This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The mastoid process is continous with the superior curved line of the occiput. It increases in size from the time of birth, but is composed of cancellous tissue until after the age of puberty, when the mastoid cells develop. The mastoid antrum, a cavity five millimetres in size at birth, which opens into the upper posterior portion of the tympanum, is relatively larger at birth than in the adult.
This triangle, so named by Macewen, is formed above by the posterior root of the zygoma, anteriorly by the bony posterior wall of the external auditory meatus and posteriorly by a line from the floor of the meatus passing upward and backward to meet the first line. The mastoid antrum is reached by operating through this triangle (see section on Ear).
The fibrous membrane which lines the interior of the skull is composed of two layers which are in most places intimately united, forming one single membrane known as the dura mater. The outer layer is applied to the bone, while the inner layer covers the brain. In certain places these two layers separate to form channels in which venous blood flows; these channels are called sinuses. In certain other places these layers separate and enclose some special structure, as the Gasserian ganglion.
The cerebral sinuses of most importance are the superior longihidinal, the lateral or transverse, and the cavernous.
The superior longihidinal sinus runs in the median line from the foramen caecum in the ethmoid bone in front, to the torcular Herophili behind. As it passes backward it inclines more to the right side, so that at the torcular Herophili the left side of the sinus is about in the median line. This sinus receives the veins from the cortex of the brain and also some from the diploe of the bones above it. A vein pierces the upper posterior angle of each parietal bone and forms a communication between the superficial veins of the scalp outside and the superior longitudinal sinus within. The deviation of the superior longitudinal sinus toward the right, as it proceeds posteriorly, is to be borne in mind in operating in this region, as one can approach the median line nearer on the left side posteriorly than the right, without wounding it. In the parietal region the Pacchionian bodies are surrounded by extensions from the longitudinal sinus and free hemorrhage will ensue if the bone is removed too close to the median line.
Fig. 18. - Posterior view of the skull, showing the relation of the superior longitudinal sinus and torcular Herophili to the median line and external occipital protuberance.
The torcular Herophili, or confluence of the sinuses, does not correspond exactly to the external occipital protuberance or inion on the exterior of the skull. It is a little above and to the right of it. This torcular Herophili is formed by the meet-ing of the longitudinal sinus from above, the lateral, or transverse sinuses from the sides, the straight sinus from in front and the occipital sinus from below.
The lateral or transverse sinuses, of which there are two, pass from the torcular Herophili toward each side in the tentorium between the cerebrum and cerebellum, following the superior curved line of the occiput until just above the upper posterior portion of the mastoid process. They then bend downward to within a centimetre of the tip of the process and again curve forward to end in the jugular foramen and be continued as the internal jugular vein. The S-shaped curve which they make in this part of their course has given rise to the name sigmoid sinus. In its course along the superior curved line the sinus rises above the level of a line drawn from the inion to the centre of the external auditory meatus.
In operating for cerebellar abscess, care should be taken to place the trephine opening sufficiently low down to avoid wounding this sinus. It is in great danger of being wounded in operating for septic conditions involving the mastoid antrum and cells. Its distance from the surface of the skull varies in different individuals, and it gets farther from it as it descends to the level of the tip of the mastoid process. It receives the blood from the posterior lower portion of the cerebrum and upper portion of the cerebellum, and communicates with the veins outside the skull through the mastoid and posterior condyloid foramina.
Fig. 19. - Exit of cranial nerves and venous sinuses at the base of the skull.
Running along the upper posterior edge of the petrous portion of the temporal bone, in the attachment of the tentorium, is the superior petrosal sinus. It connects the lateral or transverse sinus about its middle with the cavernous sinus. More deeply situated, and running from the cavernous sinus to the lateral sinus, just as it enters the jugular foramen, is the inferior petrosal sinus.
The petrosal and lateral sinuses are frequently torn in fractures of the skull. A fracture passing through the petrous portion of the temporal bone may tear the petrosal sinuses, and hemorrhage from the ear might come from this source. A fracture- through the posterior cerebral fossa may tear the lateral sinus. Leeches are sometimes applied behind the ear in inflammation of the brain, with the idea of drawing blood from the lateral sinus through the mastoid vein.
The occipital sinus is usually small and brings the blood up from the region of the foramen magnum to the torcular Herophili.
The straight sinus runs along the line of juncture of the tentorium and falx cerebri. It receives the blood from the ventricles of the brain which are drained by the veins of Galen, and the blood from the falx through the inferior longitudinal sinus. This latter is usually very small and sometimes almost lacking, the blood in that case passing upward to empty into the superior longitudinal sinus.
one on each side, - is a large, irregular space on the side of the body of the sphenoid bone. It runs from the sphenoidal fissure in front to the apex of the petrous portion of the temporal bone behind. In front it is continuous with the ophthalmic vein, and receives the sphenoparietal sinus which brings the blood from the diploŽ; behind it communicates with the superior and inferior petrosal sinuses. The two sinuses communicate across the median line around the pituitary body, forming the circular sinus, and across the basilar process, forming what is sometimes called the transverse sinus, but which is more correctly described as a plexus of veins.
The cavernous sinus has embedded in its outer wall the third and fourth nerves and the ophthalmic branch of the fifth. Farther below and to the outer side of the sinus are the superior and inferior maxillary or mandibular branches of the fifth nerve. Within the sinus and toward its lower and inner portion, is the internal carotid artery. It is surrounded by the blood-current. Between the carotid artery and outer wall of the sinus runs the sixth nerve, held in place by fine, trabecular, fibrous bands which pass from side to side in the cavity of the sinus.
Fig. 20. - Transverse section of the right cavernous sinus. showing the position of the nerves and internal carotid artery (from a dissection).
The cavernous sinuses are sometimes torn in fractures of the base of the skull, resulting in a traumatic communication between the carotid artery and the sinus. The cavernous sinus is not infrequently torn in attempting the removal of the Gasserian ganglion, particularly if its ophthalmic branch is attacked. Its interior is not one large cavity, but is subdivided by fibrous septa, which pass from side to side. It is sometimes the seat of thrombosis and infection, which may reach it through the ophthalmic vein in front.