Part 11

FIG. 9.--Gyri and Sulci, on the outer surface of the Cerebral Hemisphere.

f^1, Sulcus frontalis superior. f^2, Sulcus frontalis inferior. f.m, Sulcus frontalis medius. p.m, Sulcus paramedialis. A, Pars basilaris. B, Pars triangularis. C, Pars orbitalis. S, Sylvian fissure. s^1, Anterior horizontal limb (Sylvian fissure). s^2, Ascending limb (Sylvian fissure). s^3. Posterior horizontal limb (Sylvian fissure). s.asc, Ascending terminal part of the posterior horizontal limb of the Sylvianfissure. p.c.i, Inferior praecentral sulcus. p.c.s, Superior praecentral sulcus. r, Fissure of Rolando. g.s, Superior genu. g.i, Inferior genu. d, Sulcus diagonalis. t^1, Superior temporal sulcus (parallel sulcus). t^2, Inferior temporal sulcus. p^1, Inferior postcentral sulcus. p^2, Superior postcentral sulcus. p^3, Ramus horizontalis. p^4, Ramus occipitalis. s.o.t, Sulcus occipitalis transversus. occ. lat, Sulcus occipitalis lateralis (the sulcus lunatus of Elliot Smith). c.m, Calloso-marginal sulcus. c.t.r, Inferior transverse furrow.]

The _Temporo-Sphenoidal Lobe_ presents on the outer surface of the hemisphere three convolutions, arranged in parallel _tiers_ from above downward, and named _superior, middle and inferior temporal_ gyri. The fissure which separates the superior and middle of these convolutions is called the _parallel fissure_ (fig. 9, t^1). The _Occipital Lobe_ also consists from above downwards of three parallel gyri, named _superior, middle and inferior occipital_. The _Frontal Lobe_ is more complex; immediately in front of the fissure of Rolando, and forming indeed its anterior boundary, is a convolution named _ascending frontal_ or pre-central, which ascends obliquely backward and upward from the Sylvian to the longitudinal fissure. Springing from the front of this gyrus, and passing forward to the anterior end of the cerebrum, are three gyri, arranged in parallel _tiers_ from above downwards, and named _superior, middle and inferior frontal_ gyri, which are also prolonged on to the orbital face of the frontal lobe. The _Parietal Lobe_ is also complex; its most anterior gyrus, named _ascending parietal_ or post-central, ascends parallel to and immediately behind the fissure of Rolando. Springing from the upper end of the back of this gyrus is the supra-parietal lobule, which, forming the boundary of the longitudinal fissure, extends as far back as the parieto-occipital fissure; springing from the lower end of the back of this gyrus is the _supra-marginal_, which forms the upper boundary of the hinder part of the Sylvian fissure; as this gyrus occupies the hollow in the parietal bone, which corresponds to the eminence, it may appropriately be named the _gyrus_ of the _parietal eminence_. Above and behind the gyrus of the parietal eminence is the _angular gyrus_, which bends round the posterior extremity of the parallel fissure, while arching over the hinder end of the inferior temporo-sphenoidal sulcus is the post-parietal gyrus. Lying in the parietal lobe is the _intra-parietal_ fissure (fig. 9, p^3 and p^4), which separates the gyrus of the parietal eminence from the supra-parietal lobule.

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 opercula forming the margin of which, conceal it. It consists of four or five short gyri, which radiate from the _locus perforatus anticus_, situated at the inner end of the fissure. This lobe is almost entirely surrounded by a deep sulcus called the limiting sulcus of Reil, which insulates it from the adjacent gyri. It lies opposite the upper part of the ali-sphenoid, where it articulates with the parietal and squamous-temporal.

[Illustration: FIG. 10.--Orbital surface of the left frontal lobe and the island of Reil; the tip of the temporo-sphenoidal lobe has been removed to display the latter.

17. Convolution of the margin of the longitudinal fissure. O. Olfactory fissure, over which the olfactory peduncle and lobe are situated. TR. Orbital sulcus. 1" 1"'. Convolutions on the orbital suface. 1,1,1,1. Under surface of infero-frontal convolution. 4. Under surface of ascending frontal; and 5, of ascending parietal convolutions. C. Central lobe or insula.]

In front of the central lobe, on the base of the brain, are the _orbital gyri_, which are separated from one another by the _orbital sulcus_. This is usually H-shaped, and the gyri are therefore anterior, posterior, external and internal. Bisecting the internal orbital gyrus is an antero-posteripr sulcus (_s. rectus_), beneath which lies the olfactory lobe, bulbous in front, for the olfactory nerves to arise from.

On the mesial surface of the hemisphere, as seen when the brain is longitudinally bisected and the cerebellum and medulla removed by cutting through the crus cerebri (see fig. 11), the divided corpus callosum is the most central object, while below it are seen the fornix, septum lucidum and third ventricle, the description of which will follow. The cerebral surface, above and in front of the corpus callosum, is divided into two by a sulcus, the contour of which closely resembles that of the upper margin of the corpus callosum. This is the _calloso-marginal sulcus_, so called because it separates the callosal gyrus, which lies between it and the corpus callosum, from the marginal gyri nearer the margin of the brain. When the sulcus reaches a point vertically above the hind end of the corpus callosum it turns sharply upward and so forms the hinder limit of the marginal gyri, the posterior inch or two of which is more or less distinctly marked off to form the _paracentral lobule_, where the upper part of the central fissure of Rolando turns over the margin of the brain. The callosal gyrus, which is also called the gyrus fornicatus from its arched appearance, is continued backward round the posterior end of the corpus callosum, and so to the mesial surface of the temporal lobe. Behind the upturned end of the calloso-marginal sulcus there is a square area which is called the _precuneus_ or _quadrate lobe_; it is bounded behind by the deeply cut internal parieto-occipital fissure and this runs from the margin of the brain downward and forward to join another fissure, the calcarine, at an acute angle, thus enclosing a wedge-shaped piece of brain called the _cuneus_ between them. The _calcarine_ fissure is fairly horizontal, and is joined about its middle by the internal parieto-occipital, so that the part in front of the junction is called the _pre-calcarine_, and that behind the _post-calcarine_ fissure. The internal parieto-occipital and calcarine are real fissures, because they cause an elevation in the interior of the brain, known as the hippocampus minor. Just in front of the anterior end of the calcarine fissure the callosal gyrus is constricted to form the isthmus which connects it with the hippocampal or uncinate gyrus. Below the calcarine fissure is a gyrus called the _gyrus lingualis_, and this is bounded below by another true fissure, the _collateral_, which runs parallel to the calcarine, but is continued much farther forward into the temporal lobe and so forms the lower boundary of the hippocampal gyrus. It will thus be seen that the hippocampal gyrus is continuous posteriorly with the callosal gyrus above by means of the isthmus, and with the gyrus lingualis below. The hippocampal gyrus is bounded above by the dentate or hippocampal fissure which causes the hippocampus major in the descending cornu and so is a complete fissure. If its lips are separated the fascia dentata or gyrus dentatus and the fimbria continued from the posterior pillar of the fornix are seen. Anteriorly the fissure is arrested by the recurved process of the upper part of the hippocampal gyrus, called the _uncus_, and in front of this a slight sulcus, the _incisura temporalis_, marks off the temporal pole or tip of the temporal lobe from the region of the uncus. It will be seen that the callosal gyrus, isthmus, and hippocampal gyrus form nearly a complete ring, and to this the name of _limbic lobe_ is given.

_Interior of the Cerebrum._

If a horizontal slice be removed from the upper part of each hemisphere (see fig. 12), the peripheral grey matter of the gyri will be seen to follow their various windings, whilst the core of each gyrus consists of white matter continuous with a mass of white matter in the interior of the hemisphere. If a deeper slice be now made down to the plane of the corpus callosum, the white matter of that structure will be seen to be continuous with the white centre of each hemisphere known as the centrum ovale. The _corpus callosum_ does not equal the hemispheres in length, but approaches nearer to their anterior than their posterior ends. It terminates behind in a free rounded end, named the splenium (see fig. 11), whilst in front it forms a knee-shaped bend, and passes downwards and backwards as far as the lamina cinerea. If the dissection be performed on a brain which has been hardened in spirit, the corpus callosum is seen to consist almost entirely of bundles of nerve fibres, passing transversely across the mesial plane between the two hemispheres; these fibres may be traced into the white cores and grey matter of the gyri, and connect the gyri, though by no means always corresponding ones, in the opposite hemispheres. Hence the corpus callosum is a connecting or commissural structure, which brings the gyri of the two hemispheres into anatomical and physiological relation with each other. On the surface of the corpus callosum a few fibres, the _striae longitudinales_, run in the antero-posterior or longitudinal direction (see fig. 12, b). Their morphological interest is referred to in the section below on _Comparative Anatomy_. In the sulcus between the corpus callosum and the limbic lobe a narrow band of fibres called the _cingulum_ is seen, most of its fibres only run a short distance in it and link together adjacent parts of the brain. If the corpus callosum be now cut through on each side of its mesial line, the large cavity or _lateral ventricle_ in each hemisphere will be opened into.

[Illustration: From Cunningham, _Text-The book of Anatomy_.

FIG. 11.--The Gyri and Sulci on the Mesial Aspect of the Cerebral Hemisphere, r, Fissure of Rolando. r, o, Rostral sulcus. i, t, Incisura temporalis.]

The lateral ventricle is subdivided into a _central space_ or body, and three bent prolongations or _cornua_; the _anterior cornu_ extends forward, outward and downward into the frontal lobe; the _posterior cornu_ curves backward, outward and inward into the occipital lobe; the _descending cornu_ curves backward, outward, downward, forward and inward, behind and below the optic thalamus into the temporo-sphenoidal lobe. On the floor of the central space may be seen from before backward the grey upper surface of the pear-shaped caudate nucleus of the _corpus striatum_ (figs. 12 and 13, f), and to its inner and posterior part a small portion of the _optic thalamus_, whilst between the two is the curved flat band, the _taenia semicircularis_ (figs. 12 and 13, g). Resting on the upper surface of the thalamus is the vascular fringe of the velum interpositum, named _choroid plexus_, and immediately internal to this fringe is the free edge of the white _posterior pillar of the fornix_. The anterior cornu has the anterior end of the corpus striatum projecting into it. The posterior cornu has an elevation on its floor, the _hippocampus minor_ (fig. 12, n), and between this cornu and the descending cornu is the elevation called _eminentia collateralis_, formed by the collateral fissure (fig. 12, o).

[Illustration: FIG. 12.--To show the Right Ventricle and the left half of the Corpus Callosum.

a, Transverse fibres, and b, Longitudinal fibres of corpus callosum. c, Anterior, and d, Posterior cornua of lateral ventricle. e, Septum lucidum. f, Corpus striatum. g, Taenia semicircularis. h, Optic thalamus. k, Choroid plexus. l, Taenia hippocampi. m, Hippocampus major. n, Hippocampus minor. o, Eminentia collateralis.]

Extending down the descending cornu and following its curvature is the _hippocampus major_, which terminates below in a nodular end, the _pes hippocampi_; on its inner border is the white _taenia hippocampi_, continuous above with the posterior pillar of the fornix. If the taenia be drawn to one side the hippocampal fissure is exposed, at the bottom of which the grey matter of the gyrus hippocampi may be seen to form a well-defined dentated border (the so-called _fascia dentala_). The choroid plexus of the pia mater turns round the gyrus hippocampi, and enters the descending cornu through the lateral part of the great transverse fissure between the taenia hippocampi and optic thalamus. The lateral ventricle is lined by a ciliated epithelium called the _ependyma._ This lining is continuous through the foramen of Monro with that of the third ventricle, which again is continuous with the lining of the fourth ventricle through the aqueduct of Sylvius. A little fluid is contained in the cerebral ventricles, which, under some pathological conditions, may increase greatly in quantity, so as to occasion considerable dilatation of the ventricular cavities.

If the corpus callosum be now divided about its middle by a transverse incision, and the posterior half of this structure be turned back (see fig. 13), the body of the fornix on which the corpus callosum rests is exposed. If the anterior half of the corpus callosum be now turned forward, the grey partition, or _septum lucidum_, between the two lateral ventricles is exposed. This septum fits into the interval between the under surface of the corpus callosum and the upper surface of the anterior part of the fornix. It consists of two layers of grey matter, between which is a narrow vertical mesial space, the _fifth ventricle_ (fig. 13, e), and this space does not communicate with the other ventricles nor is it lined with ependyma. If the septum be now removed, the anterior part of the fornix is brought into view.

The _fornix_ is an arch-shaped band of nerve fibres extending in the antero-posterior direction. Its anterior end forms the _anterior_ pillars of the arch, its posterior end the _posterior pillars_, whilst the intermediate _body_ of the fornix forms the crown of the arch. It consists of two lateral halves, one belonging to each hemisphere. At the summit of the arch the two lateral halves are joined to form the _body_; but in front the two halves separate from each other, and form two anterior pillars, which descend in front of the third ventricle to the base of the cerebrum, where they form the _corpora albicantia_, and from these some white fibres called the bundle of Vicq d'Azyr ascend to the optic thalamus (see fig. 11). Behind the body the two halves diverge much more from each other, and form the posterior pillars, in the triangular interval between which is a thin lamina of commissural fibres called the _lyra_ (fig. 13, a). Each posterior pillar curves downward and outward into the descending cornu of the ventricle, and, under the name of _taenia hippocampi_, forms the mesial free border of the hippocampus major (fig. 13, l). Eventually it ends in the substance of the hippocampus and in the uncus of the temporal lobe. If the body of the fornix be now divided by a transverse incision, its anterior part thrown forward, and its posterior part backward, the great transverse fissure of the cerebrum is opened into, and the velum interpositum lying in that fissure is exposed.

The _velum interpositum_ is an expanded fold of pia mater, which passes into the anterior of the hemispheres through the great transverse fissure. It is triangular in shape; its base is a line with the posterior end of the corpus callosum, where it is continuous with the external pia mater; its lateral margins are fringed by the choroid plexuses, which are seen in the bodies and descending cornua of the lateral ventricles, where they are invested by the endothelial lining of those cavities. Its apex, where the two choroid plexuses blend with each other, lies just behind the anterior pillars of the fornix. The interval between the apex and these pillars is the aperture of communication between the two lateral ventricles and the third, already referred to as the foramen of Monro. The choroid plexuses contain the small _choroidal arteries_; and the blood from these is returned by small veins, which join to form the _veins of Galen._ These veins pass along the centre of the velum, and, as is shown in fig. 1, open into the straight sinus. If the velum interpositum be now carefully raised from before backward, the optic thalami, third ventricle, pineal body and corpora quadrigemina are exposed.

[Illustration: FIG. 13.--A deeper dissection of the Lateral Ventricle, and of the Velum Interpositum.

a, Lyra, turned back. b, b, Posterior pillars of the fornix, turned back. c, c, Anterior pillars of the fornix. d, Velum interpositum and veins of Galen. e, Fifth ventricle. f, f, Corpus striatum. g, g, Taenia semicircularis. h, h, Optic thalamus. k, Choroid plexus. l, Taenia hippocampi. m, Hippocampus major in descending cornu. n, Hippocampus minor. o, Eminentia collateralis.]

The _optic thalamus_ is a large, somewhat ovoid body situated behind the corpus striatum, and above the crus cerebri. Its upper surface is

## partly seen in the floor of the body of the lateral ventricle, but is

for the most part covered by the fornix and velum interpositum. Its postero-inferior surface forms the roof of the descending cornu of the ventricle, whilst its inner surface forms the side wall of the third ventricle. At its outer and posterior part are two slight elevations, in close relation to the optic tract, and named respectively corpus geniculatum internum and externum.

The posterior knob-like extremity of the thalamus is called the _pulvinar_; this, as well as the two corpora geniculata and the superior corpus quadrigeminum, is connected with the optic tract.

The _third ventricle_ (see fig. 6) is a cavity situated in the mesial plane between the two optic thalami. Its roof is formed by the velum interpositum and body of the fornix; its floor by the posterior perforated space, corpora albicantia, tuber cinereum, infundibulum, and optic commissure; its anterior boundary by the anterior pillars of the fornix, anterior commissure and lamina cinerea; its posterior boundary by the corpora quadrigemina and posterior commissure. The cavity of this ventricle is of small size in the living head, for the inner surfaces of the two thalami are connected together by intermediate grey matter, named the _middle_ or _soft commissure_. Immediately in front of the corpora quadrigemina, the white fibres of the _posterior commissure_ pass across between the two optic thalami. If the anterior pillars of the fornix be separated from each other, the white fibres of the _anterior commissure_ may be seen lying in front of them.

[Illustration: From Cunningham, _Text-book of Anatomy_.

FIG. 14.--Horizontal Section through the Right Cerebral Hemisphere at the Level of the Widest Part of the Lenticular Nucleus.]

The _pineal body_ is a reddish cone-shaped body situated upon the anterior pair of the corpora quadrigemina (see figs. 3 and 6). From its broad anterior end two white bands, the _peduncles_ of the _pineal body_, pass forward, one on the inner side of each optic thalamus. Each peduncle joins, along with the taenia semicircularis, the anterior pillar of the fornix of its own side. In its structure this body consists of tubular gland tissue containing gritty calcareous

## particles, constituting the _brain sand_. Its morphology will be

referred to later.

A general idea of the internal structure of the brain is best obtained by studying a horizontal section made just below the level of the Sylvian point and just above the great transverse fissure (see fig. 14). Such a section will cut the corpus callosum anteriorly at the genu and posteriorly at the splenium, but the body is above the plane of section. Behind the genu the fifth ventricle is cut, and behind that the two pillars of the fornix which here form the anterior boundary of the third ventricle. At the posterior end of this is the pineal body, which the section has just escaped. To the outer side of the fornix is seen the foramen of Munro, leading into the front of the body and anterior horn of the lateral ventricle. It will be seen that the lateral boundary of this horn is the cut caudate nucleus of the corpus striatum, while the lateral boundary of the third ventricle is the cut optic thalamus, both of which bodies have been already described, but external to these is a third triangular grey mass, with its apex directed inward, which cannot be seen except in a section. This is the lenticular nucleus of the corpus striatum, the inner or apical half of which is of a light colour and is called the _globus pallidus_, while the basal half is reader and is known as the _putamen._ External to the putamen is a long narrow strip of grey matter called the _claustrum_, which is sometimes regarded as a third nucleus of the corpus striatum. These masses of grey matter, taken together, are the basal nuclei of the brain. Internal to the lenticular nucleus, and between it and the caudate nucleus in front and the thalamus behind, is the _internal capsule_, through which run most of the fibres connecting the cerebral cortex with the crus cerebri. The capsule adapts itself to the contour of the lenticular nucleus and has an anterior limb, a bend or genu, and a posterior limb. Just behind the genu of the internal capsule is a very important region, for here the great motor tract from the Rolandic region of the cortex passes on its way to the crusta and spinal cord. Besides this there are fibres passing from the cortex to the deep origins of the facial and hypo-glossal nerves. Behind the motor tracts are the sensory, including the fillet, the superior cerebellar peduncle and the inferior quadrigeminal tract, while quite at the back of the capsule are found the auditory and optic radiations linking up the higher (cortical) and lower auditory and visual centres. Between the putamen and the claustrum is the _external capsule_, which is smaller and of less importance than the internal, while on the lateral side of the claustrum is the white and then the grey matter of the central lobe. As the fibres of the internal capsule run up toward the cortex they decussate with the transverse fibres of the corpus callosum and spread out to form the _corona radiata._ It has only been possible to deal with a few of the more important bundles of fibres here, but it should be mentioned that much of the white matter of the brain is formed of association fibres which link up different cortical areas, and which become medullated and functional after birth.

_Weight of the Brain._