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 upsidedown. Light-sensitive cells in the retina transmit the image to the brain by electrical signals. The brain "sees" the image right side up.
The ear is divided into three parts: the outer ear, the middle ear, and the inner ear. Each section performs its own separate function in a process that converts sound waves into nerve impulses, which are then transmitted to the brain. The outer ear has two parts; the pinna and the external auditory canal. The outer ear collects and channels sound. The middle ear , or tympanic cavity, is a tiny cavity hollowed out of the temporal bone. It is an intermediary in the processing of sound energy. It is responsible for increasing the intensity of incoming sound waves and transforming them into mechanical vibrations that can easily travel through the inner ear. The inner ear has two parts. One is made of bone, the other of a membrane that lies inside the bone. Both have complicated shapes, and for this reason they are called labyrinths. Each labyrinth has three parts: vestibule, semicircular canals, and cochlea. The inner ear contains the receptor cells, which receive the mechanical vibrations and transmit them to the brain.
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 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 sun rays. 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.
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.
