The Eyeball lies nestled in fat within the orbital cavities (two bony sockets) of the skull, where it is situated above and lateral to the center. Of all the senses, eyesight is often considered most important. According to one estimate, four-fifths of everything we know reaches the brain through our eyes. The eyes transmit constant streams of images to the brain by electrical signals. The eyes receive information from light rays. The light rays are either absorbed or reflected. Objects that absorb all of the light rays appear black, whereas those that reflect all the light rays appear white. Colored objects absorb certain parts of the light spectrum and reflect others. When you look at something, the light rays reflected from the object enter the eye. The light is refracted by the cornea and passes through the watery aqueous humor and pupil to the lens. The iris controls the amount of light entering the eye. Then the lens focuses the light through the vitreous humor onto the retina, forming an image in reverse and upside down. Light-sensitive cells in the retina transmit the image to the brain by electrical signals. The brain "sees" the image right side up.

Smell is the most basic and most primitive of the senses. It is some 10,000 times more acute than our sense of taste. In fact, most food flavors are smelled, not tasted, as anyone with a heavy cold will verify. Nasal congestion prevents the little eddies of air, stirred up by the action of chewing and swallowing, from reaching the receptors in the roof of the nasal cavity. Human smell receptors distinguish several thousand different types of smell. Some people have a better sense of smell than others. The nose also plays an important role in conditioning the inspired air for the lower respiratory tract. This conditioning includes: the control of temperature, the control of humidity, and the elimination of dust and infectious organisms.

The Skin has the largest surface area of any organ in the body and is the heaviest. On the surface are the sensitive papillae, and within are certain organs with special functions, the sweat glands, hair follicles, and sebaceous glands. The skin protects the internal organs of the body against infection, injury, and harmful sunrays. It also plays an important role in the regulation of body temperature. Although the skin of an average-sized adult may weigh as much as twenty pounds, it is only paper thin in some places and not much thicker in others.

The Tongue is usually flat and moderately extensible. It consists of a network of bundles of striated muscle fibers, fibrous tissue, fat and lymphoid masses, mucous-producing glands, and a covering of mucous membrane. It is an extremely mobile muscle that enables you to taste food, move it around as you chew, push it back into your pharynx (throat) when swallowing, and is an invaluable aid in speech. It is derived mostly from an outgrowth (tuberculum) in the floor of the pharynx. The tuberculum grows forward and is joined by other tissues from the region, forming this complex muscular organ of many uses.

The Lymphatic System is not really a separate system of the body. It is considered part of the circulatory system since it consists of lymph, a moving fluid that comes from the blood and returns to the blood by way of the lymphatic vessels. Lymph carries some nutrients around the body, especially fat. It also distributes germ-fighting white cells. Lymph resembles plasma, but is more diluted and contains only about 5% of proteins and 1% of salts and extractives. It is formed from bits of blood and other body liquids, called interstitial fluid or tissue fluid, that collect in the spaces between cells. Some of the interstitial fluid goes back into the body through the capillary membrane, but most enters the lymphatic capillaries to become lymph. Along with this interstitial fluid, the lymph also picks up any particles that are too big to be absorbed through the capillary membrane. These include cell debris, fat globules, and tiny protein particles. The lymph then moves into the larger lymphatic vessels and through the lymph nodes and eventually enters the blood through the veins in the neck region. The lymphatic system is thus a secondary transport system. Lymph has no pump of its own. Its flow depends on pressure from the blood system and the massaging effect of the muscles.


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