External Anatomy of the Brain





This atlas emphasizes views of the interior of the human central nervous system (CNS) , sectioned in various planes. Here in the first chapter we lay some of the groundwork for understanding the arrangements of these interior structures by presenting the surface features with which they are continuous, and by giving a broad overview of the components of the CNS.


The CNS is composed of the spinal cord and the brain , the major components of which are indicated in Fig. 1.1 . The human brain is dominated by two very large cerebral hemispheres , separated from each other by a deep longitudinal fissure . Each hemisphere is convoluted externally in a fairly consistent pattern into a series of gyri , separated from each other by a series of sulci (an adaptation that makes more area available for the cortex that covers each cerebral hemisphere). Several prominent sulci are used as major landmarks to divide each hemisphere into five lobes a


a In addition, the insula , an area of cerebral cortex buried deep in the lateral sulcus (see Fig. 5.7A ), is usually considered as a separate lobe.

frontal , parietal , occipital , temporal , and limbic —each of which contains a characteristic set of gyri ( Figs. 1.3 to 1.8 ). The two hemispheres are interconnected by a massive bundle of nerve fibers, the corpus callosum, and two smaller bundles of fibers called the anterior and posterior commissures . Finally, certain areas of gray matter are embedded in the interior of each cerebral hemisphere. These include major components of the basal ganglia (or, more properly, basal nuclei) and limbic system (primarily the amygdala and hippocampus ). They are apparent in the brain sections shown in Chapter 5, Chapter 6, Chapter 7 .


The cerebral hemispheres of humans are so massive that they overshadow or almost conceal the remaining major subdivisions of the brain—the diencephalon (made up of the thalamus, hypothalamus, epithalamus), brainstem , and cerebellum . Hemisecting a brain in the midsagittal plane, as in Fig. 1.1B , reveals these components.


The diencephalon (literally the “in-between brain”) is interposed between each cerebral hemisphere and the brainstem. The diencephalon contains the left and right thalamus , major waystations for information seeking access to the cerebral cortex; the hypothalamus , a major control center for visceral and drive-related functions; and the epithalamus, which includes the pineal gland and a set of nuclei called the habenula.


The brainstem, continuous caudally with the spinal cord, serves as a conduit for pathways traveling between the cerebellum or spinal cord and more rostral levels of the CNS. It also contains the neurons that receive or give rise to most of the cranial nerves .


The cerebellum (literally the “little brain”) is even more intricately convoluted than the cerebral hemispheres, to make room for an extensive covering of its own cortex. It plays a major role in the planning and coordination of movement. A deep transverse fissure (normally occupied over most of its extent by the tentorium cerebelli ) separates the cerebellum from the overlying occipital and parietal lobes and then continues deeper into the brain, partially separating the diencephalon from the cerebral hemispheres.


Figure 1.1


Lateral and medial surfaces of the brain. (A) The left lateral surface of the brain; anterior is to the left. (B) The medial surface of the right half of the sagittally hemisected brain; anterior is to the left.

(Dissections by Grant Dahmer, Department of Cell Biology and Anatomy, The University of Arizona College of Medicine.)





Figure 1.2


A masterful dissection of the entire CNS, with the spinal cord still encased in dura mater and arachnoid. (A) The anterior/inferior surface. Regions enlarged in the insets , after the dura mater and arachnoid were spread apart.

(B) The posterior surface of the entire CNS. The cauda equina and the caudal end of the spinal cord, enlarged in the insets after the dura mater and arachnoid were spread apart.

(Dissection by Dr. Norman Koelling, Department of Cell Biology and Anatomy, The University of Arizona College of Medicine.)





Figure 1.3


Multiple views of a brain. Only major structures are labeled here. (A) The right lateral surface (anterior toward the right). (B) The left lateral surface (anterior toward the left). (C) The anterior surface. (D) The superior surface (anterior toward the top of the page). (E) The posterior surface. (F) The inferior surface (anterior toward the top of the page). (G) The same inferior surface after removal of the cerebellum and most of the brainstem; the latter are shown in more detail in Fig. 1.9 .

(The rhinal sulcus is drawn as a dashed line to indicate that it is separate from the collateral sulcus, even though in this particular brain the two are continuous.) IFG, Inferior frontal gyrus; IPL, inferior parietal lobule; MFG, middle frontal gyrus; Occ, occipital lobe; Po, postcentral gyrus; Pr, precentral gyrus; SFG, superior frontal gyrus; SPL, superior parietal lobule; Temp, temporal lobe.

(Dissection by Grant Dahmer, Department of Cell Biology and Anatomy, The University of Arizona College of Medicine.)





Figure 1.4


Lateral and superior surfaces of the cerebral hemispheres. The arachnoid was not removed from this specimen, but major gyri and sulci are still apparent. (A) Lateral view of the left hemisphere. Numerous branches of the left middle cerebral artery can be seen emerging from the lateral sulcus and spreading over the lateral surface of the hemisphere beneath the arachnoid. (B) The same hemisphere as in (A), seen from a more anterior and superior vantage point. Anterior and posterior cerebral branches can be seen emerging from the longitudinal fissure and extending for a short distance over the superior surface of the hemisphere beneath the arachnoid. (Middle cerebral branches, although visible in this view, are not indicated.)

(C) Lateral view of the right hemisphere of the same brain shown in (A) and (B). Numerous branches of the right middle cerebral artery can be seen emerging from the lateral sulcus and spreading over the lateral surface of the hemisphere beneath the arachnoid, and a few posterior cerebral branches emerge from the longitudinal fissure. Although the two cerebral hemispheres of human brains are approximately mirror images of each other, some slight asymmetries are common, particularly in certain language-related areas. Note in this specimen how much farther posteriorly the lateral sulcus extends in the left hemisphere (A), and how much larger the triangular part of the inferior frontal gyrus is on the left (see also Fig. 1.5 ). (D) The same hemisphere as in (C) seen from a more anterior and superior vantage point. Anterior and middle cerebral branches can be seen emerging from the longitudinal fissure and extending for a short distance over the superior surface of the hemisphere beneath the arachnoid. (Middle cerebral branches, although visible in this view, are not indicated.)

(Dissection by Grant Dahmer, Department of Cell Biology and Anatomy, The University of Arizona College of Medicine.)

Dec 29, 2019 | Posted by in NEUROLOGY | Comments Off on External Anatomy of the Brain
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