The veins of the dum mater are formed simi larly to those in other parts, being derived from radicles which take their rise in the membrane itself as well as from the osseous walls of the cranium, from the diploic veins of those bones. (See BON t,figs. 187, 188, vol. i.) All of them, with the occasional exception of one or two which accompany the middle meningeal artery and pass out at the foramen spinosum, pour their blood into the great venous canals en closed between the laininm of the dura mater, which are called Sinuses.
The sinuses of the dara mater.—At certain situations, processes of the inner membrane of the venous system are included in canals formed by the separation of the laminw of the dura mater. The channels that are thus formed for the passage of the venous blood do not admit of being dilated beyond a certain size, and in this consists an important peculiarity in the venous system within the cranium. These channels empty themselves into the internal ju gular vein, which thus forms almost the sole channel by which the venous blood is returned from the brain and its membranes as well as in a great measure from the bones of the skull. And thus is explained the rapid influence which is produced upon the brain by any impediment to the passage of the blood through the superior vena cava.
It is important to notice that the sinuses communicate with and receive blood from cer tain external veins which carry blood derived from parts exterior to the cranium. Among these rnay be enumerated the ophthalmic vein, and several small veins in the neighbourhood of the mastoid and condyloid processes, and in the parietal bones.
The following sinuses may be described.
The superior longitudinal sinus. — This sinus corresponds to the superior margin of the falx cerebri. It commences very narrow by one or two small veins from the dura mater in the vicinity of the crista galli and cribriform plate of the ethmoid bone. Thence it proceeds back wards, gradually increasing in calibre, and it terminates a little above the internal occipital protuberance by cornmunicating with a small cavity or reservoir, situated between the layers of the dura mater there, vvhich is called Tor calor Herophili. If a vertical section of this sinus be made in the transverse direction, it will be seen to be triangular in shape, the apex cor responding to the falx, the base slightly curvi linear and lodged in the groove which passes along the median line of' the cranial vault.
When the sinus is laid open in its length by slitting up its superior wall, we find that its sides are perforated by a grnat number of mi nute orifices, which are the openings of veins passing- into it from the dura mater and from the brain itself. These veins pass into the sinus chiefly at right angles to it, or in the direction from behind forwards; a few, situate in front, enter the sinus from before backwards. In the interior of the sinus we observe little bands ( trabecule of Haller, chorde stretching across from right to left, connected only with the lateral walls and leaving a free space above and below them. These bands are numerous, and various as regards breadth. Haller has seen them so numerous that they appeared like a septum dividing the sinus into two portions, of which the superior was the larger.
The walls of the sinus, towards its inferior angle, have frequently a cribriform appearance, which puts on somewhat the aspect of erectile tissue. There is no appeamnce of valves in the interior of the sinus; frequently-, however, the oblique entrance of a small vein into the sinus produces a fold near the venous avrture, which, under the retrograde pressure of he co lumn of blood, might close the orifice, and probably, when the veins open into the sinus from behind forwards, they may be protected from the regurgitation of the blood by this mechanism.
Several of the small bodies, previously al luded to by the name of Pacchionian glands, project into the cavity of the sinus through apertures in its wall. They appear as if they had worn their way by pressure and friction through the walls of the sinus, and it is here that the appearance of an erectile structure is most manifest. We cannot suppose that these bodies are bathed in the blood of the sinus, but rather that they push the lining membrane of the sinus before them. It has been supposed that these bodies are natural structures destined to perform a mechanical office somewhat on the principle of the ball-valve, but they are frequently absent altogether, and when present they have no constant relation to the venous orifices.