The inferior longitudinal sinus (sinus infe rior fidcis) is a small vein lodged in the inferior part of the falx cerebri, running parallel to and a little above its inferior margin for about the two posterior thirds of its length. It gradually increases in calibre from before backwards, being formed by tributary veins from the falx ; it opens into the strait sinus.
The strait sinus corresponds to the middle line, at the place where the falx cerebri unites with the tentorium cerebelli. It seems to be enclosed between the layers of the former. Like the other large sinuses, it presents in its section the form of a triangle, whose base is inferior. Its direction is from before backwards and a little downwards, with a slight degree of curva ture corresponding to that of the tentorium. It corresponds at its commencement to the space between the posterior reflected portion of the corpus callosum and the quadrigeminal bodies, and here it receives two large veins (vene magne Galeni), which carry the blood from the inte rior of the ventricles, and a third vein, the in ferior longitudinal sinus. It opens into the conflux of the sinuses or torcular by a round opening or sometimes by two openings, sepa rated by a slip of fibrous membrane. This sinus likewise receives veins from the inferior surface of the posterior and middle lobes of the brain, and flora the superior surface of the cerebel 1 um.
At the posterior extremity of the straight sinus we find a reservoir somewhat polygonal in shape, which corresponds to the occipital pro tuberance; this is called the Torcular .Hero phili,* (the press of Herophilus,) the conflux of the principal sinuses of the dura mater; it has six openings, one for the superior longitu dinal sinus above; one for the straight sinus in front ; two for the lateral sinuses on each side ; and two for the occipital sinuses inferiorly (fig. 362, 1).
Lateral sinuses.—From each side of the con flux of the sinuses, there proceeds in a some what serpentine course outwards, downwards, and forwards, a wide canal, the largest of the sinuses. which conveys the blood from the torcular to the internal jugular vein. A groove exists on each side of the internal occipital pro tuberance, for the reception of this sinus, which marks the occipital bone, the mastoid portion of the temporal, and a small portion of the occipital bone again. In a great portion of
their course, the lateral sinuses correspond to the posterior margin of the tentorium cerebelli, as far forwards as the mastoid portion of the tem poral bone. Ilere each sinus winds downwards to reach the jugular foramen in the posterior lacerated opening. These sinuses are never equal ; that of the right side being, with few exceptions, the larger, a circumstance which Vicq d'Azyr, Sliemmering, and Itudolphi attri buted to the fact that most persons sleep on the right side, on which account the blood is apt to accumulate to that side. They are more capacious at their termination in the jugular veins than at their commencement from the torcular. The inner surface of this sinus is like that of all the others; it is not, however, tra versed by any of the bands which are found so numerous in the longitud:nal sinus. Cruveil hier states that he once saw in the horizontal portion of this sinus, a few of the Pacchionian bodies.
In its course each lateral sinus receives veins from the inferior surface of the brain and supe rior of the cerebellum ; it also receives the supe rior petrosal sinus near the base of the petrous portion of the temporal bone. A large mastoid vein communicates with this sinus and pene trates to the exterior, where it forms one of the principal sources of the occipital vein, thus establishing a free and direct communication between the circulation within and that with out the cranium.* Near the jugular foramen the lateral sinus receives the inferior petrosal.
None of the sinuses has been more fre quently the seat of inflammatory disease than the lateral. Being the principal channel for the return of the venous blood from the in terior of the skull, a slight morbid action within them can scarcely fail to induce a material de rangement of the cerebral circulation, and the nearness of their position to the cerebellum and to the posterior lobes of the brain renders it very unlikely that those parts would escape participating in any acute disease which might arise within it.