The human brain is the portion of the central nervous system that fills the cranial cavity. The different anatomical structures that make up the brain are centers of higher nervous activity and the chief regulators of all life functions of the human body. Thus, the brain is a large mass of soft grey and white tissue that occupies tightly the hollow of the skull, resting on the sphenoid and occipital bone. It extends out of the skull as the spinal cord through the foramen magnum (a hole in the base of the cranium).
The human brain is composed of the cerebrum (telencephalon), thalamus and hypothalamus (diencephalon), basal ganglia, amygdala, midbrain (mesencephalon), pons, medulla oblongata, and cerebellum. The cerebrum in turn consists of two cerebral hemispheres and the cerebral cortex. The temporal lobe of each cerebral hemisphere contains an important anatomical structure called the hippocampus, which is the center of emotion and long-term memory. Meanwhile, the midbrain, pons and medulla make up the brainstem. The cerebrum is the center of complex intellectual functions; the different areas of the cerebrum cortex are connected with one another through a network of myelinated fibers called fasciculi, which make the brain work as one functional whole, giving us humans intellectual integrity; that is to say empathic intelligence capable of insight, understanding, and self-awareness.
Among all the animal species, the human brain is the largest as it has attained its highest development through evolution in terms of the number of neurons. Its massive size is due to the enlargement and complexity of the structure of the two cerebral hemispheres, which are joined morphologically and functionally by a thick bundle of nerve fibers (the corpus callosum). On the average, the adult human brain weighs 1.470 g, with an average volume of 1,425 cm3 and surface area of 1,600 cm2. Although the human brain is second in weight after the whale brain (6,000–7,000 g) and the elephant's (5,700 g), the human brain has a thicker cerebral cortex and contains deep grooves and fissures, with many folds (gyri) and lobes, making it larger in the number of nerve cells it contains. It is as if nature and evolution had crumpled it up tightly into wrinkles so that it could fit into our cranium.
The brain is tightly enveloped by three layers of connective tissue called meninges: the pia matter, which is the thin, inner layer covering snugly the brain surface; the arachnoid, which is the middle layer; and the dura mater, which is the external coat. Cerebral spinal fluid fills a narrow space between the arachnoid and the pia mater.
Histology
The brain tissue consists of billions of neurons and their support cells (oligodendrocytes, astrocytes, and microglia). All together, these support cells are called neuroglia, while the neurons are the functional and fundamental cells of the brain and the nervous system. The neurons form the grey matter, while their axons (hair-like processes) constitute the white matter. This is so, because these extremely thin and long extensions are covered with a sheath of lipid (fat), which is called myelin. The millions of myelinated axons cluster together to form nerve fibers, and these fibers get together to form fasciculi (plural of fasciculus); fasciculi constitute the white matter of the brain.
Blood Supply
The human brain receives oxygenated blood from two sources; the internal carotid artery and the basilar artery. The internal carotid divides into the middle cerebral and the anterior cerebral artery, while the basilar artery is formed by the union of the two vertebral arteries, which in turn arise from the subclavian arteries.
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| Human brain: medial surface of right side. |
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| Right external lateral surface of brain. |
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| Left external lateral surface of human brain. |