Now that we've seen the principal arteries of the the brain, we'll move on to look at its veins, and at the channels that its veins drain into, the venous sinuses.
The brain is richly covered with veins. Over the surface of the cerebral hemispheres, the veins emerge from the sulci, join with one another, and run upwards within the arachnoid layer. Here behind the midbrain veins converge from many directions to form this great cerebral vein. We'll see where that goes shortly.
These veins drain into the venous sinuses which are a special feature of the cranial cavity. We'll look at these next. The sinuses that drain almost all the blood from the brain are the two sagittal sinuses, the straight sinus, and the two transverse sinuses. These sinuses are closely related to the major folds in the dura that we that we saw in an earlier section: the falx, and the tentorium.
In this specimen there are some openings in the falx, which is not unusual. The two sagittal sinuses run the length of the falx. The smaller inferior sagittal sinus runs within its free border, the larger superior sagittal sinus runs within its attached border. Blood in both the sagittal sinuses flows from front to back.
Here we've removed one side of the superior sagittal sinus so that we can look into it. As we saw in a previous section, the superior sagittal sinus is contained in a triangular space that's enclosed on all three sides by dura. At several places side passages called lacunae open into the sinus.
Veins from the surface of the brain open into the lacunae. The superior sagittal sinus ends where the attachments of the falx and the tentorium meet. Also running toward the same point is the straight sinus, which we'll lay open.
The straight sinus runs along the junction between the falx and the tentorium. At its upper end it receives the inferior sagittal sinus, and also the great cerebral vein.
Here there's a major joining and branching of sinuses called the confluence of sinuses. We'll look at it in a different dissection of just the back of the head. The confluence of the sinuses is here. To see it we'll remove the falx and the tentorium, leaving just their lines of attachment. Here's the confluence laid open. Leading from it on each side are the two major outflow channels for venous blood, the transverse sinuses.
Each transverse sinus runs within the attached border of the tentorium. Starting here in the mid-line, the transverse sinus follows the attachment of the tentorium round to here, then continues by turning sharply downwards in this s-shaped groove just behind the petrous temporal bone.
The sinus goes by two different names: this part is the transverse sinus, this part is the sigmoid sinus. To follow the sigmoid sinus we'll look at a different skull. Here's the groove for right sigmoid sinus. Here's the groove for the left one. They're usually unequal in size.
The sigmoid sinus leaves the cranial cavity by passing through this irregular opening, the jugular foramen, along with three cranial nerves that we saw in the previous section. Here we're looking into the posterior cranial fossa from behind. The cerebellum has been removed. We'll remove the dura that covers the sigmoid sinus.
Within the jugular foramen the end of the sigmoid sinus turns sharply downwards, becoming continuous with the the internal jugular vein.