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The grey matter of the medulla oblongata, which contains | The transverse fibres go from one hemisphere of the numerous multipolar nerve cells, is in part continuous with cerebellum to that of the opposite side ; some are situated the grey matter of the spinal cord, and in part consists of in on the anterior surface of the pons, and form its superficial dependent masses. As the grey matter of the cord enters the transverse fibres, whilst others pass through its substance medulla it loses its crescentic arrangement. The posterior and form the deep transverse fibres. The transverse fibres cornua are thrown outwards towards the surface, lose their of the pons constitute, therefore, the commissural or pointed form, and dilate into rounded masses named the connecting arrangement by which the two hemispheres of grey tubercles of Rolando, whilst portions are prolonged the cerebellum become anatomically continuous with each into both the posterior pyramid and the restiform body. other. The longitudinal fibres of the pons ascend or pass The grey matter of the anterior cornua and of the intermedio- vertically upwards from the medulla oblongata, and consist lateral tracts loses its continuity, and becomes subdivided of the fibres of the anterior pyramids, Olivary fasciculi, into numerous small masses, owing to being traversed by fasciculi teretes, and posterior pyramids. They leave the bundles of nerve fibres, which give rise to a network termed pons by emerging from its upper surface as fibres of the formatio reticularis, in the meshes of which the groups of two crura cerebri. The pons possesses a median raphe nerve cells are contained. In the lower part of the medulla continuous with that of the medulla oblongata, and formed a central canal continuous with that of the cord exists, but like it by a decussation of fibres in the mesial plane. when the restiform bodies and posterior pyramids on the The grey matter of the pons is scattered irregularly opposite sides of the medulla diverge from each other, the through its substance, and appears on its posterior surface; central canal loses its posterior boundary, and dilates into but not on the anterior surface, which is composed excluthe cavity of the 4th ventricle. The grey matter in the sively of the superficial transverse fibres. It is traversed interior of the medulla appears, therefore, on the floor of the both by the longitudinal and deep transverse fibres, which ventricle; that which corresponds to the anterior cornua form a well-defined formatio reticularis. To a portion of being situated immediately on each side of the median fur- grey matter, containing nerve cells charged with dark row, whilst that which is continuous with the grey tubercles pigment, the name of locus cæruleus is applied. The locus of Rolando and the posterior cornua is some distance ex lies on the floor of the 4th ventricle, close to the entrance ternal to it. This grey matter forms collections of nerve to the aqueduct of Sylvius, and serves as the origin of the cells, which are the centres of origin of several important sensory root of the 5th, and perhaps of the posterior root of encephalic nerves.
the 4th cranial nerve. The nerve cells of the pons are multiof the independent masses of grey matter of the medulla, polar and stellate. The pons acts as a conductor of that which forms the corpus dentatum within the olivary impressions through its nerve fibres, and as a centre of body is the most important, and constitutes the nucleus of origin of nerve fibres from nerve cells. Meynert states that the inferior olive. It is folded on itself in a zig-zag or some of the fibres of the crura cerebri end in the nerve denticulated manner, and forms a sort of capsule open on cells of the pons, which cells again give origin to fibres the inner aspect, through which opening's a bundle of nerve that pass outwards to the cerebellum. fibres from the interior of the capsule proceeds. These The CEREBELLUM, LITTLE BRAIN, or AFTER BRAIN (Pl. Cerefibres aid in the formation of the olivary fasciculus, and as XVIII. fig. 2, c), occupies the inferior pair of occipital fosse, bellum. Deiters and Meynert have pointed out, in part arch across the and, along with the pons and medulla oblongata, lies below mesial plane and join the restiform body on the opposite side, the plane of the tentorium cerebelli. It consists of two hemiwhilst some apparently join the posterior pyramid. The spheres or lateral lobes, and of a median or central lobe, which nerve cells of the olive are multipolar and flask-shaped, and in human anatomy is called the vermiform process. It is in all probability give origin to the nerve fibres proceeding connected below with the medulla oblongata by the two restifrom the interior of the capsule. Separated from the inner form bodies which form its inferior peduncles, and above to part of the olive by a layer of reticular substance is a smaller the corpora quadrigemina of the cerebrum by two bands, grey mass, called by Stilling nucleus olivaris accessorius. which form its superior peduncles; whilst the two hemiCrossing the anterior surface of the medulla oblongata, spheres are connected together by the transverse fibres of the immediately below the pons, in the majority of mammals pons, which form the middle peduncles of the cerebellum. is a transverse arrangement of fibres forming the trapezium, On the superior or tentorial surface of the cerebellum the which contains a grey nucleus, named by Van der Kolk median or vermiform lobe is a mere elevation, but on its the superior olive. In the human brain the trapezium is inferior or occipital surface this lobe forms a well-defined concealed by the lower transverse fibres of the pons, but when inferior vermiform process, which lies at the bottom of a deep sections are made through it, as L. Clarke pointed out, the fossa or vallecula; this fossa is prolonged to the posterior grey matter of the superior olive can be seen. Meynert border of the cerebellum, and forms there a deep notch states that its nerve cells give origin to some fibres, which which separates the two hemispheres from each other; in this run straight backwards to the restiform body of the same notch the falx cerebelli is lodged. Extending horizontally side, and to others which pass across the mesial plane to backwards from the middle cerebral peduncle, along the the opposite corpus restiforme.
outer border of each hemisphere is the great horizontal The Pons VAROLII or BRIDGE (PL. XVIII. figs. 1, 2, 3,N) fissure, which divides the hemisphere into its tentorial and is cuboidal in form : its anterior surface rests upon the occipital surfaces. Each of these surfaces is again subdorsum sellæ of the sphenoid, and is marked by a median divided by fissures into smaller lobes, of which the most. longitudinal groove; its inferior surface receives the pyra- important are the amygdala or tonsil, which forms the midal and olivary tracts of the medulla oblongata ; at its lateral boundary of the anterior part of the vallecula, and superior surface are the two crura cerebri; each lateral | the flocculus, which is situated immediately behind the surface is in relation to a hemisphere of the cerebellum, middle peduncle of the cerebellum. The inferior vermiform and a peduncle passes from the pons into the interior of process is subdivided into a posterior part or pyramid ; an each hemisphere; the posterior surface forms in part the elevation or uvula, situated between the two tonsils ; and upper portion of the floor of the 4th ventricle, and in part an anterior pointed process or nodule. Stretching between is in contact with the corpora quadrigemina.
the two flocculi, and attached midway to the sides of the The pons consists of white and grey matter : the nerve nodule, a thin, white, semilunar-shaped plate of nervous fibres of the white matter pass through the substance of matter, called the posterior medullary velum. the pons, either in a transverse or a longitudinal direction. The whole outer surface of the cerebellum possesses a
characteristic foliated or laminated appearance, due to its axial cylinder of a medullated nerve fibre; for the nerve subdivision into multitudes of thin plates or lamellæ by fibres of the white core enter this layer, divide into minute numerous fissures. The cerebellum consists both of grey fibres, and ramify amidst the granules. From the oppoand white matter. The grey matter forms the exterior or site aspect of each cell two peripheral processes arise, cortex of the lamellæ, and passes from one to the other and ramify in an antler-like manner in the external grey across the bottoms of the several fissures. The white layer. Obersteiner and Hadlich maintain that the finer matter lies in the interior of the organ, and extends into branches of these processes curve back towards the rust the core of each lamella. When a vertical section is made coloured layer, where, according to Boll, they form a netthrough the organ, the prolongations of white matter work of extreme minuteness, from which it is believed branching off into the interior of the several lamellæ give that nerve fibres may arise. The substratum of the grey to the section an arborescent appearance, known by the layer, in which the branched processes of the cells of fanciful name of arbor vitæ (Pl. XVIII. fig. 3,c). Independent Purkinje lie, consists of a very delicate neuroglia, in which masses of grey matter are, however, found in the interior scattered corpuscles are imbedded; but, in the outer part of the cerebellum. If the hemisphere be cut through a of this layer, delicate supporting connective tissue-like little to the outer side of the median lobe, a zig-zag arrange- fibres are also met with. ment of grey matter, similar in appearance and structure The Fourth Ventricle is the dilated upper end of the cen- Fourt' to the nucleus of the olivary body in the medulla oblongata, tral canal of the medulla oblongata. Its shape is like an Veaine. and known as the corpus dentatum of the cerebellum, is heraldic lozenge. Its floor is formed by the grey matter of seen; it lies in the midst of the white core of the hemisphere, and encloses white fibres, which leave the interior of the corpus at its inner and lower side. Stilling has de.scribed, in connection with the anterior end of the inferior vermiform process, which projects forwards into the valve of Vieussens, and aids in the formation of the roof of the 4th ventricle, two grey masses, named roof nuclei. They possess flask-shaped nerve cells like those of the corpus dentatum. The white matter is more abundant in the hemispheres than in the median lobe, and is for the most part directly continuous with the fibres of the peduncles of the cerebellum. Thus the restiform or inferior peduncles pass from below upwards through the white core, to end in the grey matter of the tentorial surface of the cerebellum, more especially in that of the central lobe; on their way they are connected both with the grey matter of the corpus dentatum and of the roof nuclei. The superior peduncles, which descend from the corpora quadrigemina of the cerebrum, reach the grey cortical matter, more especially on the inferior surface of the cerebellum, though they also Fig. 68.-Floor of the fourth ventricle and adjacent structures. 1, pine form connections with the corpus dentatum. The middle
gland; 2, the nates, and 3, the testes of the corpora quadrigemina; 4.
middle peduncles, 5, 5, superior peduncles, 9, 9, inferior peduncles of the peduncles form a large proportion of the white core, and cerebellum; 6, 6, valve of Vieussens divided; 7, 7, fasciculi teretes; 8, 8, roots
of the auditory nerves; 9', corpus dentatum; 10, 10, posterior pyramids; 11, their fibres terminate in the grey matter of the foliated
calamus scriptorius. cortex of the hemispheres. But, in addition to these peduncular fibres, which connect the cerebellum to other the posterior surfaces of the medulla oblongata and pons; subdivisions of the encephalon, its white matter contains its roof partly by the inferior vermiform process of the cerefibres proper to the cerebellum itself. The fibræ proprio bellum, the nodule of which projects into its cavity, and have been especially described by Stilling; some, which he partly by a thin layer, called valve of Vieussens, or anterior has termed the median fasciculi, lie near the mesial plane, medullary velum ; its lower lateral boundaries, by the diverand connect the grey matter on the tentorial aspect of the gent restiform bodies and posterior pyramids; its upper middle lobe with that of the inferior vermiform process, lateral boundaries, by the superior peduncles of the cerewhilst others cross directly the mesial plane to unite opposite bellum; the reflection of the arachnoid membrane from the and symmetrical regions of the hemispheres. Further, the back of the medulla to the inferior vermiform process closes auditory nerve was said by Foville to derive some of its it in below, but allows of a communication between its cavity fibres of origin from the cerebellum; the connection of this and the sub-arachnoid space; above, it communicates with nerve with the cerebellum has been strongly insisted on by the aqueduct of Sylvius, which is tunnelled through the sub. Meynert, and this anatomist has also ascribed a cerebellar stance of the corpora quadrigemina. Along the centre of origin to a portion of the sensory root of the 5th cranial the floor is the median furrow, which terminates below in
a pen-shaped form, the so-called calamus scriptorius. The grey inatter of the cortex is divided into two well Situated on its floor are the fasciculi teretes, striæ acousdefined layers, an external grey, and an inner rust coloured ticæ, and deposits of grey matter described in connection layer of about equal thickness. The rust coloured layer is with the medulla oblongata. Its endothelial lining is condistinguished by containing multitudes of so called “gran- tinuous with that of the central canal. ules,” the well-defined nucleus in which, as described by The CEREBRUM or Great BRAIN lies above the plane Strachan, is invested by a small quantity of branched of the tentorium, and forms much the largest division of protoplasm. These “ granules” are, therefore, minute
are, therefore, minute the encephalon. It is customary in human anatomy to stellate cells. Where the rust coloured layer joins the include under the name of cerebrum, not only the convogrey layer the characteristic nerve cells of the cerebel lutions, the corpora striata, and the optic thalami, developed lum, named the corpuscles of Purkinje, are situated. А in the anterior cerebral vesicle, but also the corpora quadrislender central process arising from each cell enters the gemina and crura cerebri developed in the middle cerebral rust coloured layer, and, as the observations of Hadlich vesicle. The cerebrum is ovoid in shape, and presents and Koscheunikoff show, becomes continuous with the superiorly, anteriorly, and posteriorly a deep median longitia
dinal fissure, which subdivides it into two hemispheres. volution, and passing forwards to the anterior end of the Inferiorly there is a continuity of structure between the cerebrum, are three convolutions, arranged in parallel tiers two hemispheres across the mesial plane, and if the two from above downwards, and named superior, middle, and hemispheres be drawn asunder by opening out the longi- inferior frontal convolutions, which are also prolonged on tudinal fissure, a broad white band, the corpus callosum, to the orbital face of the frontal lobe. The Parietal Lobe may be seen at the bottom of the fissure passing across the is also complex ; its most anterior convolution, named inesial plane from one hemisphere to the other. The outer ascending parietal, ascends parallel to and immediately surface of each hemisphere is convex, and adapted in shape behind the fissure of Rolando. Springing from the upper to the concavity of the inner table of the cranial bones ; end of the back of this convolution is the postero-parietal its inner surface, which bounds the longitudinal fissure, is convolution, which, forming the boundary of the longitudinal flat and is separated from the opposite hemisphere by the 'fissure, extends as far back as the parieto-occipital fissure; falx cerebri; its under surface, where it rests on the springing from the lower end of the back of this convotentorium, is concave, and is separated by that membrane lution is the supra-marginal convolution, which forms from the cerebellum and pons. From the front of the pons the upper boundary of the hinder part of tho Sylvian two strong white bands, the crura cerebri or cerebral peduncles, pass forwards and upwards to enter the optic
Fr. thalami in their respective hemispheres. Winding round
Par the outer side of each crus is a flat white band, the optic tract. These tracts converge in front, and join to form the optic commissure, from which the two optic nerves arise. The crura cerebri, optic tracts, and optic commissure enclose a lozenge shaped space, which includes—a, a grey layer,
Yos called porus Tarini, which, from being perforated by several small arteries, is often called locus perforatus posticus, Oc b, two white mammillæ, the corpora albicantia ; c, a grey nodule, the tuber cinereum, from which, d, the infundibulum projects to join the pituitary body. Immediately in front of the optic commissure is a grey layer, the lamina cinerea or lamina terminalis of the 3d ventricle; and
TS between the optic commissure and the inner end of each
Fig. 69. Sylvian fissure is a grey spot perforated by small arteries, the locus perforatus anticus.
SF The peripheral part of each hemisphere, which consists
IF of grey matter, exhibits a characteristic folded appearance, known as the convolutions or gyri of the cerebrum. These convolutions are separated from each other by fissures or
Er sulci, some of which are considered to subdivide the hemisphere into lobes, whilst others separate the convolutions in each lobe from each other. In each hemisphere of the
R human brain five lobes are recognised: the temporo-sphe
-IP noidal, frontal, parietal, occipital, and the central lobe or insula. Passing obliquely on the outer face of the hemisphere from before, upwards and backwards, is the wellmarked Sylvian fissure, which is the first to appear in the development of the hemisphere. Below it lies the temporo-sphenoidal lobe, and above and in front of it, the parietal and frontal lobes. The frontal lobe is separated from the parietal by the fissure of Rolando, which extends on the outer face of the hemisphere from the longitudinal
PO fissure obliquely downwards and forwards towards the Sylvian fissure. About two inches from the hinder end of the hemisphere is the parieto-occipital fissure, which, commencing at the longitudinal fissure, passes down the
О. inner surface of the hemisphere, and transversely outwards
Fig. 70. for a short distance on the outer surface of the hemi
Figs. 69 and 70.-Profile and vertex views of cerebrum. Fr, the frontal lobe; sphere; it separates the parietal and occipital lobes from
Par, parietal; Oc, occipital; T's, temporo-sphenoidal lobe; SS, Sylvian tissure; each other.
RR, fissure of Rolando; Po, parieto-occipital fissure; IP, intra-parietal fissure;
PP, Parallel fissure ; SF and IF, supero- and infero-frontal fissures; 1, 1, 1, The Temporo-Sphenoidal Lobe presents on the outer surface
inferior, 2, 2, 2, middle, and 3, 3, 3, superior frontal convolutions; 4, 4, of the hernispbere three convolutions, arranged in parallel
ascending frontal convolution; 5, 5, 5, ascending parietal, 5', postcro-parietal,
and 6, 6, angular convolutions; A, supra-marginal, or convolution of the parietal tiers from above downwards, and named superior, middle and eminence; 7, 7, superior, 8, 8, 8, middle, and 9, 9, 9, inferior temporoinferior temporo-sphenoidal convolutions. The fissure which sphenoidal convolutions; 10, superior, 11, middle, and 12, inferior occipital
convolutions; 2, B, 7, •, four annectent convolutions. separates the superior and middle of these convolutions is called the parallel fissure. The Occipital Lobe also con- fissure; as this gyrus occupies the hollow in the parietal sists from above downwards of three parallel convolutions, bone, which corresponds to the eminence, it may appronamed superior, middle, and inferior occipital. The priately be named the convolution of the parietal eminence. Frontal Lobe is more complex ; immediately in front of the Continuous with the convolution of the parietal eminence fissure of Rolando, and forming indeed its anterior boundary, is the angular convolution, which bends round the posterior is a convolution named ascending frontal, which ascends extremity of the Sylvian fissure. Lying in the parietal lobe obliquely backwards and upwards from the Sylvian to the is the intra-parietal fissure, which separates the convolution longitudinal fissure. Springing from the front of this con- 1 of the parietal eminence from the postero-parietal con
I. — 110
volution. The occipital is connected with the parietal lobe | then, from being co-terminous with the parietal lobe of the by two annectent or bridging gyri, which bridge across the cerebrum, is trenched on anteriorly by the frontal, postetransverse external part of the parieto-occipital fissure ; the riorly by the occipital, and inferiorly by the temporo
sphenoidal lobe. The convolutions of the parietal lobe itself are grouped around the parietal eminence, and in the interval between it and the sagittal suture. The inner table of the cranial bones is an almost exact inould of the convolutions of these lobes ; but this is not so with the exterior of the skull, the configuration of which is modified by the formation of ridges and processes for the attachment of muscles, by variations in the thickness of the diploë, and by the development of the frontal and mastoid airsinuses. Hence the outer surface of the skull does not correspond in shape to the outside of the brain.
The Central Lobe of the hemisphere, more usually called the insula or island of Reil, does not come to the surface of the hemisphere, but lies deeply within the Sylvian fissure, the convolutions forming the margin
of which conceal it.
It consists of four or
TR depth and extent of this fissure vary in different brains in
tions, which radiate
0 proportion to the size of these bridging convolutions. The from the locus persuperior annectent gyrus passes between the postero-parietal foratus anticus, situand the superior occipital convolutions, whilst the second an
ated at the inner nectent gyrus connects the middle occipital with the angular end of the fissure. gyrus. Two annectent gyri also pass from the inferior occi- This lobe is almost pital convolution to the lower convolutions of the temporosphenoidal lobe. These lobes of the cerebrum, though by a deep sulcus,
entirely surrounded named after the bones which form the vault of the skull, are
which insulates it not exactly co-terminous with them. The frontal lobe not from the adjacent only lies under cover of the frontal bone, but extends back-convolutions. ît lies wards under the anterior part of the parietal ; for the fissure
opposite the upper of Rolando, which forms its posterior boundary, lies from
part of the ali 14 to 2 inches behind the coronal suture. The occipital lobe is not limited to the upper tabular part of the occipital articulates with the
sphenoid, where it bone, but extends forwards under cover of the posterior parietal and squapart of the parietal, for the parieto-occipital fissure lies
mous-temporal. about inch in front of the apex of the lambdoidal fissure. Convolutions also and the island of Reil; the tip of the temporada
Fig. 72.-Orbital surface of the left frontal lobe The temporo-sphenoidal lobe not only lies under the
exist on the inner sphenoidal lobe has been removed to display the squamous-temporal and great wing of the sphenoid, but surface of the hemi- tudinal fissure; o, olfactory fissure, over which passes upwards under cover of the lower part of the parietal, sphere, and on the the colfactorie peduncle and lobe of use ones antena for the Sylvian fissure passes from below obliquely upwards under surface which orbital surface; 1, 1, 1, 1, under surface of inferoand backwards across the line of the squamous suture near
rests on the tento- ing frontal, and 5, of ascending parietal convolu
frontal convolution; 4, under surface of ascend. its middle. The area covered the parietal bone so far,
rium, but these have tions; C, central lobe or insula. 1 The above view of the brain in situ shows the relations of the sur
no relation to the bones of the cranial vault. They may face convolutions to the regions of the skull. R, fissure of Rolando, be studied in connection with the corpus callosum or which separates the frontal from the parietal lobe. Po, parieto-occi- great transverse commissure, which connects the two pital fissure between the parietal and occipital lobes. SS, fissure of Sylvius, which separates the temporo-sphenoidal from the frontal and
hemispheres, and with certain fissures situated on these parietal lobes. SF, MF, IF, the supero-, mid-, and infero-frontal sub- surfaces of the hemisphere. The small convolutions which divisions of the frontal area of the skull; the letters are placed on the lie behind the internal part of the parieto-occipital fissure superior, middle, and inferior frontal convolutions; the inferior frontal form the inner convolutions of the occipital lobe, or the occiregion is separated from the middle frontal by the frontal part of the pital lobule (Fig. 73). Those which lie immediately in front curved line of the temporal ridge; the mid- from the supero-frontale by 1 of the same fissure belong to the inner face of the parietal an antero-posterior line througb the frontal eminence. SĀP, the superoantero-parietal area of the skull; S is placed on the ascending parietal lobe, and form the quadrilateral lobule. It is customary, convolution, AP on the ascending frontal convolution. IAP, the infero- however, to name the convolution which extends forwards antero-parietal area of the skull; I is placed on the ascending parietal, AP
from that fissure along the margin of the longitudinal on the ascending frontal convolution. SPP, the supero-postero-parietal area of the skull; the letters are placed on the angular convolution.
fissure to the anterior end of the hemisphere, and which IPP, the infero-postero-parietal area of the skull ; the letters are placed then turns back to the locus perforatus anticus as the on the mid-temporo-sphenoidal convolution; the temporal ridge separates marginal convolution. This is separated by a fissure called the supero- and infero-parietal regions from each other; a vertical line calloso-marginal, from the callosal convolution or gyrus drawn through the parietal eminence separates the antero- and posteroparietal regions. X, the convolution of the parietal eminence, or supra- fornicatus, which, commencing at the locus perforatus anmarginal gyrus. 6, the occipital area of the skull; the letter is placed ticus, turns round the anterior end of the corpus callosum, on the mid-occipital convolution. Sq, the squamoso-temporal region extends parallel to its upper surface, and then turns round of the skull; the letters are placed on the mid-temporo-sphenoidal convolution. AS, the ali-sphenoid region of the skull; the letters are
its posterior end. It is separated from the corpus callosum placed on the tip of the supero-temporo-sphenoidal convolution. The by the callosal fissure, at the bottom of which the matter
grey black lines mark the boundaries of different cranial regions.
of the gyrus fornicatus termintes in a well-defined edge.
The callosal convolution encloses the corpus callosum | the surface of the corpus callosum a few fibres, the striæ within the concavity of its arch, and from its direction is longitudinales, run in the antero-posterior or longitudinal
TS Fig. 73.-Convolutions of the inner and tentorial surfaces of the left hemisphere. i, i, i, calloso-marginal fissure; 1, l, calcarine fissure; m, m, hippocampal fissure; m, n, collateral fissure; PO, parieto-occipital fissure ; 17, 17, marginal convolution; 18, 18, gyrus fornicatus; 18', quadrilateral lobule; 19, hippocampal gyrus; 19', its recurved end; 25, occipital lobule; 9, 9, infe
rior temporo-sphenoidal convolution. appropriately called fornicatus (arch-shaped). The posterior end of the callosal convolution curves downwards and then forwards, under the name of gyrus hippocampi, to the tip of the inner surface of the temporo-sphenoidai lobe. This gyrus is separated anteriorly by a narrow curved fissure called hippocampal fissure, from a white band, the tænia hippocampi, which band possesses a free curved border, round which the pia mater and choroidal artery enter the lateral ventricle through the great transverse fissure of the cerebrum. The hippocampal fissure is con
Fig. 74.-To show the right ventricle and the left half of the corpus callosum. tinuous round the posterior end of the corpus callosum a, transverse fibres, and b, longitudinal fibres of corpus callosum; s, anterior, with the callosal fissure, and at the bottom of the hippo
and d, posterior cornua of lateral ventricle; e, septum lucidum; fi corpus
striatum; 9, tænia semicircularis; h, optic thalamus; k, choroid plexus ; 1, tænja campal fissure the grey matter of the gyrus hippocampi hippocampi; m, hippocampus major; n, hippocampus minor; o, eminentia colterminates in a well-defined dentated border (fascia dentata). The hippocampal fissure on this surface of the direction. If the corpus callosum be now cut through on hemisphere marks the position of an eminence in the de- each side of its mesial line, the large cavity or lateral scending cornu of the ventricle called hippocampus major. ventricle in each hemisphere will be opened into. The gyrus hippocampi is separated posteriorly from the The lateral ventricle is subdivided into a central space adjacent temporo-sphenoidal convolution by a fissure, named or body, and three bent prolongations or cornua ; the collateral, which marks the position on this surface of the anterior cornu extends forwards and outwards into the hemisphere of the collateral eminence in the interior of the frontal lobe; the posterior cornu curves backwards, ventricle. From the lower end of the parieto-occipital outwards, and inwards into the occipital lobe; the defissure an offshoot, called the calcarine fissure, passes almost scending cornu curves backwards, outwards, downwards, horizontally backwards in the occipital lobe, which fissure forwards, and inwards, behind and below the optic thamarks on this surface of the hemisphere the eminence named lamus into the temporo-sphenoidal lobe. On the floor of calcar avis, or hippocampus minor, in the posterior cornu of the central space may be seen from before backwards the the ventricle.
grey upper surface of the pear-shaped corpus striatum, and If a horizontal slice be removed from the upper part of to its inner and posterior part a small portion of the optic each hemisphere, the peripheral grey matter of the convo- thalamus, whilst between the two is the curved flat band, lutions will be seen to follow their various windings, whilst the tænia semicircularis. Resting on the upper surface of the core of each convolution consists of white matter con- the thalamus is the vascular fringe of the velum interpositinuous with a mass of white matter in the interior of the tum, named choroid plexus, and immediately internal to hemisphere. If a deeper slice be now made down to the this fringe is the free edge of the white posterior pillar of plane of the corpus callosum, the white matter of that the fornix. The anterior cornu has the anterior end of the structure will be seen to be continuous with the white corpus striatum projecting into it. The posterior cornu centre of each hemisphere. The corpus callosum does not has an elevation on its floor, the hippocampus minor, and equal the hemispheres in length, but approaches nearer to between this cornu and the descending cornu is the elevatheir anterior than their posterior ends (Pl. XVIII. fig. 3, tion called eminentia collateralis. B.) It terminates behind in a free rounded end, whilst in Extending down the descending cornu and following front it forms a knee-shaped bend, and passes downwards its curvature is the hippocampus major, which terminates and backwards as far as the lamina cinerea. If the dissec- below in a nodular end, the pes hippocampi ; on its inner tion be performed on a brain which has been hardened in border is the white tænia hippocampi, continuous above spirit, the corpus callosum is seen to consist almost entirely with the posterior pillar of the fornix. If the tænia bo of bundles of nerve fibres, passing transversely across the drawn on one side the hippocampal fissure is exposed, at mesial plane between the two hemispheres ; these fibres the bottom of which the grey matter of the gyrus hippomay be traced into the white cores and grey matter of the campi may be seen to form a well-defined dentated border convolutions, and apparently connect the corresponding the so-called fascia dentata). The choroid plexus of the convolutions in the opposite hemispheres. Hence the pia mater turns round the gyrus hippocampi, and enters the corpus callosum is a connecting or commissural structure, descending cornu through the great transverse fissure bewhich brings the convolutions of the two hemispheres into tween the tænia hippocampi and optic thalamus. The anatomical and physiological relation with each other. On | lateral ventricle is lined by a cylindrical endothelium,