The brain and the spinal cord together form the central nervous system. This is the main control unit of the body. The nerves going out from the central nervous system, reaching out to all the parts of the body, are called the peripheral nervous system. This system, that also includes the five senses, consists of two systems as well: a somatic and an autonomous system. The somatic system gathers information about the surroundings and transfers this to the central nervous system, and sends out signals to the muscles of the body, to create movement, while the autonomous nerve system regulates the internal bodily functions, such as keeping the body at the right temperature and the heart pumping.
All of this system is constructed out of nerve cells, called neurons. These cells are much like the cells in the rest of the body, but one large difference between them is that neurons are able to communicate and transfer information from one to another, either through electrical or chemical impulses. Another difference is that neurons are not able to divide, as other cells do. And since the production of new nerve cells stops around the age of 18, and even though it is estimated that there are 100 billion neurons in the human brain alone, the number of nerve cells will have peaked early in life.
The brain itself is divided into three parts: The large cerebrum with its significant folded cortex, the limbic system with thalamus and hypothalamus, and last the mid- and hindbrain with pons, cerebellum and the upper spinal cord. All three parts have not always been natural parts of an organism´s brain, but have one by one been added during the evolution from micro-organism to Homo sapiens.
The oldest and most primitive part, the mid- and hindbrain, provides the basic functions and instincts such as breathing and heartbeat, and included in this part of the brain is also the cerebellum, both controlling the automatic functions, like keeping one´s balance, and coordinating the conscious movements.
The limbic system is placed right in the middle of the brain, under cerebrum, and this part is characteristic for and appeared with the first mammals. Thalamus, hypothalamus, hippocampus and the pituitary gland are parts of the limbic system, these parts are controlling the hormone production, emotions (fear, joy, aggression, love) and are also connected to storing memories.
The last, and latest, part of the brain is the cerebrum with the cerebral cortex. Cortex has a very large amount of foldings that gives it a much larger surface than the size of the scull otherwise would be able to allow. The upper layer, the grey matter, is where all the higher cognitive processes take place: communication, memory, understanding, creating. Or in one single word: Thinking. This is the part of the brain that by its complexity constitutes Homo sapiens as the intellectually superior being on this planet (although it is discussed where the dolphin, that has a far more folded cortex, is to be placed intellectually with that point of view on cortex-conditioned intelligence). Cerebrum is divided in two halves, a left and a right hemisphere connected by the 200 million nerve fibers that gives the corpus callosum. No other mammal than the human being is known to have a so-called hemisphere specialization, two seemingly symmetrical halves with different approaches to the same subject, a digital and an analogic. In most cases the two hemispheres are characterized by the left one being the digital (rational, verbal and analytic perception and thinking) and the right being the analogic ("seeing the larger perspective", sensoric, "creative"). The fine combination of the two diverse ways of perception and thinking is what has made Homo sapiens the outstanding creation it is. Even our closest "relatives", the gorilla and the chimpanzee, with whom we seem to have so much in common does not possess this hemispheric specialization, that might be what lifted us from their level.
Even though we seem to have mapped all of the brain, we are still far from solving the puzzle. There are still parts of the brain which functions have not been revealed yet, but with all the energy that is being put into the research, it can only be a matter of time.