Archive for the Category »Anatomy & Physiology «

Mouth

25 Aug 2010 | By Ramitan

Mouth, opening in an animal’s body used for taking in food. Mouths are also typically used for making sounds, such as barks, chirps, howls, and in humans, speech. In most animals, the mouth is found on the face, near the eyes and nose.

Lips, which form the mouth’s muscular opening, are an especially familiar part of the body for humans. Lips help hold food in the mouth and are used to form words during speech. They also help form facial expressions, such as smiling and frowning. Lips open wide during a yawn and squeeze together during a whistle. Lips are darker than the surrounding skin because of the many extremely small blood vessels, called capillaries, that show through the skin.

Anatomy of the Mouth
In humans, the mouth is an integral part of digestion, speech, and breathing. Food enters the mouth to be broken down both by the teeth and by enzymes secreted by three salivary glands—the sublingual gland, the submandibular gland, and the parotid gland. The tongue pushes food down the pharynx. The tongue and nasal cavity modify sound waves to produce the sounds of speech, while the tongue and teeth work together to form words.

The cheeks form the sides of the mouth. They are composed of muscle tissue that is covered on the outside by skin. Like the lips, the cheeks help hold food and they also play a role in speech.

Human Taste Buds
Seen here at 75 times their size, human taste buds line the surface of the tongue and portions of the mouth. Taste buds pick up only four sensations (sweet, sour, salty, and bitter), but these are combined with the sense of smell to give various foods their unique tastes.

Inside the mouth is the large, muscular tongue. This extremely flexible muscle is used for eating and swallowing and also for talking. It is attached to the floor, or bottom, of the mouth. Its upper surface is covered with tiny projections, called papillae, that give the tongue a somewhat rough texture. The papillae contain tiny pores that are the site of taste buds, the receptor cells responsible for our sense of taste. There are four kinds of taste buds that are grouped together on certain areas of the tongue’s surface—those that are sensitive to sweet, salty, sour, and bitter flavors.

Teeth
Teeth are anchored in the jaw by their roots, which fit into sockets of the spongy bone. In a growing child, the roots of the deciduous teeth are gradually absorbed into the bone. When a baby tooth “falls out,” it is really just the loose crown, dislodged by an incoming permanent tooth, that emerges from the gum line. Left to right are incisor, canine, premolar, and molar.

The roof, or top, of the mouth is called the palate. It separates the mouth from the nasal passages above it. The front part of the palate—the part closer to the lips—is made of bone covered with moist tissue, called mucous membrane. This part of the mouth is known as the hard palate. Behind the hard palate is the soft palate, a small area composed mainly of muscle tissue. During swallowing, the soft palate presses against the back of the throat, preventing food or liquid from moving upward into the nasal passages.

Teeth are used for biting into and chewing food. Their interaction with the lips and tongue helps a person speak clearly. Children have 20 primary teeth, which begin to erupt, or break through the gums, at about six months of age. At six years of age, the primary teeth start to fall out, as permanent teeth replace them. The number of permanent teeth is 32. The crown, or top, of each tooth is covered with enamel, the hardest substance in the human body.

The mouth also contains three pairs of salivary glands. These glands secrete a watery fluid called saliva, which moistens food and the tissues of the mouth. Saliva contains amylase, a digestive enzyme that starts to break down carbohydrates in food even before it is swallowed. Saliva also contains a specialized protein, or enzyme, called lysozyme, which fights bacteria.

Despite the presence of saliva, many kinds of bacteria live in the warm, moist environment of the mouth. Caring for the mouth, called oral hygiene, helps keep these bacteria from multiplying and causing illness. Daily brushing of the teeth and tongue, flossing between the teeth, and regular checkups with a dentist help keep the mouth clean and the teeth and gums healthy.

The most common ailment of the mouth is tooth decay. Other disorders affecting the mouth include gingivitis, a condition marked by inflamed, infected gums; trench mouth, a severe form of gingivitis that causes bleeding ulcers in the mouth; and thrush, a fungal infection characterized by white sores in the mouth. Oral cancer is a risk for individuals who smoke or chew tobacco or who drink alcohol excessively. A small lump or thickened tissue in the mouth may indicate cancer. It should be checked by a doctor or dentist without delay, as many oral cancers can be cured if treated early.

Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.


Category: Anatomy & Physiology, Human Body  One Comment

Skeleton (anatomy)

25 Aug 2010 | By Ramitan

Skeleton (anatomy), term applied to all the rigid or semirigid structures supporting the soft tissues of an animal’s body and providing leverage for muscular action. In vertebrates, the skeleton is known as the endoskeleton and is formed within the body. Some invertebrate animals, such as insects and crustaceans, have skeletons known as exoskeletons on the outside of the body.

A form of exoskeleton is the shell of calcium or silica secreted by certain protozoans known as foraminiferans. Commercial sponges have an exoskeleton consisting of spongin, which is a tough, elastic substance. Cnidarians secrete a wide variety of exoskeletal substances, ranging from the elastic covering of the jellyfish to the stony material deposited by coral. The familiar shells of most mollusks are composed of calcium carbonate and an organic ground substance known as conchiolin. Among insects, each of the three principal divisions of the body—the head, the thorax, and the abdomen—is enclosed in a framework of horny plates. The plates of each primary division are separated from those of the next division by elastic tissue that permits flexibility of motion. The appendages are enclosed by sheaths projecting from the exoskeleton; elastic tissue similar to that between the plates joins the segments of the appendages and attaches them to the body.

Vertebrates have a more or less rigid group of structures composed of cartilage or bone or of a combination of these two connective tissues. The most primitive of these structures is the notochord, which is a backbone of cartilage occurring in fishes. Animals higher on the evolutionary scale have an axial skeleton, consisting of the skull, spinal column, and ribs, and an appendicular skeleton, made up of the pelvic and pectoral girdles and the appendages.

Skeleton of a Cat
The domestic cat is well adapted for life as a carnivore. It has long canine teeth for capturing and holding prey; its spine is somewhat flexible; the claws on its toes are retractable, enabling it to seize its prey and defend itself against predators; and the long tail assists with balance as the cat stalks its prey.

Fish Skeleton
The skeleton of the fish has the same general structural components as other vertebrates: a skull, spinal column, limbs, and a tail. Fish limbs have developed into fins, which are adapted for swimming or providing stability while swimming.

In higher animals, the skeleton formed in the embryo is initially cartilaginous; bone and calcium are deposited as the organism matures. In humans, the process of bone hardening, or ossification, is completed at about the age of 25. The last bone to ossify is the breastbone.

Skeleton of a Snake
All snakes have at least 100 vertebrae, and some species have more than 400. A large number of vertebrae provides snakes with great flexibility of movement. The vertebrae also allow for the attachment of the powerful trunk muscles, which are responsible for locomotion, capturing prey, and swallowing.

The total number of bones in any animal varies with its age; many bones fuse together during the ossification process. The average number of distinct skeletal structures in a young human is 200, exclusive of the 6 ossicles found in the ears. The human skeleton is subject to a number of pathological conditions, most important of which are fracture and a deficiency disease that is known as rickets.

Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.

Category: Anatomy & Physiology, Human Body  3 Comments

Protein

18 Aug 2010 | By Ramitan

Protein, any of a large number of organic compounds that make up living organisms and are essential to their functioning. First discovered in 1838, proteins are now recognized as the predominant ingredients of cells, making up more than 50 percent of the dry weight of animals. The word protein is coined from the Greek proteios, or “primary.”

Protein molecules range from the long, insoluble fibers that make up connective tissue and hair to the compact, soluble globules that can pass through cell membranes and set off metabolic reactions. They are all large molecules, ranging in molecular weight from a few thousand to more than a million, and they are specific for each species and for each organ of each species. Humans have an estimated 30,000 different proteins, of which only about 2 percent have been adequately described. Proteins in the diet serve primarily to build and maintain cells, but their chemical breakdown also provides energy, yielding close to the same 4 calories per gram as do carbohydrate.

Besides their function in growth and cell maintenance, proteins are also responsible for muscle contraction. The digestive enzymes are proteins, as are insulin and most other hormones. The antibodies of the immune system are proteins, and proteins such as hemoglobin carry vital substances throughout the body.

Whether found in humans or in single-celled bacteria, proteins are composed of units of about 20 different amino acids, which, in turn, are composed of carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur. In a protein molecule these acids form peptide bonds—bonds between amino and carboxyl (COOH) groups—in long strands (polypeptide chains). The almost numberless combinations in which the acids line up, and the helical and globular shapes into which the strands coil, help to explain the great diversity of tasks that proteins perform in living matter.

To synthesize its life-essential proteins, each species needs given proportions of the 20 main amino acids. Although plants can manufacture all their amino acids from nitrogen, carbon dioxide, and other chemicals through photosynthesis, most other organisms can manufacture only some of them. The remaining ones, called essential amino acids, must be derived from food. Nine essential amino acids are needed to maintain health in humans: leucine, isoleucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine, and histidine. All of these are available in proteins produced in the seeds of plants, but because plant sources are often weak in lysine and tryptophan, nutrition experts advise supplementing the diet with animal protein from meat, eggs, and milk, which contain all the essential acids.

Most diets—especially in the United States, where animal protein is eaten to excess—contain all the essential amino acids. (Kwashiorkor, a wasting disease among children in tropical Africa, is due to an amino acid deficiency.) For adults, the Recommended Dietary Allowance (RDA) for protein is 0.79 g per kg (0.36 g per lb) of body weight each day. For children and infants this RDA is doubled and tripled, respectively, because of their rapid growth.

The most basic level of protein structure, called the primary structure, is the linear sequence of amino acids. Different sequences of the acids along a chain, however, affect the structure of a protein molecule in different ways. Forces such as hydrogen bonds, disulfide bridges, attractions between positive and negative charges, and hydrophobic (“water-fearing”) and hydrophilic (“water-loving”) linkages cause a protein molecule to coil or fold into a secondary structure, examples of which are the so-called alpha helix and the beta pleated sheet. When forces cause the molecule to become even more compact, as in globular proteins, a tertiary protein structure is formed. When a protein is made up of more than one polypeptide chain, as in hemoglobin and some enzymes, it is said to have a quaternary structure.

Secondary, Tertiary, and Quaternary Structure of a Protein
Interactions between the molecules that make up a protein cause the coiled polypeptide chain (left) to fold into a three-dimensional structure (center) which may join others to form a large, complex protein (right).
Polypeptide chains are sequenced and coiled in such a way that the hydrophobic amino acids usually face inward, giving the molecule stability, and the hydrophilic amino acids face outward, where they are free to interact with other compounds and especially other proteins. Globular proteins, in particular, can join with a specific compound such as a vitamin derivative and form a coenzyme, or join with a specific protein and form an assembly of proteins needed for cell chemistry or structure.
The major fibrous proteins, described below, are collagen, keratin, fibrinogen, and muscle proteins.
Collagen, which makes up bone, skin, tendons, and cartilage, is the most abundant protein found in vertebrates. The molecule usually contains three very long polypeptide chains, each with about 1000 amino acids, that twist into a regularly repeating triple helix and give tendons and skin their great tensile strength. When long collagen fibrils are denatured by boiling, their chains are shortened to form gelatin.
Keratin, which makes up the outermost layer of skin and the hair, scales, hooves, nails, and feathers of animals, twists into a regularly repeating coil called an alpha helix. Serving to protect the body against the environment, keratin is completely insoluble in water. Its many disulfide bonds make it an extremely stable protein, able to resist the action of proteolytic (protein-hydrolyzing) enzymes. In beauty treatments, human hair is set under a reducing agent, such as thioglycol, to reduce the number of disulfide bonds, which are then restored when the hair is exposed to oxygen.
Fibrinogen is a blood plasma protein responsible for blood clotting. With the catalytic action of thrombin, fibrinogen is converted into molecules of the insoluble protein fibrin, which link together to form clots.
Myosin, the protein chiefly responsible for muscle contraction, combines with actin, another muscle protein, forming actomyosin, the different filaments of which shorten, causing the contracting action.

Unlike fibrous proteins, globular proteins are spherical and highly soluble. They play a dynamic role in body metabolism. Examples are albumin, globulin, casein, hemoglobin, all of the enzymes, and protein hormones. The albumins and globulins are classes of soluble proteins abundant in animal cells, blood serum, milk, and eggs. Hemoglobin is a respiratory protein that carries oxygen throughout the body and is responsible for the bright red color of red blood cells. More than 100 different human hemoglobins have been discovered, among which is hemoglobin S, the cause of sickle-cell anemia, a hereditary disease suffered mainly by blacks.

All of the enzymes are globular proteins that combine rapidly with other substances, called substrate, to catalyze the numerous chemical reactions in the body. Chiefly responsible for metabolism and its regulation, these molecules have catalytic sites on which substrate fits in a lock-and-key manner to trigger and control metabolism throughout the body.These proteins, which come from the endocrine glands, do not act as enzymes. Instead they stimulate target organs that in turn initiate and control important activities—for example, the rate of metabolism and the production of digestive enzymes and milk. Insulin, secreted by the islands of Langerhans, regulates carbohydrate metabolism by controlling blood glucose levels. Thyroglobulin, from the thyroid gland, regulates overall metabolism; calcitonin, also from the thyroid, lowers blood calcium levels. Angiogenin, a protein structurally determined in the mid-1980s, directly induces the growth of blood vessels in tissues.
Also called immunoglobulins, antibodies  make up the thousands of different proteins that are generated in the blood serum in reaction to antigens (body-invading substances or organisms). A single antigen may elicit the production of many antibodies, which combine with different sites on the antigen molecule, neutralize it, and cause it to precipitate from the blood.Also called immunoglobulins, antibodies (see Antibody) make up the thousands of different proteins that are generated in the blood serum in reaction to antigens (body-invading substances or organisms). A single antigen may elicit the production of many antibodies, which combine with different sites on the antigen molecule, neutralize it, and cause it to precipitate from the blood.
Globular proteins can also assemble into minute, hollow tubes that serve both to structure cells and to conduct substances from one part of a cell to another. Each of these microtubules, as they are called, is made up of two types of nearly spherical protein molecules that pair and join onto the growing end of the microtubule, adding on length as required. Microtubules also make up the inner structure of cilia, the hairlike appendages by which some microorganisms propel themselves.

Protein Structure of a Microtubule
Microtubules are tiny hollow tubes that make up cilia and flagella, the threadlike organs of locomotion and feeding that are embedded in the cell wall of some eukaryotic organisms. Forming the walls of the tube are two types of globular protein subunits, alpha and beta tubulin, that come together to form a compound called a dimer. The dimers self-assemble, coiling to form a tube of the required length. Inside each cilium (short) or flagellum (long), nine pairs of microtubules encircle a tenth, central pair.

Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.

Category: Anatomy & Physiology, Human Body  4 Comments

The Noticeable Differences Between Genital Warts and Genital Herpes

10 Aug 2010 | By Ramitan

The numbers are frightening when you consider that there are approximately 19 million sexually transmitted disease infections every year reported to the Center for Disease Control (CDC). Most of these infections affect people in the age groups between 15 and 24. There are two specific types of virus that often go unreported because they may never actual be diagnosed by a physician; Genital Warts or Human Papillomavirus (HPV) and Genital Herpes or Herpes Simplex Virus (HSV). There are approximately 200 known types of HPV that can cause the outbreak of genital warts, while there are only two variations of HSV. The two variations were first divided into two distinct categories of above and below the waist by HSV-1 and HSV-2, but this is no longer the case, as it was realized that cross contamination was possible. There are noticeable differences between the symptoms related to genital warts and genital herpes. This is most likely due to the fact that both are caused by two completely unrelated viruses. Both viruses are contracted through sexual contact, but outbreaks, related symptoms and duration are very different.

Certain types of HPV cause genital warts and according to the CDC, are responsible for nearly every case of cervical cancer. More specifically, HPV 6 and 11 are responsible for approximately almost 90 percent of all genital wart cases. In most of the younger women who develop warts in the genital area, they often clear up over a period of time however, in cases where the conditions persist precancerous cells remain present on the cervix and cancer becomes the most likely outcome. Unfortunately, a person can become infected with the virus and never realize it as some do not even see evidence of warts, but the virus will always remain. It is stated that they are easy to detect during an exam and are popular, flat, or predunculated growths that are located on the genital mucosa. Warts may also be found in the mouth, anus, on the cervix, in the vagina and urethra. Treating genital warts can be challenging.

The current figures from the CDC on Herpes state that one out of every six people are infected with the virus between the ages of 14 and 49. Genital herpes is transmitted through sores on the skin that may or may not be visible at the time of sexual interaction. It can also be transmitted through sores on the mouth. Those that have the infection may not even be aware of its presence at the time if they have not yet had an outbreak. The first outbreak can be very difficult with sores on the genitalia and may be accompanied by symptoms of what seems to be the flu. Four or five outbreaks per year is the common number for most infected individuals and they will decrease over a period of years. Often some individuals do not even have outbreaks of sores, or their symptoms are very mild and almost unnoticeable. According to the CDC, Herpes may play some sort of role in the continued spread of HIV, and in making those infected with HIV more infectious.

Category: Anatomy & Physiology, Human Body  55 Comments

Eye

09 Aug 2010 | By Ramitan

Eye, light-sensitive organ of vision in animals. The eyes of various species vary from simple structures that are capable only of differentiating between light and dark to complex organs, such as those of humans and other mammals, that can distinguish minute variations of shape, color, brightness, and distance. The actual process of seeing is performed by the brain rather than by the eye. The function of the eye is to translate the electromagnetic vibrations of light into patterns of nerve impulses that are transmitted to the brain.

The entire eye, often called the eyeball, is a spherical structure approximately 2.5 cm (about 1 in) in diameter with a pronounced bulge on its forward surface. The outer part of the eye is composed of three layers of tissue. The outside layer is the sclera, a protective coating. It covers about five-sixths of the surface of the eye. At the front of the eyeball, it is continuous with the bulging, transparent cornea. The middle layer of the coating of the eye is the choroid, a vascular layer lining the posterior three-fifths of the eyeball. The choroid is continuous with the ciliary body and with the iris, which lies at the front of the eye. The innermost layer is the light-sensitive retina.

The cornea is a tough, five-layered membrane through which light is admitted to the interior of the eye. Behind the cornea is a chamber filled with clear, watery fluid, the aqueous humor, which separates the cornea from the crystalline lens. The lens itself is a flattened sphere constructed of a large number of transparent fibers arranged in layers. It is connected by ligaments to a ringlike muscle, called the ciliary muscle, which surrounds it. The ciliary muscle and its surrounding tissues form the ciliary body. This muscle, by flattening the lens or making it more nearly spherical, changes its focal length.

Structure of the Eye
The amount of light entering the eye (right) is controlled by the pupil, which dilates and contracts accordingly. The cornea and lens, whose shape is adjusted by the ciliary body, focus the light on the retina, where receptors convert it into nerve signals that pass to the brain. A mesh of blood vessels, the choroid, supplies the retina with oxygen and sugar. Lacrimal glands (left) secrete tears that wash foreign bodies out of the eye and keep the cornea from drying out. Blinking compresses and releases the lacrimal sac, creating a suction that pulls excess moisture from the eye’s surface.

The pigmented iris hangs behind the cornea in front of the lens, and has a circular opening in its center. The size of its opening, the pupil, is controlled by a muscle around its edge. This muscle contracts or relaxes, making the pupil larger or smaller, to control the amount of light admitted to the eye.

Behind the lens the main body of the eye is filled with a transparent, jellylike substance, the vitreous humor, enclosed in a thin sac, the hyaloid membrane. The pressure of the vitreous humor keeps the eyeball distended.

Light-Sensitive Organ
The eye is an organ. Behind its visible portions are a complicated array of delicate mechanisms that work in unison to transmit an image to the brain.

The retina is a complex layer, composed largely of nerve cells. The light-sensitive receptor cells lie on the outer surface of the retina in front of a pigmented tissue layer. These cells take the form of rods or cones packed closely together like matches in a box. Directly behind the pupil is a small yellow-pigmented spot, the macula lutea, in the center of which is the fovea centralis, the area of greatest visual acuity of the eye. At the center of the fovea, the sensory layer is composed entirely of cone-shaped cells. Around the fovea both rod-shaped and cone-shaped cells are present, with the cone-shaped cells becoming fewer toward the periphery of the sensitive area. At the outer edges are only rod-shaped cells.

Where the optic nerve enters the eyeball, below and slightly to the inner side of the fovea, a small round area of the retina exists that has no light-sensitive cells. This optic disk forms the blind spot of the eye.

Eye Movement
Eye movement is controlled by six muscles that are directly attached to the eyeball. The four rectus muscles form a relatively straight line from their points of origin, while the two oblique muscles approach the surface of the eye at an angle. All the muscles combine to keep the eyeball in nearly constant motion in order to maximize human vision, which is capable of focusing on about 100,000 distinct points in the visual field. These muscles also enable both eyes to focus on the same point simultaneously, thereby creating effective depth perception.

In general the eyes of all animals resemble simple cameras in that the lens of the eye forms an inverted image of objects in front of it on the sensitive retina, which corresponds to the film in a camera.

Focusing the eye, as mentioned above, is accomplished by a flattening or thickening (rounding) of the lens. The process is known as accommodation. In the normal eye accommodation is not necessary for seeing distant objects. The lens, when flattened by the suspensory ligament, brings such objects to focus on the retina. For nearer objects the lens is increasingly rounded by ciliary muscle contraction, which relaxes the suspensory ligament. A young child can see clearly at a distance as close as 6.3 cm (2.5 in), but with increasing age the lens gradually hardens, so that the limits of close seeing are approximately 15 cm (about 6 in) at the age of 30 and 40 cm (16 in) at the age of 50. In the later years of life most people lose the ability to accommodate their eyes to distances within reading or close working range. This condition, known as presbyopia, can be corrected by the use of special convex lenses for the near range.

Structural differences in the size of the eye cause the defects of hyperopia, or farsightedness, and myopia, or nearsightedness.

Focusing the Eye
Light rays entering the eye are refracted, or bent, when they pass through the lens. Normal vision requires that the rays focus on the retina. If the eyeball is too long, an accurately focused image falls short of the retina. This is called myopia, or nearsightedness. A nearsighted person sees distant objects unclearly. Farsighted focus, or hyperopia, results when the eyeball is too short. In this case, an accurately focused image would fall behind the retina. These conditions can also occur if the muscles of the eye are unable to alter the shape of the lens to focus light rays accurately.

The eye sees with greatest clarity only in the region of the fovea, due to the neural structure of the retina. The cone-shaped cells of the retina are individually connected to other nerve fibers, so that stimuli to each individual cell are reproduced and, as a result, fine details can be distinguished. The rod-shaped cells, on the other hand, are connected in groups so that they respond to stimuli over a general area. The rods, therefore, respond to small total light stimuli, but do not have the ability to separate small details of the visual image. The result of these differences in structure is that the visual field of the eye is composed of a small central area of great sharpness surrounded by an area of lesser sharpness. In the latter area, however, the sensitivity of the eye to light is great. As a result, dim objects can be seen at night on the peripheral part of the retina when they are invisible to the central part.

The mechanism of seeing at night involves the sensitization of the rod cells by means of a pigment, called visual purple or rhodopsin, that is formed within the cells. Vitamin A is necessary for the production of visual purple; a deficiency of this vitamin leads to night blindness (see Vitamin). Visual purple is bleached by the action of light and must be reformed by the rod cells under conditions of darkness. Hence a person who steps from sunlight into a darkened room cannot see until the pigment begins to form. When the pigment has formed and the eyes are sensitive to low levels of illumination, the eyes are said to be dark-adapted.

A brownish pigment present in the outer layer of the retina serves to protect the cone cells of the retina from overexposure to light. If bright light strikes the retina, granules of this brown pigment migrate to the spaces around the cone cells, sheathing and screening them from the light. This action, called light adaptation, has the opposite effect to that of dark adaptation.

Subjectively, a person is not conscious that the visual field consists of a central zone of sharpness surrounded by an area of increasing fuzziness. The reason is that the eyes are constantly moving, bringing first one part of the visual field and then another to the foveal region as the attention is shifted from one object to another. These motions are accomplished by six muscles that move the eyeball upward, downward, to the left, to the right, and obliquely. The motions of the eye muscles are extremely precise; the estimation has been made that the eyes can be moved to focus on no less than 100,000 distinct points in the visual field. The muscles of the two eyes, working together, also serve the important function of converging the eyes on any point being observed, so that the images of the two eyes coincide. When convergence is nonexistent or faulty, double vision results. The movement of the eyes and fusion of the images also play a part in the visual estimation of size and distance.

Several structures, not parts of the eyeball, contribute to the protection of the eye. The most important of these are the eyelids, two folds of skin and tissue, upper and lower, that can be closed by means of muscles to form a protective covering over the eyeball against excessive light and mechanical injury. The eyelashes, a fringe of short hairs growing on the edge of either eyelid, act as a screen to keep dust particles and insects out of the eyes when the eyelids are partly closed.

Inside the eyelids is a thin protective membrane, the conjunctiva, which doubles over to cover the visible sclera. Each eye also has a tear gland, or lacrimal organ, situated at the outside corner of the eye. The salty secretion of these glands lubricates the forward part of the eyeball when the eyelids are closed and flushes away any small dust particles or other foreign matter on the surface of the eye.

Normally the eyelids of human eyes close by reflex action about every six seconds, but if dust reaches the surface of the eye and is not washed away, the eyelids blink more often and more tears are produced. On the edges of the eyelids are a number of small glands, the Meibomian glands, which produce a fatty secretion that lubricates the eyelids themselves and the eyelashes.

The eyebrows, located above each eye, also have a protective function in soaking up or deflecting perspiration or rain and preventing the moisture from running into the eyes. The hollow socket in the skull in which the eye is set is called the orbit. The bony edges of the orbit, the frontal bone, and the cheekbone protect the eye from mechanical injury by blows or collisions.


Compound Eyes of a Fly
The head of the common house fly is dominated by a pair of large compound eyes containing approximately 4,000 image-forming elements called ommatidia. Each cone-shaped ommatidium consists of a lens, a crystalline rod, and a collection of light-sensitive cells. Compound eyes are only found in arthropods such as insects and crustaceans.

The simplest animal eyes occur in the cnidarians and ctenophores, phyla comprising the jellyfish and somewhat similar primitive animals. These eyes, known as pigment eyes, consist of groups of pigment cells associated with sensory cells and often covered with a thickened layer of cuticle that forms a kind of lens. Similar eyes, usually having a somewhat more complex structure, occur in worms, insects, and mollusks.

Two kinds of image-forming eyes are found in the animal world, single and compound eyes. The single eyes are essentially similar to the human eye, though varying from group to group in details of structure. The lowest species to develop such eyes are some of the large jellyfish. Compound eyes, confined to the arthropods , consist of a faceted lens, each facet of which forms a separate image on a retinal cell, creating a moasic field. In some arthropods the structure is more sophisticated, forming a combined image.

The eyes of other vertebrates are essentially similar to human eyes, although important modifications may exist. The eyes of such nocturnal animals as cats, owls, and bats are provided only with rod cells, and the cells are both more sensitive and more numerous than in humans. The eye of a dolphin has 7,000 times as many rod cells as a human eye, enabling it to see in deep water. The eyes of most fish have a flat cornea and a globular lens and are hence particularly adapted for seeing close objects. Birds’ eyes are elongated from front to back, permitting larger images of distant objects to be formed on the retina.

Eye disorders may be classified according to the part of the eye in which the disorders occur.

The most common disease of the eyelids is hordeolum, known commonly as a sty, which is an infection of the follicles of the eyelashes, usually caused by infection by staphylococci. Internal sties that occur inside the eyelid and not on its edge are similar infections of the lubricating Meibomian glands. Abscesses of the eyelids are sometimes the result of penetrating wounds. Several congenital defects of the eyelids occasionally occur, including coloboma, or cleft eyelid, and ptosis, a drooping of the upper lid. Among acquired defects are symblepharon, an adhesion of the inner surface of the eyelid to the eyeball, which is most frequently the result of burns. Entropion, the turning of the eyelid inward toward the cornea, and ectropion, the turning of the eyelid outward, can be caused by scars or by spasmodic muscular contractions resulting from chronic irritation. The eyelids also are subject to several diseases of the skin such as eczema and acne, and to both benign and malignant tumors. Another eye disease is infection of the conjunctiva, the mucous membranes covering the inside of the eyelids and the outside of the eyeball. See Conjunctivitis; Trachoma.

Disorders of the cornea, which may result in a loss of transparency and impaired sight, are usually the result of injury but may also occur as a secondary result of disease; for example, edema, or swelling, of the cornea sometimes accompanies glaucoma.

The choroid, or middle coat of the eyeball, contains most of the blood vessels of the eye; it is often the site of secondary infections from toxic conditions and bacterial infections such as tuberculosis and syphilis. Cancer may develop in the choroidal tissues or may be carried to the eye from malignancies elsewhere in the body. The light-sensitive retina, which lies just beneath the choroid, also is subject to the same type of infections. The cause of retrolental fibroplasia, however—a disease of premature infants that causes retinal detachment and partial blindness—is unknown. Retinal detachment may also follow cataract surgery. Laser beams are sometimes used to weld detached retinas back onto the eye. Another retinal condition, called macular degeneration, affects the central retina. Macular degeneration is a frequent cause of loss of vision in older persons. Juvenile forms of this condition also exist.

The optic nerve contains the retinal nerve fibers, which carry visual impulses to the brain. The retinal circulation is carried by the central artery and vein, which lie in the optic nerve. The sheath of the optic nerve communicates with the cerebral lymph spaces. Inflammation of that part of the optic nerve situated within the eye is known as optic neuritis, or papillitis; when inflammation occurs in the part of the optic nerve behind the eye, the disease is called retrobulbar neuritis. When the pressure in the skull is elevated, or increased in intracranial pressure, as in brain tumors, edema and swelling of the optic disk occur where the nerve enters the eyeball, a condition known as papilledema, or chocked disk.

Eye banks are organizations that distribute corneal tissue taken from deceased persons for eye grafts. Blindness caused by cloudiness or scarring of the cornea can sometimes be cured by surgical removal of the affected portion of the corneal tissue. With present techniques, such tissue can be kept alive for several weeks by freezing the tissue. Eye banks also preserve and distribute vitreous humor, the liquid within the larger chamber of the eye, for use in treatment of detached retinas. The first eye bank was opened in New York City in 1945. The Eye Bank Association of America, in Washington, D.C., acts as a clearinghouse for information.

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Category: Anatomy & Physiology, Human Body  33 Comments
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