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Outside the cranium these prolongations of the membrane blend with the fibrous sheaths of the nerves, and likewise become connected with the periosteum on the exterior of the skull. In the child, during the growth of the cranial bones, and also in old age, the dura mater is more adherent to the cranial wall than during the intermediate portion of life.
The cranial dura mater is composed of two layers intimately connected with each other, but yet capable of being demonstrated in most regions of the cranium. Along certain lines these two layers separate from each other so as to form channels lined with endothelium. These channels are the venous blood-sinuses which receive the blood from veins which come from various parts of the brain. They are described in the section dealing with the Vascular System.
Strong fibrous partitions or septa are given off along certain lines from the deep
FIG. 595.-SAGITTAL SECTION THROUGH THE SKULL, A LITTLE TO THE LEFT OF THE MEDIAN PLANE
to show the arrangement of the dura mater.
The cerebral nerves are indicated by numerals.
surface of the dura mater. These project into the cranial cavity, and subdivide it partially into compartments which all freely communicate with each other, and each of which contains a definite subdivision of the brain. These septa are: (1) the falx cerebri; (2) the tentorium cerebelli; (3) the falx cerebelli; and (4) the diaphragma sellæ.
The falx cerebri is a sickle-shaped partition which descends in the great longitudinal fissure between the two hemispheres of the cerebrum. In front it is narrow, and attached to the crista galli of the ethmoid bone. As it is followed backwards it increases in breadth, and posteriorly it is attached, along the median plane, to the upper surface of the tentorium. The anterior narrow part of the falx is frequently cribriform, and is sometimes perforated by apertures to such an extent that it almost resembles lace-work. Along each border it splits into two layers, so as to enclose a blood-sinus. Along its superior convex attached border runs the superior sagittal sinus; along its concave free border sometimes courses the much smaller inferior
sagittal sinus; whilst along its attachment to the tentorium is enclosed the straight sinus.
The tentorium cerebelli is a large crescentic partition of dura mater, which forms a membranous tent-like roof for the posterior cranial fossa, and thus intervenes between the posterior portions of the cerebral hemispheres and the cerebellum. It is accurately applied to the superior surface of the cerebellum. Thus, its highest point is in front and in the median plane, and from this it slopes downwards towards its attached border. It is kept at a high degree of tension, and this depends on the integrity of the falx cerebri, which is attached to its superior aspect in the median plane.
The posterior border of the tentorium is convex, and is attached to the horizontal ridge which marks the deep surface of the occipital bone. Beyond this, on each side, it is fixed to the postero-inferior angle of the parietal bone, and then forwards and medially along the superior border of the petrous portion of the temporal bone. From the internal occipital protuberance to the postero-inferior angle of the parietal bone this border encloses the sinus transversus, whilst along the superior border of the petrous bone it encloses the superior petrosal sinus. The anterior border of the tentorium is sharp, free, and concave, and forms with the dorsum sellæ an oval opening shaped posteriorly like a pointed arch. This opening receives the name of the incisura tentorii, and within it is placed the mesencephalon, or the stalk of connexion between the parts which lie in the posterior cranial fossa and the cerebrum. Beyond the apex of the petrous part of the temporal bone the two margins of the tentorium cross each other like the limbs of the letter X; the free margin is continued forwards, to be attached to the anterior clinoid process, whilst the attached border proceeds medially, to be fixed to the posterior clinoid process.
The falx cerebelli is a small, sickle-shaped process of dura mater placed below the tentorium, which projects forwards in the median plane from the internal occipital crest. It occupies the notch which separates the two hemispheres of the cerebellum posteriorly. Inferiorly it bifurcates into two small diverging ridges. which gradually fade away as they are traced forwards on each side of the foramen magnum.
The diaphragma sella is a small circular fold of dura mater which forms a roof for the fossa hypophyseos. A small opening is left in its centre for the transmission of the infundibulum.
Dura Mater Spinalis.-In the vertebral canal the dura mater forms a tube which encloses the spinal medulla, and which extends from the foramen magnum above to the level of the second or third piece of the sacrum below. It is very loosely applied to the spinal medulla and the nerve-roots which form the cauda equina; in other words, it is very capacious in comparison with the volume of its contents. Moreover, its calibre is not uniform. In the cervical and lumbar regions it is considerably wider than in the thoracic region, whilst in the sacral canal it rapidly contracts, and finally ends by blending with the filum terminale externum, the chief bulk of which it forms. At the superior end of the vertebral canal the spinal dura mater is firmly fixed to the third cervical vertebra, to the epistropheus vertebra, and around the margin of the foramen magnum. In the sacral canal the filum terminale externum, with which it blends, extends downwards to the back of the coccyx, to the periosteum of which it is fixed. The inferior end of the tube is thus securely anchored and held in its place.
Within the cranial cavity the dura mater is closely adherent to the bones, and forms for them an internal periosteum. As it is followed into the vertebral canal its two constituent layers separate. The inner layer is carried downwards as the long cylindrical tube which encloses the spinal medulla. The outer layer, which is much thinner, becomes continuous behind and on each side of the foramen magnum with the periosteum on the exterior of the cranium, whilst in front it is prolonged. downwards into the vertebral canal in connexion with the periosteum and ligaments on the anterior wall of the canal. The spinal dura mater, therefore, corresponds to the inner layer of the cranial dura mater, and to it alone. It is separated from the walls of the vertebral canal by an interval, the cavum epidurale, which is occupied by
soft fat and a plexus of thin-walled veins. In connexion with the spinal dura mater there are no blood-sinuses such as are present in the cranial cavity, but it should be noted that the veins in the epidural space, placed as they are between the periosteum of the vertebral canal and tube of dura mater, occupy the same morphological plane as the cranial blood-sinuses. Another feature which serves to distinguish the spinal dura mater from the cranial dura mater consists in the fact that it gives off from its deep surface no partitions or septa..
The cylindrical tube of spinal dura mater does not lie quite free within the vertebral canal. Its attachments, however, are of such a character that they in no way interfere with the free movement of the vertebral column. On each side the spinal nerve-roots, as they pierce the dura mater, carry with them into the intervertebral foramina tubular sheaths of the membrane, whilst in front loose fibrous prolongations—more numerous above and below than in the thoracic regionconnect the tube of dura mater to the posterior longitudinal ligament of the vertebral column. No connexion of any kind exists between the dura mater and the posterior wall of the vertebral canal.
When the interior of the tube of spinal dura mater is inspected, the series of apertures of exit for the roots of the spinal nerves is seen. These are ranged in pairs opposite each intervertebral foramen.
Viewed from the inside of the tube of dura mater, each of the two roots of a spinal nerve is seen to carry with it a special and distinct sheath. When examined on the outside, however, the appearance is such that one might be led to conclude that both roots are enveloped in one sheath of dura mater. This is due to the fact that the two sheaths are firmly held together by intervening connective tissue. The two tubular sheaths remain distinct as far as the ganglion on the posterior root, and then blend with each other.
Cavum Subdurale.-The dura mater and the arachnoid are closely applied to each other, and the capillary interval between them is termed the subdural space. It contains a minute quantity of fluid, which is just sufficient in amount to moisten the opposed surfaces of the two bounding membranes.
The subdural space in no way communicates with the subarachnoid space. The fluid which it contains is led into the venous blood-sinuses around the arachnoideal granulations (O.T. Pacchionian bodies), and thus gains exit. The subdural space is carried outwards for a very short distance on the various nerves which are connected with the brain and the spinal medulla, and it has a free communication with the lymph-paths present in these nerves. In the case of the optic nerve the sheath of dura mater is carried along its whole length, and with it the subdural space is likewise prolonged to the back of the eyeball.
The arachnoid is a very thin membrane, remarkable for its delicacy and transparency, which envelops both the brain and the spinal medulla between the dura mater and the pia mater. The cranial part of the arachnoid or the arachnoidea encephali, except in the case of the longitudinal and the lateral fissures, does not dip into the sulci on the surface of the brain. In this respect it differs from the pia mater. It bridges over the inequalities on the surface of the brain. Consequently, on the basal aspect of the encephalon it is spread out in the form of a very distinct sheet over the medulla oblongata, the pons, and the hollow which lies in front of the pons, and in certain of these regions it is separated from the brainsurface by wide intervals.
The spinal part of the arachnoid or arachnoidea spinalis, which is directly continuous with the cranial arachnoidea, forms a loose wide investment for the spinal medulla. This arachnoideal sac is most capacious towards its inferior part where it envelopes the inferior end of the spinal medulla and the collection of long nerve-roots which constitute the cauda equina.
As the nerves, both from the brain and the spinal medulla, pass outwards they receive an investment from the arachnoid, which runs for a short distance upon them and then comes to an end.
Cavum Subarachnoideale. The interval between the arachnoidea and the pia mater receives the name of the subarachnoid space. It contains the cerebro-spinal fluid, and communicates freely, through certain well-defined apertures, with the ventricular cavities in the interior of the brain (aperturæ ventriculi quarti).
Within the cranium the subarachnoid space is broken up by a meshwork of fine filaments and trabeculæ, which connects the two bounding membranes (viz., the arachnoidea and the pia mater) in the most intimate manner, and forms a delicate sponge-like interlacement between them. Where the arachnoidea passes over the summit of a cerebral gyrus, and is consequently closely applied to the subjacent pia mater, the meshwork is so dense and the trabeculæ so short that it is hardly possible to discriminate between the two membranes. To all intents and purposes they form in these localities one lamina. In the intervals between the rounded margins of adjoining gyri, however, distinct angular spaces exist, where the subarachnoid trabecular tissue can be studied to great advantage. These
FIG. 596.-DIAGRAM to show the relations of the membranes of the brain to the cranial wall and the cerebral gyri, and also of the arachnoideal granulations to the superior sagittal sinus and the lateral lacunæ.
intervals on the surface of the cerebrum constitute numerous communicating channels which serve for the free passage of the subarachnoid fluid from one part of the brain to another. The larger branches of the arteries and veins of the brain traverse the subarachnoid space; their walls are directly connected with the subarachnoid trabeculae, and are bathed by subarachnoid fluid.
In certain situations within the cranium the arachnoidea is separated from the pia mater by intervals of considerable width and extent. These expanded portions of the subarachnoid space are termed cisternæ subarachnoideales. In these the subarachnoid tissue is much reduced. There is no longer a close meshwork; the trabeculæ connecting the two bounding membranes take the form of long filamentous intersecting threads which traverse the spaces. All the subarachnoid cisterns communicate in the freest manner with each other and also with the narrow channels on the surface of the cerebrum.
Certain of these cisterns require special mention. The largest and most conspicuous is the cisterna cerebellomedullaris. It is formed by the arachnoid membrane bridging over the wide interval between the posterior part of the inferior surface of the cerebellum and the medulla oblongata. It is continuous
through the foramen magnum with the posterior part of the wide subarachnoid space of the spinal medulla.
The cisterna pontis is the continuation upwards on the floor of the cranium of the anterior part of the subarachnoid space of the spinal medulla. In the region of the medulla oblongata it is continuous behind with the cisterna cerebellomedullaris, so that this subdivision of the brain, like the spinal medulla, is
surrounded by a wide subarachnoid space.
In front of the pons the arachnoidea bridges across between the projecting temporal lobes, and covers in the deep hollow in this region of the brain. This space is called the cisterna interpeduncularis, and within it are placed the large arteries which take part in the formation of the arterial circle (of Willis). Leading out from the interpeduncular cistern there are certain wide subarachnoid channels. Two of these are prolonged into the lateral fissures, and in these are accommodated the middle cerebral arteries. Anteriorly the interpeduncular cistern. passes into a space in front
of the optic chiasma (cisterna chiasmatis), and from this it is continued into the longitudinal fissure above the corpus callosum. In this subarachnoid passage the anterior cerebral arteries are lodged.
The spinal part of the subarachnoid space is a very wide interval which is partially subdivided into compartments by three incomplete septa. One of these is a median partition called the septum posterius, which connects the pia mater covering the posterior aspect of the spinal medulla with the arachnoid. In the upper part of the cervical region the septum posterius is imperfect, and is represented merely by some strands passing between the two membranes; in the inferior part of the cervical region and in the thoracic region it becomes more complete. The other two septa are formed by the ligamenta denticulata which spread laterally one from each side of the spinal medulla. These will be described with the pia mater.
Granulationes Arachnoideales.-When the surface of the dura mater is inspected after the removal of the calvaria, a number of small fleshy-looking excrescences, purplish-red in colour, are seen ranged in clusters on each side of the superior sagittal sinus, and when this sinus is opened they are also observed protruding in considerable numbers into its interior. These are the arachnoideal granulations (O.T. Pacchionian bodies), and they are found also, in smaller numbers and distinctly smaller size, in connexion with other blood-sinuses, such as the transverse sinus, the straight sinus, and the cavernous sinus. At first sight they appear to belong to the dura mater, but in reality they are projections from the arachnoid. In the child they are exceedingly small and rudimentary, and it is only as life advances that they become large and conspicuous.
Each granulation is a bulbous protrusion of the arachnoid. It is attached to the arachnoid by a narrow pedicle, and into its interior there is prolonged, through this pedicle, a continuation of the subarachnoid space and its characteristic meshwork. The granulations do not pierce the dura mater. As they push their way into a blood-sinus they carry before them a thin covering continuous with the