It performs the double function of a periosteum to the interior of the skull and of affording support and protection to the brain. In correspondence with this double function it may be regarded as consisting of two layers, an outer or periosteal lamina, and an inner or supporting lamina. These layers are inseparable for the greater part of their extent, but along certain lines the inner lamina leaves the periosteal lamina and forms shelf -like projections (of which the falx cerebri and the tentorium cerebelli are examples) into the cranial cavity. Along the lines where these layers divaricate spaces occur which form cranial sinuses. A cranial sinus may be defined as a space formed by the cleavage of the dura mater, lined by a prolongation of the lining membrane of a vein and conveying venous blood. Some of the sinuses are placed between the periosteal and supporting layers of the dura mater - the lateral sinus is a good example of this class. Others (as the straight sinus) are bounded wholly by the inner or supporting layer.
The cranial sinuses form two principal systems, which, however, communicate with one another. The following is a list of the sinuses which unite to form the larger of these two systems : - superior longitudinal, inferior longitudinal, straight, occipital, superior petrosal. These form a system which converges at the torcular Herophili, a dilated blood-space situated on the right side (rarely on the left) of the internal occipital protuberance. The blood from the torcular is drained away by the lateral sinuses (more particularly by the right sinus), and leaves the cranium through the posterior compartment of the jugular foramen. The straight sinus usually passes into the left lateral sinus. The smaller system comprises the spheno-parietal, cavernous, circular, transverse, and inferior petrosal sinuses. The blood from this system passes into the internal jugular vein through the anterior compartment of the jugular foramen. A portion of the blood conveyed by the cavernous sinus, however, passes into the lateral sinus through the superior petrosal sinus.
These veins are possibly extracranial tributaries of the cranial sinuses under normal conditions, but, in cases of engorgement of the sinuses, permit the flow of blood in the opposite direction, and become, in accordance with their name, emissary. The mastoid emissary is the most important of these veins ; it passes through the mastoid foramen to enter the lateral sinus, and explains the value of applying leeches behind the ears in cases of cerebral congestion. Two small emissaries enter the superior longitudinal sinus, one of these passes through the foramen caecum ; another traverses the parietal foramen. An additional vein occasionally passes to the torcular Herophili through a foramen in the occipital bone near the external occipital protuberance. Emissary veins pass through the foramen ovale and foramen of Vesalius, and place the cavernous sinus in com- munication with the pterygoid venous plexus ; others traverse the cartilage which occupies the foramen lacerum medium. Small veins enter the lateral sinuses through the anterior and posterior condyloid foramina. A minute venous plexus, which accompanies the internal carotid artery through the carotid canal, establishes a communication between the cavernous sinus and the internal jugular vein. (Eektorzik.)
The dura mater of the brain is continuous through the foramen magnum with the dura mater of the cord. It is also continuous, through the various foramina in the cranium, with the external periosteum of the skull. This is most easily observed at the sphenoidal fissure, but may be made out with ordinary care at any of the other foramina. It sends a strong prolongation through the optic foramen which splits into two layers, one of which forms the sheath of the optic nerve, and the other is continued into the periosteum of the orbit. On account of the number of foramina at the base of the skull, it follows that the dura mater is more firmly connected to the bone here than at the roof and sides. In the region of the basilar process of the occipital bone, the dura mater splits into supporting and periosteal layers. The supporting layer passes through the foramen magnum to become continuous with the dura mater of the spinal cord. The periosteal layer clothes the bone and is thickened into a ligamentous band, the occipito-cervical or cervico- basilar ligament (page 198), which passes through the foramen magnum to join the posterior common ligament of the vertebrae and to gain attachment to the body of the axis. Thus a connection is established between the dura mater and the periosteum of the vertebrae. At the back and sides of the foramen magnum the dura mater does not split into periosteal and supporting layers until it reaches the margin of that opening, where its layers divaricate, the one to become continuous with the external periosteum of the skull, the other to be continued into the spinal dura mater. In the middle fossa of the skull there is a separation of the periosteal and supporting layers to form the cavernous sinus ; on the inner side of this the supporting layer is moulded round the pituitary body and then folds on itself to form the diaphragma sellae. External to the cavernous sinus the two layers remain separate for some distance, forming a space (Meckel's space), which encloses the Gasserian ganglion. The outer surface of the dura mater is covered with a number of arteries, which ramify between it and the inner table of the skull. These vessels break up into small twigs which penetrate the bone. The term meningeal, applied to these arteries, is somewhat misleading, inasmuch as they do not supply any of the meninges, except the dura mater, being chiefly destined for the nutrition of the cranial bones. These small vessels, together with a number of fibrous retinacula, are torn across when the dura mater is forcibly detached from the bone, and give rise to the rough appearance which is presented by its outer surface. An examination under water will, however, reveal smooth spots interspersed among the bundles of ruptured vessels ; these are subperiosteal lymph-spaces.
The middle or great meningeal artery, which enters the middle cranial fossa through the foramen spinosum, supplies the dura mater which lines the vault of the cranium. In addition to this there are meningeal arteries, mostly of small size, which are limited in their distribution to the base of the cranium. In the anterior cranial fossa there are anterior meningeal branches of the anterior and posterior ethmoidal arteries, which enter the cranium through the anterior and posterior internal orbital canals ; also twigs from the middle meningeal which usually pierce the great wing of the sphenoid near the outer angle of the sphenoidal fissure. In the middle cranial fossa the following arteries are met with : - a branch of the ascending pharyngeal which perforates the cartilage which occupies the foramen lacerum medium ; the meningea parva which enters the cranium through the foramen ovale ; the meningeal branch of the internal carotid, and twigs from the middle meningeal. In the posterior cranial fossa, two meningeal branches, derived from the occipital and ascending pharyngeal, enter through the posterior compartment of the jugular foramen, another twig from the occipital passes through the mastoid foramen, and a twig from the ascending pharyngeal makes its way through the anterior condyloid foramen. A meningeal branch of the vertebral is given off from that artery, in the interval between the occipital bone and the posterior arch of the atlas, and enters the skull through the foramen magnum.
A branch of the occipital artery (ramus parietalis, Cruveilhier) occasionally passes through the parietal foramen to the dura mater lining the calvaria.
Inner or supporting layer of the dura mater
The following are the infoldings formed by the inner or supporting layer of the dura mater, taken in their order of magnitude : the falx cerebri, the tentorium cerebelli, the falx cerebelli, and the diaphragma sellge. There are also two smaller paired folds ; viz. the folds which project from the lesser wings of the sphenoid bone into the Sylvian fissm-es of the brain, and the crescentic folds which are placed over the optic nerves at the upper margin of the optic foramina.
The Falx Cerebri
The Falx Cerebri is a large sickle-shaped process, deeper behind than in front, which is placed in the great longitudinal fissure of the brain. It is attached by its base to the tentorium cerebelli, in the middle line, and maintains by its tension the vaulted character of the latter. The straight sinus follows this line of attachment. Its apex is firmly fixed to the crista galli of the ethmoid, and has also an attachment to the ethmoidal spine of the sphenoid bone. Its convex or upper margin corresponds to the superior longitudinal sinus, and is attached to the periosteal layer opposite to the edges of the groove for that sinus in the frontal, the two parietal, and the occipital bones. The inferior longitudinal sinus courses along its concave or free margin. This margin closely approaches the corpus callosum behind, but is separated from it by a considerable interval in front.
The Tentorium Cerebelli
The Tentorium Cerebelli occupies the interval between the cerebrum and cerebellum. It presents for examination an upper surface, a lower surface, a free or concave border, and an attached or convex border. The upper surface is in contact with the occipital lobes of the cerebrum, and is strongly convex upwards, forming a roof-like structure, to the ridge of which the base of the falx cerebri is attached. This ridge also slopes upwards and forwards, so that its highest point is situated at the free border. The under surface is in apposition to the cerebellum. The free border bounds an opening usually described as oval, but which is more properly triangular with curved sides, the apex of the triangle being turned back-wards, and corresponding to the point where the veins of Galen enter the straight sinus. This opening transmits the crura cerebri and the superior peduncles of the cerebellum, these parts forming an isthmus connecting the masses of brain sub- stance above and below the tentorium ; the basilar artery and the third and fourth pairs of cranial nerves also pass upwards through this opening. Followed for- wards, the free border is seen to be attached to the anterior clinoid processes of the sphenoid bone. The attached border follows the lateral sinus along the occipital and parietal bones, and then corresponds to the superior petrosal sinus along the superior border of the petrous portion of the temporal, and finally dips under the free border, to be attached to ihe posterior clinoid process of the sphenoid bone.
The Falx Cerebelli
The Falx Cerebelli is a small prominent fold, which is placed between the cerebellar hemispheres. Its attached border corresponds to the occipital sinus along the internal occipital crest. Above this it is attached for a short distance to the under surface of the tentorium. Its free border looks upwards and forwards.
The diaphragma sellae
The diaphragma sellae is a small, annular, shelf-like fold of the dura mater which roofs in the pituitary fossa, leaving a small aperture in the centre, which transmits the infundibulum.
The subdural space
The subdural space is a narrow lymph-space between the dura mater and the arachnoid. It is occupied by a small portion of the cerebro-spinal fluid, the greater part of which, however, is lodged in the meshes of the subarachnoid tissue, and in the ventricles of the brain. This space separates the arachnoid from the dura mater, except where it is crossed by the veins of the brain passing to the cranial sinuses, by the Pacchionian bodies, and by the cranial nerves at their points of exit from the skull.
by H. ST. John Brooks M.D.