Photographic view with brainstem

This is a photographic view of the same brain seen from below, the inferior view, a view that includes the brainstem and the cerebellum. The medulla and pons, parts of the brainstem can be identified (see Figure 6 and Figure 7), but the midbrain is mostly hidden from view. The cranial nerves are still attached to the brainstem, and some of the arteries to the brain are also present.

The frontal lobe occupies the anterior cranial fossa of the skull. The inferior surface of the frontal lobe extends from the frontal pole to the anterior tip of the temporal lobe (and the beginning of the lateral fissure). These gyri rest on the roof of the orbit and are sometimes referred to as the orbital gyri. This is association cortex and these gyri have strong connections with the limbic system (discussed in Section D).

The next area is the inferior surface of the temporal lobe. This lobe occupies the middle cranial fossa of the skull. The temporal lobe extends medially toward the mid-brain and ends in a blunt knob of tissue known as the uncus. Moving laterally from the uncus, the first sulcus visible is the collateral sulcus/fissure (seen clearly on the left side of this photograph). The parahippocampal gyrus is the gyurus medial to this sulcus; it is an extremely important gyrus of the limbic system (discussed with Figure 74). It should be noted that the uncus is the most medial protrusion of this gyrus. (The clinical significance of the uncus and uncal herniation will be discussed with the next illustration.)

The olfactory tract and optic nerve (and chiasm) are seen on this view. Both are, in fact, CNS pathways and are not peripheral cranial nerves, even though they are routinely called CN I and CN II. The olfactory bulb is the site of termination of the olfactory nerve filaments from the nose; these filaments are, in fact, the peripheral nerve

CN I (see Figure 79). Olfactory information is then carried in the olfactory tract to various cortical and subcortical areas of the temporal lobe (discussed with Figure 79). The optic nerves (CN II) exit from the orbit and continue to the optic chiasm, where there is a partial crossing of visual fibers, which then continue as the optic tract (see Figure 41A). Posterior to the chiasm is the area of the hypothalamus, part of the diencephalon, including the pituitary stalk and the mammillary bodies, which will be seen more clearly in the next illustration.

The brainstem and cerebellum occupy the posterior part of this brain from this inferior perspective. These structures occupy the posterior cranial fossa of the skull. In fact, the cerebellum obscures the visualization of the occipital lobe (which is shown in the next photograph, after removing most of the brainstem and cerebellum). Various cranial nerves can be identified as seen previously (see Figure 7). The oculomotor nerve, CN III, should be noted as it exits from the midbrain; the slender trochlear nerve (CN IV) can also be seen.

Part of the arterial system is also seen in this brain specimen (the arterial supply is discussed with Figure 58-Figure 62). The initial part, vertebral arteries and the formation of the basilar artery, are missing, as are the three pairs of cerebellar arteries. The basilar artery, which is situated in front of the pons, ends by dividing into the posterior cerebral arteries to supply the occipital regions of the brain. The cut end of the internal carotid artery is seen, but the remainder of the arterial circle of Willis is not dissected on this specimen (see Figure 58); the arterial supply to the cerebral hemispheres will be fully described in Section C (see Figure 60 and Figure 61).

Note to the Learner: The specimen of the brainstem and diencephalon shown in Figure 7 was created by dissecting these parts of the brain free of the hemispheres. This has been done by cutting the fibers going to and from the thalamus, as well as all the fibers ascending to and descending from the cerebral cortex (called projection fibers, discussed with Figure 16). The diagrams of such a specimen are shown in Figure 6, Figure 8A, and Figure 8B.

Olfactory bulb

Olfactory tract

Optic chiasm

Parahippocampal gyrus

Bulb Spinal Cord

Optic nerve (CN II)

Trochlear nerve (CN IV)

Basilar artery

Olfactory bulb

Olfactory tract

Optic chiasm

Uncus

Collateral sulcus

Parahippocampal gyrus

Optic nerve (CN II)

Lateral fissure

Internal carotid artery

Oculomotor nerve (CN III)

Posterior cerebral artery

Trochlear nerve (CN IV)

Basilar artery

F = Frontal lobe T = Temporal lobe Po = Pons M = Medulla SC = Spinal cord Ch = Cerebellar hemisphere

FIGURE 15A: Cerebral Hemispheres 4 — Inferior View with Brainstem (photograph)

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