(Sinus Duræ Matris). Ophthalmic Veins and Emissary Veins
The sinuses of the dura mater are venous channels which drain the blood from the brain; they are devoid of valves, and are situated between the two layers of the dura mater and lined by endothelium continuous with that which lines the veins. They may be divided into two groups:
(1) a postero-superior, at the upper and back part of the skull
(2) an antero-inferior, at the base of the skull.
The postero-superior group comprises the :
Superior Sagittal – Straight.
Inferior Sagittal – Two Transverse.
Superior sagittal sinus laid open after remova of the skull cap. The chordæ Willisii are clearly seen. The venous lacunæ are also well shown; from two of them probes are passed into the superior sagittal sinus. (Poirier and Charpy.)
The superior sagittal sinus (sinus sagittalis superior; superior longitudinal sinus) occupies the attached or convex margin of the falx cerebri. Commencing at the foramen cecum, through which it receives a vein from the nasal cavity, it runs from before backward, grooving the inner surface of the frontal, the adjacent margins of the two parietals, and the superior division of the cruciate eminence of the occipital; near the internal occipital protuberance it deviates to one or other side (usually the right), and is continued as the corresponding transverse sinus. It is triangular in section, narrow in front, and gradually increases in size as it passes backward. Its inner surface presents the openings of the superior cerebral veins, which run, for the most part, obliquely forward, and open chiefly at the back part of the sinus, their orifices being concealed by fibrous folds; numerous fibrous bands (chordæ Willisii) extend transversely across the inferior angle of the sinus; and, lastly, small openings communicate with irregularly shaped venous spaces (venous lacunæ) in the dura mater near the sinus. There are usually three lacunæ on either side of the sinus: a small frontal, a large parietal, and an occipital, intermediate in size between the other two. Most of the cerebral veins from the outer surface of the hemisphere open into these lacunæ, and numerous arachnoid granulations (Pacchionian bodies) project into them from below. The superior sagittal sinus receives the superior cerebral veins, veins from the diploë and dura mater, and, near the posterior extremity of the sagittal suture, veins from the pericranium, which pass through the parietal foramina.
The numerous communications exist between this sinus and the veins of the nose, scalp, and diploë.
Dura mater and its processes exposed by removing part of the right half of the skull, and the brain.
The inferior sagittal sinus (sinus sagittalis inferior; inferior longitudinal sinus) is contained in the posterior half or two-thirds of the free margin of the falx cerebri. It is of a cylindrical form, increases in size as it passes backward, and ends in the straight sinus. It receives several veins from the falx cerebri, and occasionally a few from the medial surfaces of the hemispheres.
The straight sinus (sinus rectus; tentorial sinus) is situated at the line of junction of the falx cerebri with the tentorium cerebelli. It is triangular in section, increases in size as it proceeds backward, and runs downward and backward from the end of the inferior sagittal sinus to the transverse sinus of the opposite side to that into which the superior sagittal sinus is prolonged. Its terminal part communicates by a cross branch with the confluence of the sinuses. Besides the inferior sagittal sinus, it receives the great cerebral vein (great vein of Galen) and the superior cerebellar veins. A few transverse bands cross its interior.
Sagittal section of the skull, showing the sinuses of the dura
Tentorium cerebelli from above
The sinuses at the base of the skull
The transverse sinuses (sinus transversus; lateral sinuses) are of large size and begin at the internal occipital protuberance; one, generally the right, being the direct continuation of the superior sagittal sinus, the other of the straight sinus. Each transverse sinus passes lateralward and forward, describing a slight curve with its convexity upward, to the base of the petrous portion of the temporal bone, and lies, in this part of its course, in the attached margin of the tentorium cerebelli; it then leaves the tentorium and curves downward and medialward to reach the jugular foramen, where it ends in the internal jugular vein. In its course it rests upon the squama of the occipital, the mastoid angle of the parietal, the mastoid part of the temporal, and, just before its termination, the jugular process of the occipital; the portion which occupies the groove on the mastoid part of the temporal is sometimes termed the sigmoid sinus. The transverse sinuses are frequently of unequal size, that formed by the superior sagittal sinus being the larger; they increase in size as they proceed from behind forward. On transverse section the horizontal portion exhibits a prismatic, the curved portion a semicylindrical form. They receive the blood from the superior petrosal sinuses at the base of the petrous portion of the temporal bone; they communicate with the veins of the pericranium by means of the mastoid and condyloid emissary veins; and they receive some of the inferior cerebral and inferior cerebellar veins, and some veins from the diploë. The petrosquamous sinus, when present, runs backward along the junction of the squama and petrous portion of the temporal, and opens into the transverse sinus.
The occipital sinus (sinus occipitalis) is the smallest of the cranial sinuses. It is situated in the attached margin of the falx cerebelli, and is generally single, but occasionally there are two. It commences around the margin of the foramen magnum by several small venous channels, one of which joins the terminal part of the transverse sinus; it communicates with the posterior internal vertebral venous plexuses and ends in the confluence of the sinuses.
The Confluence of the Sinuses (confluens sinuum; torcular Herophili) is the term applied to the dilated extremity of the superior sagittal sinus. It is of irregular form, and is lodged on one side (generally the right) of the internal occipital protuberance. From it the transverse sinus of the same side is derived. It receives also the blood from the occipital sinus, and is connected across the middle line with the commencement of the transverse sinus of the opposite side.
The antero-inferior group of sinuses comprises the:
Two Cavernous – Two Superior Petrosal.
Two Intercavernous – Two Inferior Petrosal.
The cavernous sinuses (sinus cavernosus) are so named because they present a reticulated structure, due to their being traversed by numerous interlacing filaments. They are of irregular form, larger behind than in front, and are placed one on either side of the body of the sphenoid bone, extending from the superior orbital fissure to the apex of the petrous portion of the temporal bone. Each opens behind into the petrosal sinuses. On the medial wall of each sinus is the internal carotid artery, accompanied by filaments of the carotid plexus; near the artery is the abducent nerve; on the lateral wall are the oculomotor and trochlear nerves, and the ophthalmic and maxillary divisions of the trigeminal nerve (Fig. 571). These structures are separated from the blood flowing along the sinus by the lining membrane of the sinus. The cavernous sinus receives the superior ophthalmic vein through the superior orbital fissure, some of the cerebral veins, and also the small sphenoparietal sinus, which courses along the under surface of the small wing of the sphenoid. It communicates with the transverse sinus by means of the superior petrosal sinus; with the internal jugular vein through the inferior petrosal sinus and a plexus of veins on the internal carotid artery; with the pterygoid venous plexus through the foramen Vesalii, foramen ovale, and foramen lacerum, and with the angular vein through the ophthalmic vein. The two sinuses also communicate with each other by means of the anterior and posterior intercavernous sinuses.
Oblique section through the cavernous sinus
The ophthalmic veins , two in number, superior and inferior, are devoid of valves.
The Superior Ophthalmic Vein (v. ophthalmica superior) begins at the inner angle of the orbit in a vein named the nasofrontal which communicates anteriorly with the angular vein; it pursues the same course as the ophthalmic artery, and receives tributaries corresponding to the branches of that vessel. Forming a short single trunk, it passes between the two heads of the Rectus lateralis and through the medial part of the superior orbital fissure, and ends in the cavernous sinus.
The Inferior Ophthalmic Vein (v. ophthalmica inferior) begins in a venous net-work at the forepart of the floor and medial wall of the orbit; it receives some veins from the Rectus inferior, Obliquus inferior, lacrimal sac and eyelids, runs backward in the lower part of the orbit and divides into two branches. One of these passes through the inferior orbital fissure and joins the pterygoid venous plexus, while the other enters the cranium through the superior orbital fissure and ends in the cavernous sinus, either by a separate opening, or more frequently in common with the superior ophthalmic vein.
Veins of orbit. (Poirier and Charpy.)
The intercavernous sinuses (sini intercavernosi) are two in number, an anterior and a posterior, and connect the two cavernous sinuses across the middle line. The anterior passes in front of the hypophysis cerebri, the posterior behind it, and they form with the cavernous sinuses a venous circle (circular sinus) around the hypophysis. The anterior one is usually the larger of the two, and one or other is occasionally absent.
The superior petrosal sinus (sinus petrosus superior) small and narrow, connects the cavernous with the transverse sinus. It runs lateralward and backward, from the posterior end of the cavernous sinus, over the trigeminal nerve, and lies in the attached margin of the tentorium cerebelli and in the superior petrosal sulcus of the temporal bone; it joins the transverse sinus where the latter curves downward on the inner surface of the mastoid part of the temporal. It receives some cerebellar and inferior cerebral veins, and veins from the tympanic cavity.
The inferior petrosal sinus (sinus petrosus inferior) is situated in the inferior petrosal sulcus formed by the junction of the petrous part of the temporal with the basilar part of the occipital. It begins in the postero-inferior part of the cavernous sinus, and, passing through the anterior part of the jugular foramen, ends in the superior bulb of the internal jugular vein. The inferior petrosal sinus receives the internal auditory veins and also veins from the medulla oblongata, pons, and under surface of the cerebellum.
The exact relation of the parts to one another in the jugular foramen is as follows: the inferior petrosal sinus lies medially and anteriorly with the meningeal branch of the ascending pharyngeal artery, and is directed obliquely downward and backward; the transverse sinus is situated at the lateral and back part of the foramen with a meningeal branch of the occipital artery, and between the two sinuses are the glossopharyngeal, vagus, and accessory nerves. These three sets of structures are divided from each other by two processes of fibrous tissue. The junction of the inferior petrosal sinus with the internal jugular vein takes place on the lateral aspect of the nerves.
The basilar plexus (plexus basilaris; transverse or basilar sinus) consists of several interlacing venous channels between the layers of the dura mater over the basilar part of the occipital bone, and serves to connect the two inferior petrosal sinuses. It communicates with the anterior vertebral venous plexus.
Emissary Veins (emissaria).—The emissary veins pass through apertures in the cranial wall and establish communication between the sinuses inside the skull and the veins external to it. Some are always present, others only occasionally so. The principal emissary veins are the following:
(1) A mastoid emissary vein, usually present, runs through the mastoid foramen and unites the transverse sinus with the posterior auricular or with the occipital vein.
(2) A parietal emissary vein passes through the parietal foramen and connects the superior sagittal sinus with the veins of the scalp.
(3) A net-work of minute veins (rete canalis hypoglossi) traverses the hypoglossal canal and joins the transverse sinus with the vertebral vein and deep veins of the neck.
(4) An inconstant condyloid emissary vein passes through the condyloid canal and connects the transverse sinus with the deep veins of the neck.
(5) A net-work of veins (rete foraminis ovalis) unites the cavernous sinus with the pterygoid plexus through the foramen ovale.
(6) Two or three small veins run through the foramen lacerum and connect the cavernous sinus with the pterygoid plexus. (7) The emissary vein of the foramen of Vesalius connects the same parts.
(8) An internal carotid plexus of veins traverses the carotid canal and unites the cavernous sinus with the internal jugular vein.
(9) A vein is transmitted through the foramen cecum and connects the superior sagittal sinus with the veins of the nasal cavity.