Chapter 9

The diencephalon includes the thalamus and its geniculate bodies, the hypothalamus, the subthalamus, and the epithalamus (Fig 9–1). The third ventricle lies between the halves of the diencephalon. The development of the diencephalon is reviewed in Chapter 10 (see Figs 10–1, 10–3, and 10–4).

###### Figure 9–1

Midsagittal section through the diencephalon.

A small groove on the lateral wall of the slim third ventricle—the hypothalamic sulcus—separates the thalamus dorsally and the hypothalamus and subthalamus inferiorly.

### Landmarks

Each half of the brain contains a thalamus, a large, ovoid, gray mass of nuclei (Fig 9–2). Its broad posterior end, the pulvinar, extends over the medial and lateral geniculate bodies. The rostral thalamus contains the anterior thalamic tubercle. In many individuals, there is a short interthalamic adhesion (massa intermedia) between the thalami, across the narrow third ventricle (see Fig 9–1).

###### Figure 9–2

Dorsal aspect of the diencephalon after partial removal of the overlying corpus callosum. The thalamus is shown in blue.

### White Matter

The thalamic radiations are the fiber bundles that emerge from the lateral surface of the thalamus and terminate in the cerebral cortex. The external medullary lamina is a layer of myelinated fibers on the lateral surface of the thalamus close to the internal capsule. The internal medullary lamina is a thin vertical sheet of white matter that bifurcates in its anterior portion and thus divides the gray matter of the thalamus into lateral, medial, and anterior portions (Fig 9–3).

###### Figure 9–3

Diagrams of the thalamus. Oblique lateral and medial views.

### Thalamic Nuclei

There are five major groups of thalamic nuclei, each with specific fiber connections (Figs 9–3 and 9–4; Table 9–1).

###### Figure 9–4

Schematic lateral view of the thalamus with afferent fiber systems.

Table 9–1 Functional Divisions of Thalamic Nuclei.

#### Anterior Nuclear Group

This group of clusters of neurons forms the anterior tubercle of the thalamus and is bordered by the limbs of the internal lamina. It receives fibers from the mamillary bodies via the mamillothalamic tract and projects to the cingulate cortex ...

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