This chapter, the second of three showing sections of entire human brains, illustrates axial planes approximating those used in computed tomography scans (see Chapter 9 ). Forebrain structures continue to be emphasized, but parts of the brainstem and cerebellum are indicated as well. The organization of various functional systems in the forebrain (e.g., thalamus, hippocampus) is presented in Chapter 8 .
Colored diagrams showing typical areas of arterial supply in each section are provided in this and the preceding and following chapter. We simplified these in two major ways. First, arterial territories are shown as sharply demarcated from each other, when in reality there is significant interdigitation and overlap. Second, perforating arteries arise from all the vessels of the circle of Willis, but we lumped together those from the posterior communicating and posterior cerebral arteries, and those from the anterior communicating and anterior cerebral arteries.
The hemisected brain from Fig. 1.7 , used in much of this chapter to indicate planes of section.
The planes of section shown in this chapter, indicated on three-dimensional reconstructions.
(Courtesy Dr. John W. Sundsten, Department of Biological Structure, University of Washington School of Medicine.)
(A–X) Twenty-four axial sections of a brain, arranged in an inferior-to-superior sequence extending from the orbital surface of the frontal lobe to just above the corpus callosum. Anterior is toward the top, as in the conventional orientation of computed tomography (CT) and magnetic resonance (MR) images ( Chapter 9 ); as a result the brainstem, when present, is inverted relative to its orientation in Chapter 3 .
(A) The first section just reaches the orbital surface of the frontal lobe, including gyrus rectus (2) and the beginning of other parts of the orbital frontal cortex (1) , and passes through the olfactory sulcus (8) and olfactory tract (3) . The temporal pole (4) can also be seen. The brainstem is cut at an odd angle in these sections, with more rostral parts toward the top. This section passes through the basal pons (5) and the inferior olivary nucleus (7) of the medulla. The corticospinal tract (6) can be seen passing from the basal pons into the medullary pyramid.
(B) Gyrus rectus (2) is still present, joined by a little more of the orbital frontal cortex (1) . The section again passes through the basal pons (4) and the basilar artery (3) anterior to it, as well as parts of the rostral medulla. It includes the cerebellar flocculus (5) , the inferior cerebellar peduncle (6) , the vestibulocochlear (8) nerve, and choroid plexus (7) in the lateral aperture of the fourth ventricle.
(C) The olfactory tract (1) has now reached the posterior end of the olfactory sulcus, near where it attaches to the base of the forebrain. The optic nerve (2) moves posteriorly toward the optic chiasm; the internal carotid artery (3) is just lateral to where the optic chiasm will soon be located. The inferior horn of the lateral ventricle (5) and adjacent amygdala (4) begin to appear in the temporal lobe. The inferior cerebellar peduncle (6) turns posteriorly toward the cerebellum.
(D) The middle cerebral artery (1) moves laterally into the lateral sulcus. The amygdala (3) is larger, and the hippocampus (4) appears just posterior to it; both structures underlie the uncus (2) . The plane of section moves closer to the hypothalamus and passes through the infundibulum (9) . The middle cerebellar peduncle (6) connects the basal pons (5) to the cerebellum. The inferior cerebellar peduncle (7) has completed its posterior turn and is now cut in cross section as it moves into the cerebellum. The oculomotor nerve (8) moves anteriorly from its point of emergence from the brainstem.
Axial sections.
(E) The anterior cerebral artery (2) moves into the longitudinal fissure (1) , and the middle cerebral artery (4) continues on its course toward the insula. The optic nerves partially decussate in the optic chiasm (3) . The amygdala (5) and hippocampus (6) continue to increase in size. The cerebellar vermis (8) and hemispheres (9) can be distinguished, and the middle cerebellar peduncle (7) still connects the basal pons to the cerebellum. Shown enlarged in Fig. 6.4 .
(F) The optic tract (1) begins to move posteriorly from the optic chiasm, and the plane of section reaches the tuberal zone of the hypothalamus (2) . The superior cerebellar peduncle (5) leaves the deep cerebellar nuclei (represented here by the dentate nucleus [6] ), forms part of the wall of the fourth ventricle (4) , and enters the pons. The first part of the midbrain to appear in this plane of section is the cerebral peduncle (3) .
(G) The base of the forebrain, beginning to pass through the head of the caudate nucleus (1) , the putamen (5) , the nucleus accumbens (6) , and the anterior limb of the internal capsule (2) . The insula (3) , buried in the lateral sulcus (4) , overlies the putamen. The mammillary bodies (8) and other parts of the hypothalamus border the third ventricle (7) . The cerebral peduncle (9) and substantia nigra (10) are apparent in the midbrain. The superior cerebellar peduncle (12) leaves the dentate nucleus (13) , enters the brainstem, and decussates (11) . Shown enlarged in Fig. 6.5 .
(H) The lateral ventricle is now cut twice, through the anterior (2) and inferior (7) horns, and the corpus callosum (1) makes its first appearance. The limbic lobe is also cut twice, through the cingulate (18) and parahippocampal (13) gyri. The head of the caudate nucleus (3) , the putamen (5) , the nucleus accumbens (17) and the anterior limb of the internal capsule (4) all increase in size. Fibers that will cross in the anterior commissure (6) begin to move toward the midline, and the optic tract (15) continues to move posteriorly. The column of the fornix (16) and the mammillothalamic tract (14) are transected just above each mammillary body. All of the deep cerebellar nuclei (12) are now apparent. The vast majority of efferents from these nuclei travel through the superior cerebellar peduncle (11) and decussate (10) , and then most of them (9) pass through or around the red nucleus (8) . Shown enlarged in Fig. 6.6 .
Axial sections.
