in

CAUDATE

is sometimes termed the retro-lenticular part of the internal capsule. It is somewhat interrupted, especially in front, by strands of grey matter which unite the caudate and lenticular nuclei. In front, behind, and above it is continuous with the white substance of the hemispheres, its fibres diverging in a fan-like manner towards the cortex (corona radiata of Reil). Below it passes directly into the crusta, of which it is the immediate continuation. In horizontal sections (fig. 301) the internal capsule shows a bend (genu) opposite the stria terminalis, the anterior third forming an angle of about 120° with the posterior two-thirds; these two parts are known as the anterior and posterior limbs respectively. In vertical sections (fig. 300) it appears to take a straight course upwards and outwards from the pedunculus cerebri to the coronal radiata. It has been determined as the result of experimental observations animals, and from pathological and clinical observations in man, that most of the fibres which course in the posterior twothirds of the internal capsule (i.e. which are opposite the globus pallidus of the lenticular nucleus) are connected with the part of the cerebral cortex, excitation of which gives rise to movements in the various parts of the body, whilst those of the anterior third are connected with the prefrontal region. Those of the posterior. third are intermingled with fibres coming from and passing to the occipito-temporal region.

NUCLEUS

OPTIC THALAM

LENTICUL

FIG. 303.-HORIZONTAL SECTION THROUGH
INTERNAL CAPSULE OF MONKEY, SHOW-
ING BY SHADING THE AREA WHICH IS
INVADED BY DEGENERATION WHEN THE
WHOLE OF THE MOTOR CORTEX OF THE
HEMISPHERE OF THE SAME SIDE IS

DESTROYED. (Simpson and Jolly.)

The parts which are invaded by degeneration when the face, arm, and leg regions of the motor cortex respectively are destroyed are indicated. It will be seen that these show considerable overlap. Notice also the invasion of the thalamus (but not of the corpus striatum) by the degeneration.

Besides these fibres which connect the cortex of the hemisphere with the crusta, the internal capsule contains others derived from various sources-viz. from the caudate and lenticular nuclei, the thalamus, and hypothalamus, and also, through the corpus callosum, from the cortex of the opposite hemisphere (Hamilton). The exact localisation within the internal capsule of most of these fibres has not as yet been determined. The nucleus amygdalæ is a collection of grey matter situated above and in front of the anterior end of the inferior cornu of the lateral ventricle, in the roof of which it forms a prominence. It is continuous below and on its side with the anterior end of the hippocampus and the uncus. Its upper surface is partly in relation with the base of the lenticular nucleus and partly free where it forms the lower boundary of the vallecula Sylvii and shows the elevations described by Retzius as the gyrus semilunaris and the gyrus ambiens. The tail of the caudate nucleus passes into it.

THE MEMBRANES OF THE BRAIN AND SPINAL CORD.

The cerebrospinal axis is covered by three membranes, named also meninges. They are: (1) An external fibrous membrane, named the dura mater, which lines the interior of the skull and forms a loose sheath in the spinal canal ; (2) An internal areolar and vascular tunic, the pia mater, which closely covers the brain and spinal cord; and (3) An intermediate non-vascular membrane, the arachnoid, which lies over the pia mater, the two being in some places in close connexion, in others separated by a considerable space.

Some authors describe only two meninges, considering the arachnoid and pia mater to constitute one (leptomeninx), and the dura mater the other (pachymeninx).

THE DURA MATER.

The

The dura mater is a very strong dense inelastic fibrous tunic of considerable thickness (5 mm. or more in the cranium, less in the spinal canal). Its inner surface, turned towards the brain and spinal cord, is smooth and lined with epithelium (endothelium), which was formerly regarded as the outer or parietal portion of a serous sac. It is here described as a part of the dura mater. space between the dura mater and arachnoid was formerly in like manner regarded as the sac of the arachnoid, but is now termed the subdural space. arrangement of the dura mater in the cranium differs in several respects from that in the spinal canal.

The

CRANIAL OR ENCEPHALIC DURA MATER. In the cranium the dura mater adheres to the inner surface of the bones, and forms their internal periosteum. The connexion between the two depends, in a great measure, on blood-vessels and small fibrous processes, which pass from one to the other; and the membrane, when detached and allowed to float in water, presents a flocculent appearance on its outer surface, in consequence of the torn parts projecting from it. The adhesion between the dura mater and the bone is more intimate opposite the sutures and at the base of the skull than at the other parts of the cranial wall. During infancy this adhesion is very firm at the sutures and fontanelles. At the sutures on the vault it is continuous through the sutural ligaments with the pericranium, opposite the sphenoidal fissure it joins the periosteum of the orbit, and while the petro-squamous suture persists it is united with the lining membrane of the tympanum. At the base it presents apertures for the cerebral nerves and sends outwards upon them tubular processes which blend with the areolar coat of the nerves. The two roots of the fifth cerebral nerve are enclosed along with the semilunar (Gasserian) ganglion in a common tube (cavum Meckelii) which subsequently divides to invest the main divisions. of the nerve. The relation of the dura mater to the optic nerve differs from that to the other nerves as the dural sheath remains separate from the nerve and joins in front the sclerotic coat of the eyeball. At various places the fibrous tissue of the dura mater separates into two distinct layers, leaving intervening channels called sinuses. These sinuses, which are elsewhere described, are channels for venous blood, and are lined with a continuation of the endothelium of the veins. Between the two cavernous sinuses the pituitary body is received into a depression of the membrane, which closely surrounds the organ in question except where the infundibulum enters it. There is further a fissure immediately over the orifice of the aquæductus vestibuli, and here the prolongation of the membranous labyrinth of the ear, known as the saccus endolymphaticus, is received between the two layers.

The dura mater also sends inwards into the cavity of the skull three strong membranous processes or partitions. Of these, one descends vertically in the

median plane, and is received into the longitudinal fissure between the two hemispheres of the cerebrum. This is the falx cerebri. The second is a sloping vaulted partition, stretched across the back part of the skull, between the cerebrum and the cerebellum, named the tentorium cerebelli. Below this, another vertical partition, named the falx cerebelli, of small extent, passes down between the hemispheres of the cerebellum. Lastly, the portion of dura mater which stretches over the sella turcica, and, pierced by a small hole for the infundibulum,

FIG. 304.-VIEW OF THE CRANIAL DURA MATER FROM BELOW. (J. Symington.) The specimen was prepared by breaking up the bones forming the floor of the cranium and removing them in pieces, so as to preserve the tubular sheaths of the dura mater investing the nerves and the continuity of the dura mater with the periosteum of the orbit. The internal carotid arteries were divided after their exit from the carotid canals. The deep groove behind the pituitary body was occupied by the dorsum sellæ of the sphenoid bone and the depression on each side passing from this groove outwards and backwards contained the petrous part of the temporal bone.

covers the pituitary body, is sometimes spoken of as the diaphragma selle or tentorium hypophysis.

The falx cerebri (fig. 306, 1) is narrow in front, where it is fixed to the crista galli, and broader behind, where it is attached to the middle of the upper surface of the tentorium, along which line of attachment the straight sinus is

VOL. III.

Y

attached. Along its upper convex border, which is attached to the middle line of the inner surface of the cranium, runs the superior sagittal (superior longitudinal) sinus. Its under edge is free, and reaches to within a short distance of the corpus callosum, approaching nearer to it behind. This border contains the inferior sagittal (inferior longitudinal) sinus.

The tentorium cerebelli or tent (fig. 306, 8) is elevated in the middle, and declines downwards in all directions towards its circumference, thus following the form of the upper surface of the cerebellum. Its inner border is free and concave, and leaves in front of it a shield-shaped opening (incisura tentorii)

FIG. 305.-VIEW OF THE CRANIAL DURA MATER FROM THE RIGHT LATERAL ASPECT.

(J. Symington.) The specimen was prepared by making a median section of the head, removing the right half of the brain, and then separating the dura mater from the inner surface of the skull and dividing the tubular prolongations of the dura mater on to the nerves. The transverse sinus was subsequently laid open.

through which the isthmus encephali extends. It is attached behind and at the sides by its convex border to the horizontal part of the crucial ridges of the occipital bone and to the posterior inferior angles of the parietal bones, and there encloses the transverse (lateral) sinuses. Farther forward it is connected with the upper edge of the petrous portion of the temporal bone, the superior petrosal sinus running along this line of attachment. At the point of the pars petrosa the external and internal borders meet, and may be said to intersect each other, the former being then continued inwards to the posterior, and the latter forwards to the anterior clinoid process.

The falx cerebelli (falx minor, fig. 306, 13) descends from the middle of the posterior border of the tentorium, with which it is connected, along the vertical ridge named the internal occipital crest towards the foramen magnum, bifurcating there into two smaller folds. Its attachment to the bony ridge marks the course of the occipital sinus or sinuses.

SPINAL DURA MATER. In leaving the skull the dura mater is intimately attached. to the margin of the foramen magnum, and within the vertebral canal it forms a loose sheath (theca) round the cord and is not adherent directly to the bones, which have an independent periosteum. The theca ends opposite the second sacral vertebra; below this the dura mater is prolonged downwards as a fibrous band (filum terminale externum) which spreads out below and is attached

FIG. 306.-THE CRANIUM OPENED TO SHOW THE FALX OF THE CEREBRUM AND TENTORIUM OF 1HE CEREBELLUM, AND THE PLACES OF EXIT OF THE CEREBRAL NERVES.. (Sappey.)

1, falx; 2, superior sagittal sinus; 3, concave border of the falx; 4 inferior sagittal sinus; 5, base of the falx; 6, straight sinus; 7, anterior part of the falx; 8, right side of the tentorium cerebelli, seen from below; 9, transverse sinus; 10, superior petrosal sinus; 11, inferior petrosal sinus; 12, occipital sinus; 13, falx cerebelli; 14, 15, 16, 17, 18, second, third, fourth, fifth, and sixth cerebral nerves; 19, seventh and eighth nerves; 20, ninth, tenth, and eleventh nerves; 21, twelfth nerve; 22, 23, first and second cervical nerves; 24, upper end of the ligamentum denticulatum; 25, veins of Galen.

to the periosteum of the lower end of the sacral canal or to the back of the соссух.

Opposite each intervertebral foramen the dura-matral theca has two openings, placed side by side, which give passage to the two roots of the corresponding spinal nerve. It is continued as a tubular prolongation on each nerve (fig. 307), and is lost in its sheath. Besides this, it is connected with the circumference of the foramen by areolar tissue.

The spinal dura mater is fixed in position mainly by its lateral prolongations on to the spinal nerve-roots, but also by various ligamentous bands passing from it to the wall of the spinal canal. The most important of these connect the