Brainstem and cerebellum dorsal inferior photographic view

This is a photograph of the same specimen as Figure 9A, but the specimen is tilted to reveal the inferior aspect of the cerebellum and the posterior aspect of the medulla. The posterior aspect of the pons is still covered by the cerebellum (see Figure 10). The posterior aspect of the midbrain can no longer be seen. The upper end of the thalamus is still in view.

The horizontal fissure of the cerebellum is now clearly seen; it is used as an approximate divider between the superior and inferior surfaces of the cerebellum (see Figure 54). The vermis of the cerebellum is clearly seen between the hemispheres. Just below the vermis is an opening into a space — the space is the fourth ventricle (which will be described with Figure 20A, Figure 20B, and Figure 21) The opening is between the ventricle and the subarachnoid space outside the brain (discussed with Figure 21); the name of the opening is the Foramen of Magendie.

The part of the brainstem immediately below the foramen is the medulla, its posterior or dorsal aspect. The most significant structure seen here is a small elevation, repre senting an important sensory relay nucleus, the nucleus gracilis. The pathway for discriminative touch sensation, called the gracilis tract (or fasciculus) continues up the posterior aspect of the spinal cord and synapses in the nucleus of the same name; the pathway then continues up to the cerebral cortex. (The details of this pathway will be discussed with Figure 33 and Figure 40). Beside it is another nucleus for a similar pathway with the same function, the nucleus cuneatus (see Figure 10). These nuclei will be discussed with the brainstem cross-sections in Section C (see Figure 67C). The medulla ends and the spinal cord begins where the C1 nerve roots emerge.

The cerebellar lobules adjacent to the medulla are known as the tonsils of the cerebellum (see ventral view of the cerebellum, Figure 7). The tonsils are found just inside the foramen magnum of the skull.

Clinical Aspect

Should there be an increase in the mass of tissue occupying the posterior cranial fossa (e.g., a tumor, hemorrhage), the cerebellum would be pushed downward. This would force the cerebellar tonsils into the foramen magnum, thereby compressing the medulla. The compression, if severe, may lead to a compromising of function of the vital centers located in the medulla (discussed with Figure 6). The complete syndrome is known as tonsillar herniation, or coning. This is a life-threatening situation that may cause cardiac or respiratory arrest.

Thalamus

Foramen of

Magendie

(to 4th ventricle)

Spinal cord

Cerebellum Dorsal

Horizontal fissure

Vermis of cerebellum

Tonsil of cerebellum

Nucleus gracilis C1 nerve roots

Ch = Cerebellar hemisphere

Horizontal fissure

Vermis of cerebellum

Tonsil of cerebellum

Nucleus gracilis C1 nerve roots

Ch = Cerebellar hemisphere

FIGURE 9B: Brainstem 6 — Dorsal Inferior View with Cerebellum (photograph)

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