial. Plate XV. HORIZONTAL SECTION. Through the anterior, middle, and posterior commisures.