(4.48)

Now we'll move on to look at the brain. The internal structure of the brain, which is extremely complex, lies outside the scope of this atlas. In this section we'll look at the main external features of the brain, and also at the cavities within it, the ventricles.

This model shows the shape of the ventricular system. It's formed by two small cavities in the mid-line, the third ventricle, and fourth ventricle, and two much larger cavities, the lateral ventricles, which connect to the third ventricle here. It's the third ventricle, because the lateral ventricles are counted as the first two.

The ventricles are filled with cerebrospinal fluid. We'll see more of them as we go along. To understand the external features of the brain we'll start with the central stalk, which is known as the brainstem. To look at it, we'll take the rest of the brain out of the picture.

Here's the brainstem. It consists of the medulla, the pons, and the midbrain. The brainstem contains tracts that connect the cerebrum, the cerebellum and the spinal cord; and it contains nuclei that serve basic autonomic functions. It's also the origin of nearly all the cranial nerves.

The medulla is cone shaped. It tapers down to become continuous with the spinal cord. The medulla becomes continuous with the spinal cord here at the foramen magnum.

The medulla, the pons and the midbrain are located just behind the basilar part of the occipital bone, and the dorsum sellae. The dorsal aspect of the medulla faces almost directly backwards. The back of the upper part of the medulla forms the floor of the fourth ventricle. On the model, this is the fourth ventricle, this is the floor.

This arch of tissue is the superior medullary velum, which forms the roof of this part of the ventricle. This delicate tissue, the inferior medullary velum, forms this part of the roof.

This cut surface is the attachment of the cerebellum. It's described as consisting of the superior, middle, and inferior cerebellar peduncles, which are somewhat fused together. The ventral aspect of the medulla is marked on each side by these bulges, the pyramid, and the olive.

Emerging from the ventral and lateral surfaces of the medulla are the filaments of the four lowest cranial nerves, the twelfth, the hypoglossal; the eleventh, the accessory; the tenth, the vagus; and the ninth, the glossopharyngeal.

Here's the brain stem in situ, seen from behind,. The tentorium has been removed to give us this view. Here's the cerebellum, divided in the midline. Here's the divided cerebellar peduncle. Here are the filaments of the hypoglossal nerve making their exit from the cranium.

Here are the accessory, vagus, and glossopharyngeal nerves making their exit together through one opening. Above the medulla is the pons. On each side the pons becomes continuous with the middle cerebellar peduncle.

Arising from the groove between the pons and the medulla are the next three cranial nerves. They're the eighth, the vestibulo-cochlear; the seventh, the facial; and the sixth, just visible, the abducent. The fifth cranial nerve, the trigeminal emerges from the upper part of the pons.

Here's the middle cranial fossa, here's the petrous temporal bone, here's the pons. Here are the facial and vestibulo-cochlear nerves together, here's the trigeminal nerve, here's the abducent nerve.