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Old Tuesday, August 07, 2007
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Sureshlasi Sureshlasi is offline
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Default Topic # 15

nose




nose, olfactory and respiratory organ, located between the eyes. The external nose, composed of bone and cartilage, is the most prominent feature of the face in humans. The internal nose is a hollow structure above the roof of the mouth, divided by the septum into two nasal cavities that extend from the nostrils to the pharynx. The mucous membrane that lines the nasal cavities is covered with fine hairs known as cilia that help to filter dust and impurities from the air before it reaches the lungs; the air is also moistened as it passes over the sticky nasal membrane. In the human nose, there are three horizontal folds on the walls of the nasal cavities, called the conchae: other mammals may have more conchae. The uppermost concha is densely supplied with capillaries that warm the air passing over them to near body temperature. High in the nasal cavity is a small tract of mucous membrane containing the nerve cell endings of the olfactory nerve, which impart the sense of smell. Therefore, inflammation of the nasal mucous membranes, which commonly accompanies colds and other infections, not only obstructs breathing but also impairs the sense of smell.




handedness


handedness, habitual or more skillful use of one hand as opposed to the other. Approximately 90% of humans are thought to be right-handed. It was traditionally argued that there is a slight tendency toward asymmetrical physiological development favoring the right side of the body, and that the center of gravity is to the right of the body's midline. This, however, would seem to be the consequence of greater dependence upon the right hand rather than the cause of right-handedness.

The neurological argument holds that since the right and left sides of the body are controlled by the opposite hemispheres of the brain, the greater development of the left hemisphere results in right-handedness. Anatomical studies have demonstrated that Broca's center, the area of the cerebral cortex that controls speech and muscular coordination, is almost always better developed in the left hemisphere in right-handed individuals; in 70% of left-handed individuals these centers are located in the right brain. Psychologists have raised the possibility of a cultural explanation. Although young children can be trained to prefer the right hand against a natural inclination, there is evidence that handedness is hereditary and that denser neurological connections extending from one side of the brain or the other are present from birth. A cultural explanation is also challenged by the evidence that some other vertebrates demonstrate a preference for one hand or paw over the other.

Although it is not clear that culture is a causative agent in handedness, it is certain that the high incidence of right-handedness has shaped human society in almost every conceivable aspect. Tools, machinery, and even clothing are largely designed for the right-handed, and until fairly recently, many left-handed individuals were strongly encouraged to switch to right-handedness. In some cultures the left-handed were thought to be evil or to bring bad luck.






stomach




stomach, saclike dilation in the gastrointestinal tract between the esophagus and the intestines, forming an organ of digestion. The stomach is present in virtually all vertebrate animals and in many invertebrates. In ruminants such as the cow, the stomach is divided into four separate chambers. One of these, called the rumen, breaks down complex plant materials, particularly cellulose. In birds, the stomach forms a thick-walled gizzard that is capable of grinding food. The human stomach is a muscular, elastic, pear-shaped bag, lying crosswise in the abdominal cavity beneath the diaphragm. It is capable of gross alterations in size and shape, depending on the position of the body and the amount of food inside. The stomach is about 12 in. (30.5 cm) long and is 6 in. (15.2 cm) wide at its widest point. Its capacity is about 1 qt (0.94 liters) in the adult. Food enters the stomach from the esophagus, through a ring of muscles known as the cardiac sphincter that normally prevents food from passing back to the esophagus. The other end of the stomach empties into the first section of the small intestine, or duodenum; the pyloric sphincter, which separates the two, remains closed until the food in the stomach has been modified and is in suitable condition to pass into the small intestine. The wall of the stomach is composed of four layers, or tunics: an outer fibrous membrane called the serosa, a three-ply layer of muscle, a submucous layer, and, forming the stomach lining, a mucous layer called the gastric mucosa. The surface of the mucosa is honeycombed with over 35,000 gastric glands and is folded into numerous ridges that almost disappear when the stomach is distended with food. The muscular action of the stomach and the digestive action of the gastric juice convert food in the stomach into a semiliquid state (chyme). The stomach comprises complex interconnections of neurons formed into intrinsic nerve plexuses, including the submucosal, subserous, or myenteric plexuses. The stomach is believed to be independent of the central nervous system.






tongue




tongue, muscular organ occupying the floor of the mouth in vertebrates. In some animals, such as lizards, anteaters, and frogs, it serves a food-gathering function. In humans, the tongue functions principally in chewing, swallowing, and speaking. The human tongue is covered by a mucous membrane containing small projections called papillae, which give it a rough surface. Tiny taste organs, or buds, are scattered over the entire surface of the papillae, with large numbers concentrated on the circumvallate papillae, toward the middle of the tongue. The appearance of the tongue is often an indication of body health; a pinkish-red color is normal. In impairment of the digestion and in certain feverish diseases, a yellowish coating forms. Local infection of the tongue is called thrush.





smell



smell, sense that enables an organism to perceive and distinguish the odors of various substances, also known as olfaction. In humans, the organ of smell is situated in the mucous membrane of the upper portion of the nasal cavity near the septum. It is made up of the olfactory cells, which are actually nerve cells that function as receptors for the sense of smell. The free ends of the cells project outward from the epithelial tissue in the form of numerous hairlike processes. These fibers are buried in the mucus that coats the inner surface of the nasal cavity and are stimulated by various odors. Nerve fibers extend from the olfactory cells to an area of the brain called the olfactory bulb. Any disturbance of the nasal cavity—such as the common cold—in which the olfactory hairs are covered with excess mucus or other material, interferes with the sense of smell. Most physiologists agree that although a substance must be volatile to be sniffed by the nose, it must subsequently be dissolved in the mucous lining of the nasal cavity to be smelled. It is also believed that there are only a few basic odors (perhaps about seven), and that all other odors are a combination of these. Attempts at classifying the so-called primary sensations of smell have not yet been successful. The sense of smell is not as strongly developed in humans as in many other vertebrates, particularly carnivores which employ olfactory organs to locate food and detect dangerous predators. To many invertebrates (especially insects) as well, smell is a highly developed sensory mechanism, necessary in obtaining food, in finding mating partners, and in recognizing other animals.






taste


taste, response to chemical stimulation that enables an organism to detect flavors. In man and most vertebrate animals, taste is produced by the stimulation by various substances of the taste buds on the mucous membrane of the tongue. A taste bud consists of about 20 long, slender cells; a tiny hair projects from each cell to the surface of the tongue through a tiny pore. The taste cells contain the endings of nerve filaments that convey impulses to the taste center in the brain. Only four fundamental tastes, or a combination of these, can be detected by the buds: sweet, sour, salt, and bitter. Only the buds most sensitive to salty flavor are scattered evenly over the tongue. Sweet-sensitive taste buds are concentrated on the tip of the tongue, sour flavors are detected at the sides of the tongue, and bitter flavors at the back. The close relationship of taste to smell gives the impression that a greater variety of tastes exists. This is also why an impairment of smell, as during a cold, may impart the feeling that the sense of taste is diminished.







tears




tears, watery secretion of the lacrimal gland, which is located at the outer corner of the eye socket immediately above the eyeball. Tearing, or lacrimation, is a continuous and largely involuntary process stimulated by the autonomic nervous system. Fluid is secreted into the lacrimal lake, the area between the eyeball and the upper eyelid, and spread across the surface of the eye by blinking. Tears serve to bathe and lubricate the cornea, the sensitive outer covering of the eyeball. Typically, the fluid either evaporates or is drained off through tiny canals at the inner corner of the eye, but in times of excessive tearing the apparatus is overwhelmed and tears overflow the eyes.







thirst


thirst, sensation indicating the body's need for water. Dry or salty food and dry, dusty air may induce such a sensation by depleting moisture in the mucous membranes of the mouth and throat. Relief through ingestion of water is only temporary, however, if thirst results from a generalized depletion of water in the system. About three fourths of the body is composed of fluids, and the average adult requires 2 1/2 qt (2.4 liters) of fluid per day, supplied by water, other beverages, and foods. Depriving the body of water interferes with its metabolism and functions, causing dehydration, which is eventually fatal. The unnatural thirst that accompanies fever, diabetes, and other disorders is caused by a rapid reduction of the body fluids. The sensation of thirst is controlled by osmoreceptors in the hypothalamus in the brain. Dehydration of the cells triggers the posterior pituitary to releast the antidiuretic hormone (ADH).








touch


touch, tactile sensation received by the skin, enabling the organism to detect objects or substances in contact with the body. End organs (nerve endings) in the skin convey the impression to the brain. Touch sensitivity varies in different parts of the body, depending on the number of end organs present in any one area. The tip of the tongue, lips, and fingertips are three of the most sensitive areas, the back and parts of the limbs the least so. The sense of touch is very closely related to the other four sensations received by the skin: pain, pressure, heat, and cold. There is a specific kind of sensory receptor for each of the five so-called cutaneous senses. For example, light-touch receptors convey only the sensation that an object is in contact with the body, while pressure receptors convey the force, or degree, of contact. The blind learn to read by the Braille system by making use of the sensitivity to touch of the fingertips.
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Last edited by Sureshlasi; Saturday, September 01, 2007 at 04:07 AM.
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