MODULE 14
The DIENCEPHALON - HOW IT DEVELOPED and WHAT IT LOOKS LIKE
- - - (This section is a review of material that has been covered in earlier modules, so you can skip it if you think you are familiar with the subject).
- - - Recall that the brain starts out as the simple tube-like structure shown in Figure 1-7. Within the cranial cavity, the more caudal part of the tube develops into the medulla, pons and midbrain - as shown in Figure 1-8. Collectively these regions are known as the brain stem. The most rostral division of the neural tube, the forebrain, develops into the diencephalon and telencephalon (or cerebral hemisphere). As pictured schematically in Figure 2-2, both these regions are complex and may be subdivided into several components.
- - - In this module we will start by considering the diencephalon as a whole, but then focus on the dorsal thalamus which - from this point on - we will refer to simply as the thalamus. Views such as Figure 1-8 would suggest that we ought to be able to see the diencephalon "from above". But, because of the way in which the hemispheres expand to cover the more caudal part of the brain (Figure 1-45, Figure 2-3) the diencephalon is hidden from view both dorsally and laterally, as shown in the schematic frontal section of Figure 1-46. This is also obvious in sections through the real brain such as:
- - - In fact, only a very limited ventral part of this region, the hypothalamus, can be viewed without removing brain tissue (Figure 2-13, Figure 2-21).
- - - If we had the luxury of working with real brains, we would expose the dorsal surface of the thalamus by cutting away the overlying hemisphere. In conceptual terms, we would make cut #1 of Figure 2-2 on both sides and lift off the hemispheres. The result would be to "uncover" the thalamus , as shown by the sections before and after such a cut in Figure 14-1 and Figure 14-1a. A drawing of the dorsal surface of the thalamus, so exposed, is shown in Figure 2-5; Compare it, if you like, with a similar view of a real brain seen in Figure 2-7.
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