The Human Brain
The brain is a group of neurons (nerve cells) that in vertebrate animals is located within the cranium or skull. Great differences can be found in the size and complexity of the brain of different animal species. In primitive animals like worms, the brain's functions are performed by just a few ganglia; an average human brain is composed of about 10 billion neurons and weighs 1.3 kg; finally, the brain of a whale may weigh more than 5 kg.
The brain is the main control organ in the nervous system. It receives information from the sensory organs through the nerves, and gives orders to the rest of the body's organs. It is thought that the evolutionary position of each species is due to the evolution of its brain's abilities, which supposedly reach a peak in the human species. The higher brain's abilities, in turn, are attributed to its degree of encephalization, that is, the amount of brain cells in excess of that needed by the functions of perceiving and controlling. They have also been attributed to the development of a part of the brain called the forebrain, which in humans is very important.
The development of the human embryo, including its brain, resembles the evolution of the human species. The one-month embryo is similar to a fish, the first true vertebrate. When it develops, it passes through stages that are supposed to correspond to similar stages of the human evolution, and the same happens to the embryo's brain. In the embryo, the position of the brain is marked by protuberances in the neural tube. Eventually, these protuberances give rise to the main anatomical divisions of the adult brain: the hindbrain, the midbrain, and the forebrain, whose names correspond to their position along the axis of the brain.
In the evolutionary scale, from fishes to men, the different brain structures vary with respect to their development. Certain old structures, as the olfactory cortex, are well developed in fish and amphibians, while newer structures--as the forebrain--are not so developed. In reptiles and birds, descendants of the amphibians, the corpus striatum (which coordinates sight and hearing) is well developed. Particularly in birds, the larger size of the cerebellum (a part of the hindbrain) is attributed to its accomplishing essential functions in flying. In general, the brain of these species can be characterized as having a large stem--that may weight more than the rest of the organ--and a small cortex.
In mammalians, the olfactory cortex is integrated into a larger structure called the limbic system. In this system reside, not only the sense of smell, but also emotional and sexual responses. The mammalian brain is characterized by the expansion of the neocortex, which reaches its peak in the primates. The neocortex is composed of the frontal, parietal, occipital, and temporal lobes. Humans are distinguished from other primates by the more extensive development of the frontal, parietal, and temporal areas. The human brain cortex is much more convoluted and folded: seventy percent of it is not visible from the outside.
The brain's growth, from embryo to adult, is supposed to pass trough the same stages that the adult brain followed along the evolution path. Thus, the forebrain enlarges and the diencephalon develops as the brains of adults should have done over a period of many thousands years. The cerebral hemispheres expand in such a manner that finally they cover the thalamus completely.
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