Essentially all animals have a brain, in one form or another, which enables them to function. In invertebrates, animals which have no backbone, like earthworms and insects, the workings of their various body parts are coordinated by clusters of nerve cells called "ganglia." The ganglia receive information from the sense organs, which they use as a basis for controlling the animal's behavior, such as its feeding processes.
Vertebrates, which are animals which have a backbone, have a more well-developed brain. The brain of vertebrates may be divided into three main regions: the forebrain region, the midbrain region, and the hindbrain region. The more advanced the animal, the larger and more highly developed is the forebrain region relative to the other regions of the brain. Human beings have the most highly developed brain of all animal species, especially in the forebrain region.
Some "thinking" occurs even in lower animals, such as birds. They are able to process, store, and analyze information, which give them an impressive ability to learn new behavior. The brain reaches its highest degree of development in mammals. The brains of chimpanzees and other apes are the most highly developed, and more closely resemble the human brain than that of any other animal species. Some chimpanzees have been known to exhibit almost human behavior, and have been taught to communicate with people, to an extent, through the use of sign language.
On a functional basis, the human brain, as well as the brain of other vertebrates, may be divided into three main parts: the cerebrum, which is essentially the forebrain; the cerebellum; and the brain stem. The cerebrum, among other things, processes sensory information, and is believed to be involved in the carrying out of the higher mental functions, like reasoning and language formation. The cerebellum is concerned with the maintaining of proper body balance and muscular coordination. And the brain stem relays sensory information to various parts of the brain, as well as performing other important functions.
Following is a sketch indicating the three main functional components of the human brain and those of a bird. The cerebrum comprises a greater proportion to the total brain size in human beings than it does in any other animal.
The cerebrum comprises about eighty-five percent of the weight of the human brain. The outer-most part of the cerebrum is called the "cerebral cortex," or just the "cortex." The cerebral cortex is also referred to as the "neocortex," meaning "new cortex," as it differs from that in lower vertebrates. The cortex folds upon itself many times, thus forming a surface having many ridges and grooves, which gives it a greatly increased surface area. The cerebrum of lower animals have less folds and, therefore, a smoother cortex surface and less surface area. Most of the cerebrum below the cortex consists of nerve cell fibers connecting various parts of the cortex to itself and to other parts of the brain.
Although we think of our brain as being a single whole, the cerebrum of human beings, as well as that of all mammals, is divided physically into two hemispheres. The cerebral hemispheres are interconnected by a large tract of nerve bundles containing over a quarter-billion nerve cells, which acts as a communication network between the two hemispheres. The left and right hemispheres are almost a mirror image of each other physically, but differ significantly in some of the functions each performs.
The following is an underside view of the human brain. It shows the left and right cerebral hemispheres of the brain, as well as the cerebellum and the brain stem. Also shown are the thalami, hypothalamus, and pons segments of the brain stem, which are responsible for many important functions performed by the brain.
If one were to cut through the cerebral cortex, that is, the outer layer of the cerebrum, there would be observed a whitish-yellow substance and a pinkish colored substance. After exposure to formaldehyde, the pinkish substance becomes a white color, and the whitish-yellow one becomes gray; hence, the term "gray matter" commonly being used to denote the intellectual capacity of a person.
The white matter makes up the bulk of the space of the brain and forms a vast network of cross connections between various parts of the brain. The gray matter is only a few millimeters thick and contains nerve cells called "neurons." Each of the major functional parts of the brain, i.e., the cerebrum, the cerebellum, and the brain stem, contain certain neurons that are grouped together to form centers that are responsible for specific functions of the brain.
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