The human brain weighs about three pounds, or 1.5 kg. In its natural state it is very soft, having approximately the consistency of pudding, although surrounded by leathery membranes and interspersed with stringy blood vessels. When alive it is pinkish on the outside, and mostly white on the inside, with subtle variations in color. Anatomists, in order to make it solid enough to work with, usually apply fixatives that give the tissue a rubbery texture and grayish color. The brains of other species have generally similar properties, but smaller sizes in relation to the body.
The largest portion of the human brain comprises the cerebral hemispheres, situated at the top and covered with a very convoluted "cortex". Underneath the cerebrum lies the brainstem, appearing somewhat like a stalk on which the cerebrum is mounted. At the back of the brain, beneath the cerebrum and behind the brainstem, is the cerebellum, a structure with a horizontally furrowed surface that makes it look different from anything else. These are the main structures visible from the outside, but a great deal more lies hidden below the surface. In other mammals, the same structures are present, but the cerebrum is not so large in relation to the brain as a whole. As a rule, the smaller the cerebrum, the less convoluted the cortex. The cortex of a rat or mouse is almost completely smooth. The cortex of a dolphin or whale, on the other hand, is substantially more convoluted than the cortex of a human.
In vertebrates, the brain is surrounded by connective tissues called meninges, a system of membranes that separate the skull from the brain. This three-layered covering is composed of (from the outside in) the dura mater ("hard mother"), arachnoid mater ("spidery mother"), and pia mater ("soft mother"). The arachnoid and pia are physically connected and thus often considered as a single layer, the pia-arachnoid. Below the arachnoid is the subarachnoid space which contains cerebrospinal fluid, a substance that protects the nervous system. Blood vessels enter the central nervous system through the perivascular space above the pia mater. The cells in the blood vessel walls are joined tightly, forming the blood-brain barrier which protects the brain from toxins that might enter through the blood. The brain is bathed in cerebrospinal fluid (CSF), which circulates in the narrow spaces between cells, and through cavities called ventricles. CSF is important both metabolically and mechanically: it provides neutrients to the brain, supports it, and cushions it against shocks.
The cortex is the part of the brain that most strongly distinguishes mammals from other vertebrates, primates from other mammals, and humans from other primates. In non-mammalian vertebrates, the surface of the cerebrum is lined with a comparatively simple layered structure called the pallium. In mammals, the pallium evolves into a complex 6-layered structure called neocortex. In primates, the neocortex is greatly enlarged in comparison to its size in non-primates, especially the part called the frontal lobes. In humans, this enlargement of the frontal lobes is taken to an extreme, and other parts of the cortex also become quite large and complex.
In other vertebrates—fish, amphibians, reptiles, birds—the same basic structures are present, but may be reshaped to give a dramatically different appearance. In non-vertebrates—insects, molluscs—the components and their arrangements differ so greatly that it is impossible to make meaningful comparisons except on the basis of genetics. In all, three groups of animals have notably complex brains: the arthropods (insects, crustaceans, arachnids, and others), the cephalopods (octopuses, squids, and similar mollusks), and the craniates (vertebrates and hagfish).[1] The brains of arthropods and cephalopods arise from twin parallel nerve cords that extend through the body of the animal. Arthropods have a central brain with three divisions and large optical lobes behind each eye for visual processing.[1] Cephalopods have the largest brains of any invertebrates. The brain of the octopus in particular is very highly developed, apparently comparable in complexity to the brain of a bird or reptile.
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