In order to see the sigmoid and the other venous sinuses as well as several other important structures, we need to examine the floor of the cranial vault with the brain removed. However, try and imagine the brain in place as you work through this area. It is also a good idea to have a skull in hand so you can cross reference the soft structures of the cadaver's cranial vault floor with the bony landmarks in the skull. (Figure 7-7)

Remember, the floor of the cranial vault is divided into an anterior, middle and posterior cranial fossa.

Let's start with the posterior cranial fossa as we are already there looking for the sigmoid sinuses. These continuations of the transverse sinus wind themselves down the anterior part of the posterior cranial fossa forming a groove in the temporal bone. Follow these sigmoid sinuses until they seemingly disappear through the jugular foramen .

Now, as a group, identify cranial nerves IX, X and XI that also pass through the jugular foramen. On the anterior wall of the posterior cranial fossa locate the internal acoustic meatus and identify cranial nerves VII and VIII as they enter the substance of the petrous part of the temporal bone. If you can find the foramen magnum, which should not be too difficult, you should be able to identify the cranial nerve XII exiting the skull through the hypoglossal canal .

Moving to the middle cranial fossa , identify the prominent optic nerves and the cut stub of the infundibulum. Realize the pituitary (hypophysis) sits in the sella turcica and the infundibulum, on its way from the hypothalamus to the pituitary, pierces a layer of dura which covers and hides the pituitary gland.

On the lateral side of the pituitary gland, look for a cavernous sinus. This visually obscure sinus is drained by a small superior petrosal sinus on the superior border of the petrous portion of the temporal bone. This sinus forms the posterior boundary of the middle cranial fossa. The superior petrosal sinus empties into the transverse sinus where the latter becomes the sigmoid sinus.

The inferior petrosal sinus takes a short cut to the jugular foramen and can be found near the lateral edge of the foramen magnum. The majority of the inferior petrosal sinus is located in the posterior cranial fossa. If the brain has been removed properly, you can see the stubs of cranial nerves III, IV, V and VI . They are in the posterior cranial fossa, but if you reflect the dura anteriorly you will see that the majority of these nerves reside in the middle cranial fossa. You will also expose the internal carotid artery and the ganglion of the trigeminal nerve. Can you find the three divisions of the trigeminal anterior to the ganglion?

Coursing laterally away from the cavernous sinuses, the middle meningeal vessels work their way up from the foramen spinosum to the lateral cranial vault wall between the dura and the bony vault. The anterior cranial fossa is dull compared to the others. Locate the cut edge of the falx cerebri on the crista galli.