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Veins of the Medulla Oblongata.-Deep veins of the medulla oblongata issue from its substance and end in a superficial plexus. This plexus is drained by an anterior and a posterior median vein and by radicular veins.
The anterior median vein is continuous below with the corresponding vein of the spinal medulla; it communicates above with the plexus on the surface of the pons. The posterior median vein is continuous below with the posterior median vein of the spinal medulla, from which it ascends to the lower end of the fourth ventricle, where it divides into two branches which join the inferior petrosal sinus or basilar plexus.
The radicular veins issue from the lateral parts of the plexus and run with the roots of the last four cerebral nerves; they end in the inferior petrosal and occipital sinuses or in the upper part of the internal jugular vein.
SINUS DURÆ MATRIS.
The venous sinuses of the cranium are spaces between the layers of the dura mater; and they are lined with an endothelium which is continuous with the endothelium of the veins. They receive the veins of the brain, communicate frequently with the
FIG. 787.-DISSECTION OF THE HEAD AND NECK, showing the cranial blood sinuses and the upper part of
the internal jugular vein.
meningeal veins and with veins external to the cranium, and terminate directly or indirectly in the internal jugular vein. Some of the cranial blood sinuses are unpaired, others are paired.
Unpaired Sinuses. These are the superior sagittal, the inferior sagittal, the straight, the anterior and posterior intercavernous, and the basilar.
Sinus Sagittalis Superior.-The superior sagittal sinus commences in the anterior fossa of the cranium, at the crista galli, where it communicates, through the foramen cæcum, with the veins of the nasal cavity or with the angular vein. It passes backwards in the convex margin of the falx cerebri, grooving the frontal, both the parietal bones, and upper part of the occipital. As it descends along the occipital bone it usually passes slightly to the right side, and it ends,
FIG. 788.-THE LOWER BLOOD SINUSES OF THE DURA MATER.
In the specimen represented the superior sagittal sinus opened into both transverse sinuses and chiefly into the left. The straight sinus also opened into both transverse sinuses. The medial part of the left transverse sinus was divided by a horizontal septum into upper and lower parts. The arrow in the figure passes below the septum.
at the level of the internal occipital protuberance, by becoming the right transverse sinus. Instead of passing to the right, it occasionally turns to the left, and ends in the left transverse sinus, and in some cases it bifurcates and ends in both transverse sinuses. When it ends wholly in the right or the left transverse sinus its termination is associated with a well-marked dilatation, the confluens sinuum, which is lodged in a depression at one side of the internal occipital protuberance. The confluens sinuum is connected, across the protuberance, by an anastomosing channel, with a similar dilatation which marks the junction of the straight sinus with the
lateral sinus of the opposite side. Opening into the superior sagittal sinus are the superior cerebral veins, and it communicates on each side by small openings with a series of spaces in the dura mater, the lacunæ laterales, into which the arachnoideal granulations project. It also communicates, by emissary veins, which pass through the foramen cæcum and through each parietal foramen, with the veins on the exterior of the cranium. Its cavity, which is triangular in transverse section, is crossed by several fibrous strands called the chorda Willisii.
Sinus Sagittalis Inferior. The inferior sagittal sinus lies, usually, in the posterior two-thirds of the lower free margin of the falx cerebri. It terminates posteriorly by joining with the great cerebral vein (Galen) to form the straight sinus. It receives tributaries from the falx cerebri and from the medial surface of the middle third of each cerebral hemisphere.
Sinus Intercavernosi.—The anterior intercavernous sinus is a small transverse channel which crosses from one cavernous sinus to the other in the anterior border of the diaphragma sellæ.
The posterior intercavernous sinus also connects the two cavernous sinuses together. It lies in the posterior border of the diaphragma sellæ.
The anterior and posterior intercavernous sinuses and the intervening parts of the cavernous sinuses form collectively the circular sinus.
Plexus Basilaris.-The basilar plexus (O.T. basilar sinus) is situated in the dura mater on the basilar part of the occipital bones. It connects the posterior ends of the cavernous or the anterior ends of the inferior petrosal sinuses together, and communicates below with the anterior spinal veins.
Sinus Rectus.—The straight sinus is formed by the union of the inferior sagittal sinus with the great cerebral vein (Galen). It runs downwards and backwards, along the line of union between the falx cerebri and the tentorium cerebelli. As a general rule it turns to the left at the internal occipital protuberance, dilates somewhat, and becomes continuous with the left transverse sinus, its dilatation being united with the corresponding dilatation on the lower end of the superior sagittal sinus, the "confluens sinuum," by a transverse anastomosing channel. Occasionally the straight sinus terminates in the right lateral sinus; in that case the superior sagittal sinus ends in the left transverse sinus; and sometimes it bifurcates to join both transverse sinuses. It receives some of the superior cerebellar veins and a few tributaries from the falx cerebri.
Paired Sinuses.-There are six pairs of sinuses, viz., the transverse, the occipital, the cavernous, the superior petrosal, the inferior petrosal, and the spheno-parietal
Sinus Transversi (O.T. Lateral Sinuses). Each transverse sinus commences at the internal occipital protuberance, the right usually as the continuation of the superior sagittal, and the left as the continuation of the straight sinus. Each passes laterally in the postero-lateral part of the attached border of the tentorium cerebelli and in a groove in the occipital bone. From the lateral angle of the occipital bone it passes to the posterior inferior angle of the parietal bone, which it grooves; then it leaves the tentorium and turns downwards on the inner surface of the mastoid portion of the temporal bone; from the latter it passes to the upper surface of the jugular process of the occipital bone, and turns forwards and then downwards into the jugular foramen, where it becomes continuous with the internal jugular vein. The part which descends on the temporal bone and turns forwards on the jugular process of the occipital is called the sigmoid sinus.
Its tributaries are some of the superior and inferior cerebellar veins, a posterior diploic vein, and the superior petrosal sinus. It is connected with the veins outside the cranium by emissary veins which pass through the mastoid foramen and the condyloid canal.
Sinus Occipitales.-The occipital sinuses lie in the attached border of the falx cerebelli and in the dura mater along the postero-lateral boundaries of the foramen magnum; frequently they unite above and open by a single channel into the commencement of either the right or the left transverse sinus, but their upper extremities may remain separate, and then each communicates with the commencement of the transverse sinus of its own side. On the other hand either the right or the left sinus may be absent. Each opens below into the terminal part of the corre
sponding transverse sinus, and both communicate with the posterior spinal veins. Each occipital sinus is an anastomosing channel between the upper and lower extremities of the transverse sinus of the same side, and each receives a few inferior cerebellar veins.
Sinus Cavernosi.-The cavernous sinuses lie at the sides of the body of the sphenoid bone. Each sinus commences, anteriorly, at the medial end of the superior orbital fissure, where it receives the corresponding ophthalmic veins, and it terminates, at the apex of the petrous portion of the temporal bone, by dividing into the superior and the inferior petrosal sinuses. Its cavity, which is irregular in size and shape, is so divided by numerous fibrous strands that it assumes the appearance of cavernous tissue; and in its lateral wall are embedded the internal carotid artery with its sympathetic plexus, the oculomotor, the trochlear, the ophthalmic and maxillary divisions of the trigeminal and the abducent nerves. Its tributaries are the ophthalmic vein, the spheno-parietal sinus and the inferior cerebral veins, including the middle cerebral vein (O.T. superficial Sylvian vein). It communicates with the opposite cavernous sinus by means of the anterior and posterior intercavernous sinuses; with the pterygoid plexus, in the infra-temporal fossa, by an emissary vein which passes either through the foramen ovale or through the foramen Vesalii; with the internal jugular vein by small venous channels which accompany the internal carotid artery through the carotid canal, and by the inferior petrosal sinus; with the transverse sinus by the superior petrosal sinus; and through the superior ophthalmic vein with the angular vein.
The spheno-parietal sinuses are lodged in the dura mater on the under surfaces of the small wings of the sphenoid bone close to their posterior borders. Each sinus communicates with the middle meningeal veins, receives veins from the dura mater, and terminates in the anterior part of the corresponding cavernous sinus.
Sinus Petrosi Superiores.-Each superior petrosal sinus commences at the apex of the petrous portion of the temporal bone, in the posterior end of the corresponding cavernous sinus, and it runs backwards and laterally, in the attached margin of the tentorium cerebelli, above the trigeminal nerve. It grooves the superior angle of the petrous portion of the temporal bone, at the lateral end of which it terminates in the transverse sinus, at the point where the latter is turning downwards on the medial surface of the mastoid portion of the temporal bone. receives inferior cerebral, superior cerebellar, tympanic, and diploic veins.
Sinuus Petrosi Inferiores.-An inferior petrosal sinus commences at the posterior end of each cavernous sinus; it runs backwards, laterally, and downwards, in the posterior fossa of the cranium, in a groove formed by the lower angle of the petrous part of the temporal bone and the adjacent border of the basilar part of the occipital bone, to the anterior compartment of the jugular foramen of the same side, through which it passes. It crosses the last four cerebral nerves either on their lateral or on their medial sides, and it terminates in the internal jugular vein. Its tributaries include inferior cerebellar veins and veins from the internal ear, which pass to it through the internal acoustic meatus, the aquæductus cochleæ, and the aquæductus vestibuli.
Emissaria. The emissary veins are veins which convey blood from the blood sinuses in the interior of the cranium to the veins which lie outside the walls of the cranium. They may be single veins, or plexiform channels surrounding other structures which are passing through the walls of the cranium.
(1) Frontal. In the child, and sometimes in the adult, an emissary vein passes from the anterior end of the superior sagittal sinus through the foramen cæcum. Its lower end divides
into two channels which either terminate in the veins of the roof of the nasal cavities or they perforate the nasal bones and join the angular veins.
(2) Parietal. The parietal emissary veins, one on each side, pass through the parietal foramina, from the superior sagittal sinus to the occipital veins.
(3) Occipital. An occipital emissary vein is only occasionally present. It passes from the "confluens sinuum" through the occipital protuberance to one of the tributaries of an occipital vein, and it receives the occipital diploic vein.
(4) Condyloid. When the condyloid canals are present in the occipital bone each is traversed by a condyloid emissary vein, which connects the lower end of the corresponding transverse sinus with the plexus of veins in the sub-occipital triangle.
(5) Emissary Plexus of the Foramen Ovale. This plexus surrounds the mandibular nerve, as it passes through the foramen ovale, and connects the cavernous sinus with the corresponding
pterygoid plexus in the infratemporal fossa. If the foramen Vesalii is present, the plexus of the foramen ovale is replaced or supplemented by an emissary vein which passes through that foramer
(6) Internal Carotid Plexus.-The internal carotid plexus accompanies the internal carotid artery through the carotid canal of the temporal bone, and connects the cavernous sinus either with the pharyngeal plexus or with the upper part of the internal jugular vein.
(7) Plexus of the Hypoglossal Canal. As the hypoglossal nerve passes through the hypoglossal canal (O.T. anterior condyloid foramen) it is accompanied either by a venous plexus or by a large vein which connects the veins of the medulla oblongata and the lower part of the occipita sinus with the upper end of the internal jugular vein, or with the extra-cranial part of the inferior petrosal sinus.
Venæ Basivertebrales.-The basi-vertebral veins are venous channels, enclosed by endothelial walls, which lie in the interiors of the bodies of the vertebræ. They communicate anteriorly with the plexuses of veins on the anterior surfaces of the bodies of the vertebra, and they converge, radially, towards the posterior surfaces of the bodies of the vertebra where they open into the transverse anastomoses between the longitudinal vertebral sinuses.
Plexus Venosi Vertebralis Externi. -The external vertebral plexuses, (a) anterior and (b) posterior.
(a) The anterior external vertebral plexuses are formed by anastomosing venous channels which lie on the anterior surfaces of the vertebra. They communicate with the basi-vertebral veins and with the intervertebral veins.
(b) The posterior external vertebral plexuses lie around the postero-lateral aspects of the vertebræ, in the vertebral grooves, around the spines, the articular and the transverse processes of the vertebræ. They communicate with the internal plexuses and with the intervertebral veins, and they open into the vertebral, intercostal, and lumbar veins.
Sinus Vertebrales Longitudinales.-The Longitudinal Vertebral Sinuses.-The veins in the interior of the vertebral canal form a network, the vertebral venous network, which lies external to the dura mater and covers the internal surfaces of the arches and the posterior surfaces of the bodies of the vertebræ. The network communicates laterally with the intervertebral veins, posteriorly with the posterior external venous plexuses, whilst anteriorly it receives the basi-vertebral veins. In the anterior part of the network, on the posterior surfaces of the bodies of the vertebra, at the sides of the posterior longitudinal ligament, there are two large longitudinal channels, the anterior longitudinal vertebral sinuses. Two less marked longitudinal channels, the posterior longitudinal vertebral sinuses, can sometimes be distinguished on the internal surfaces of the vertebral arches.
The anterior longitudinal vertebral sinuses communicate above with the basilar plexus, the terminal parts of the transverse sinuses, and with the network of veins which accompanies each hypoglossal nerve through the hypoglossal canal.
The posterior longitudinal vertebral sinuses, when they are well established. communicate above with the occipital sinuses.
Venæ Intervertebrales. - The Intervertebral Veins. The internal vertebral venous network is drained not only above into the cranial venous sinuses by the longitudinal vertebral sinuses, but also by a series of intervertebral veins which pass through the intervertebral foramina. In the cervical region the intervertebral veins open externally into the vertebral veins, in the thoracic region into the intercostal veins, in the lumbar region into the lumbar veins, and in the sacral region into the lateral sacral veins. The intervertebral veins convey blood both from the interna! vertebral venous plexus and also from the anterior and the posterior external vertebral plexuses.