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Membrane (Latin membrana,”parchment”), in biology, any thin layer of connective tissue coating individual cells and organs of the body, or lining the joints and the ducts and tracts that open to the exterior of the body. The membrane surrounding single-celled animals and plants and individual cells in multicellular organisms is important in the nutritive, respiratory, and excretory processes of these cells. Such cell membranes are semipermeable; that is, they allow the passage of small molecules, such as those of sugars and salts, but not large molecules, such as those of proteins. Structures inside cells, such as the nucleus, may also have membranes. Each organ in the animal body is surrounded by a membrane, extensions of which often anchor the organ to the body wall. Three membranes, known as meninges, surround the brain and spinal cord; the outermost is known as the dura mater, the middle layer as the arachnoid, and the innermost as the pia mater. Each lung is coated with a membrane...

Tissue, group of associated, similarly structured cells that perform specialized functions for the survival of the organism (see Physiology ). Animal tissues, to which this article is limited, take their first form when the blastula cells, arising from the fertilized ovum, differentiate into three germ layers: the ectoderm, mesoderm, and endoderm (see Embryology ). Through further cell differentiation, or histogenesis, groups of cells grow into more specialized units to form organs made up, usually, of several tissues of similarly performing cells. Animal tissues are classified into four main groups. EPITHELIAL TISSUES These tissues include the skin and the inner surfaces of the body, such as those of the lungs, stomach, intestines, and blood vessels. Because its primary function is to protect the body from injury and infection, epithelium is made up of tightly packed cells with little intercellular substance between them. About 12 kinds of epithelial tissue occur. One kind is stratifi...

Pulse (physiology), rhythmic expansion of the arteries resulting from passage of successive surges of blood, produced by continuing contractions of the heart. The arteries resemble elastic tubes, and at each contraction of the heart, 30 to 60 g (2 to 4 oz) of blood are forced into the already-filled vessels. The consequent distension passes along the arterial system at a rate of about 7 m (about 23 ft) a second until it reaches the capillaries, in which it is lost because of peripheral resistance to blood flow and lack of elasticity in the vessel walls. The pulse may be felt wherever an artery passes over a solid structure, such as a bone or cartilage. The crest of the pulse wave represents the systolic pressure; the trough, the diastolic (see Blood Pressure ). The rate of the pulse varies from 150 beats per minute in the embryo, to about 60 in the aged. Autosuggestion and certain training programs may alter the rate substantially (see Biofeedback ; Consciousness, States of ; Sports Me...

Cellular Respiration, process in which cells produce the energy they need to survive. In cellular respiration, cells use oxygen to break down the sugar glucose and store its energy in molecules of adenosine triphosphate (ATP). Cellular respiration is critical for the survival of most organisms because the energy in glucose cannot be used by cells until it is stored in ATP. Cells use ATP to power virtually all of their activities—to grow, divide, replace worn out cell parts, and execute many other tasks. Cellular respiration provides the energy required for an amoeba to glide toward food, the Venus fly trap to capture its prey, or the ballet dancer to execute stunning leaps. Cellular respiration occurs within a cell constantly, day and night, and if it ceases, the cell—and ultimately the organism—dies. Two critical ingredients required for cellular respiration are glucose and oxygen. The glucose used in cellular respiration enters cells in a variety of ways. Plants, algae, and cer...

. Respiratory System, in anatomy and physiology, organs that deliver oxygen to the circulatory system for transport to all body cells. Oxygen is essential for cells, which use this vital substance to liberate the energy needed for cellular activities. In addition to supplying oxygen, the respiratory system aids in removing of carbon dioxide, preventing the lethal buildup of this waste product in body tissues. Day-in and day-out, without the prompt of conscious thought, the respiratory system carries out its life-sustaining activities. If the respiratory system’s tasks are interrupted for more than a few minutes, serious, irreversible damage to tissues occurs, followed by the failure of all body systems, and ultimately, death. While the intake of oxygen and removal of carbon dioxide are the primary functions of the respiratory system, it plays other important roles in the body. The respiratory system helps regulate the balance of acid and base in tissues, a process crucial for the norma...

Nervous System, those elements within the animal organism that are concerned with the reception of stimuli, the transmission of nerve impulses, or the activation of muscle mechanisms. Nervous System Organization The nervous system is composed of the central nervous system and the peripheral nervous system. The central nervous system, which includes the brain and spinal cord, processes and coordinates all incoming sensory information and outgoing motor commands, and it is also the seat of complex brain functions such as memory, intelligence, learning, and emotion. The peripheral nervous system includes all neural tissue outside of the central nervous system. It is responsible for providing sensory, or afferent, information to the central nervous system and carrying motor, or efferent, commands out to the body’s tissues. Voluntary motor commands, such as moving muscles to walk or talk, are controlled by the somatic nervous system, while involuntary motor commands, such as digestion and h...

Urinary System, system of organs that produces and excretes urine from the body. Urine is a transparent yellow fluid containing unwanted wastes, mostly excess water, salts, and nitrogen compounds. The major organs of the urinary system are the kidneys , a pair of bean-shaped organs that continuously filter substances from the blood and produce urine. Urine flows from the kidneys through two long, thin tubes called ureters. With the aid of gravity and wavelike contractions, the ureters transport the urine to the bladder , a muscular vessel. The normal adult bladder can store up to about 0.5 liter (1 pt) of urine, which it excretes through the tubelike urethra. An average adult produces about 1.5 liters (3 pt) of urine each day, and the body needs, at a minimum, to excrete about 0.5 liter (1 pint) of urine daily to get rid of its waste products. Excessive or inadequate production of urine may indicate illness and doctors often use urinalysis (examination of a patient’s urine) as part o...

Kidney, paired organ whose functions include removing waste products from the blood and regulating the amount of fluid in the body. The basic units of the kidneys are microscopically thin structures called nephrons, which filter the blood and cause wastes to be removed in the form of urine. Together with the bladder, two ureters, and the single urethra, the kidneys make up the body’s urinary system . Human beings, as well as members of all other vertebrate species, typically have two kidneys. Like kidney beans, the body’s kidneys are dark red in color and have a shape in which one side is convex, or rounded, and the other is concave, or indented. The kidneys of adult humans are about 10 to 13 cm (4 to 5 in) long and about 5 to 7.5 cm (2 to 3 in) wide—about the size of a computer mouse. The kidneys lie against the rear wall of the abdomen, on either side of the spine. They are situated below the middle of the back, beneath the liver on the right and the spleen on the left. Each kidney i...

Heart, in anatomy , hollow muscular organ that pumps blood through the body. The heart, blood , and blood vessels make up the circulatory system , which is responsible for distributing oxygen and nutrients to the body and carrying away carbon dioxide and other waste products. The heart is the circulatory system’s power supply. It must beat ceaselessly because the body’s tissues—especially the brain and the heart itself—depend on a constant supply of oxygen and nutrients delivered by the flowing blood. If the heart stops pumping blood for more than a few minutes, death will result. The human heart is shaped like an upside-down pear and is located slightly to the left of center inside the chest cavity. About the size of a closed fist, the heart is made primarily of muscle tissue that contracts rhythmically to propel blood to all parts of the body. This rhythmic contraction begins in the developing embryo about three weeks after conception and continues throughout an individual’s life. T...

Histamine, also histamine phosphate, an amine (beta-imidazolyl-ethylamine, ergamine, or ergotidime) that is a normal constituent of almost all animal body cells. Histamine is also found in minute quantities in ergot and putrefied meat products and is produced synthetically for medicinal purposes. In the body, it is synthesized in a type of leukocyte (see Blood ) called a basophil or mast cell. In response to certain stimuli these cells release histamine, which immediately effects a dilation of the blood vessels. This dilation is accompanied by a lowering of blood pressure and an increased permeability of the vessel walls, so that fluids escape into the surrounding tissues. This reaction may result in a general depletion of vascular fluids, causing a condition known as histamine poisoning or histamine shock. Allergic reactions in which histamine is released, resulting in the swelling of body tissue, show similarities to histamine poisoning; the two may be basically allied, and the two c...

Stomach, organ of the digestive system . Most animals, like humans, have a single stomach, but birds and ruminants have digestive organs composed of two or more chambers. The outer surface of the stomach is smooth; the inner surface is folded into numerous complex ridges, which assist in the mixing of food with digestive juices and channel this material through the stomach into the intestines. Only water, alcohol, and certain drugs seem to be absorbed from the stomach; most food absorption takes place in the small intestine . In humans the stomach is situated in the upper part of the abdominal cavity (see Abdomen ), mostly to the left of the midline. The large, domed end of the stomach, the fundus, lies in the left vault of the diaphragm ; the esophagus enters the upper side, or lesser curvature, a short distance below the fundus. The region immediately below the fundus is called the body. The upper part of the stomach, spoken of as the cardiac portion, includes the fundus and body. T...

Muscle, tissue or organ of the animal body characterized by the ability to contract, usually in response to a stimulus from the nervous system. The basic unit of all muscle is the myofibril, a minute, threadlike structure composed of complex proteins. Each muscle cell, or fiber, contains several myofibrils, which are composed of regularly arranged myofilaments of two types, thick and thin. Each thick myofilament contains several hundred molecules of the protein myosin. Thin filaments contain two strands of the protein actin. The myofibrils are made up of alternating rows of thick and thin myofilaments with their ends interleaved. During muscular contractions, these interdigitated rows of filaments slide along each other by means of cross bridges that act as ratchets. The energy for this motion is generated by densely packed mitochondria that surround the myofibrils. Three types of muscular tissue are recognized: smooth muscle -> Human smooth muscle is composed of slender, spindle-sh...

Joints, in anatomy, regions of union between bones or cartilages in the skeleton . Synarthroses are rigid, immovable joints, such as the connections between the bones of the skull; symphyses are slightly movable joints, such as the junction of the bones making up the front of the pelvis; and diarthroses are movable joints, such as the meeting of the bones of the limbs with those of the trunk. Immovable joints are held together by actual intergrowth of bone or by strong fibrous cartilage. Slightly movable joints are held together by elastic cartilage. Typical movable joints consist of an external layer of fibrous cartilage giving rise to strong ligaments that support the separate bones. The bones of movable joints are covered with smooth cartilage and are lubricated by a thick fluid, called synovial fluid, produced between the bones in membranous sacs, known as bursae. Bursitis, or inflammation of the bursae, is a common painful condition of movable joints. See also Arthritis . The huma...

Lung, either of a pair of elastic, spongy organs used in breathing and respiration. Lungs are present in all mammals, birds, and reptiles. Most amphibians and a few species of fish also have lungs. In humans the lungs occupy a large portion of the chest cavity from the collarbone down to the diaphragm . The right lung is divided into three sections, or lobes. The left lung, with a cleft to accommodate the heart, has only two lobes. The two branches of the trachea , called bronchi, subdivide within the lobes into smaller and smaller air vessels known as bronchioles . Bronchioles terminate in alveoli, tiny air sacs surrounded by capillaries. When the alveoli inflate with inhaled air, oxygen diffuses into the blood in the capillaries to be pumped by the heart to the tissues of the body. At the same time carbon dioxide diffuses out of the blood into the lungs, where it is exhaled. Air travels to the lungs through a series of air tubes and passages. It enters the body through the nostrils...

Circulatory System, or cardiovascular system, in humans, the combined function of the heart , blood , and blood vessels to transport oxygen and nutrients to organs and tissues throughout the body and carry away waste products. Among its vital functions, the circulatory system increases the flow of blood to meet increased energy demands during exercise and regulates body temperature. In addition, when foreign substances or organisms invade the body, the circulatory system swiftly conveys disease-fighting elements of the immune system , such as white blood cells and antibodies , to regions under attack. Also, in the case of injury or bleeding, the circulatory system sends clotting cells and proteins to the affected site, which quickly stop bleeding and promote healing. Topics: Components of the Circulatory System Operation and Function Disorders of the Circulatory System

Blood, vital fluid found in humans and other animals that provides important nourishment to all body organs and tissues and carries away waste materials. Sometimes referred to as “the river of life,” blood is pumped from the heart through a network of blood vessels collectively known as the circulatory system . An adult human has about 5 to 6 liters (1 to 2 gal) of blood, which is roughly 7 to 8 percent of total body weight. Infants and children have comparably lower volumes of blood, roughly proportionate to their smaller size. The volume of blood in an individual fluctuates. During dehydration, for example while running a marathon, blood volume decreases. Blood volume increases in circumstances such as pregnancy, when the mother’s blood needs to carry extra oxygen and nutrients to the baby. Blood carries oxygen from the lungs to all the other tissues in the body and, in turn, carries waste products, predominantly carbon dioxide, back to the lungs where they are released into the air...

Diuretic, chemical compound that increases the flow of urine and thus eliminates accumulations of water in cells, tissues, blood, and organs. The retention of excess water may occur following injury, as when water accumulates in the knee; in congestive heart failure, when the heart pumps insufficient blood to eliminate a normal volume of fluid; and in a variety of other disabilities, including hypertension , cirrhosis of the liver, and kidney diseases. Heart stimulants, such as digitalis , produce a diuretic effect by increasing blood pressure and thus increasing the flow of blood through the kidneys. Certain alkaloids found in coffee and tea ( caffeine , theobromine, and theophylline, in increasing order of strength) increase urine output by counteracting the tendency of blood proteins to prevent the removal of water from the blood by the kidneys. A class of drug known as loop diuretics, such as furosemide, are extremely effective when taken orally. All diuretics may have severe side...

Neurotransmitter, chemical made by neurons, or nerve cells . Neurons send out neurotransmitters as chemical signals to activate or inhibit the function of neighboring cells. Within the central nervous system, which consists of the brain and the spinal cord, neurotransmitters pass from neuron to neuron. In the peripheral nervous system, which is made up of the nerves that run from the central nervous system to the rest of the body, the chemical signals pass between a neuron and an adjacent muscle or gland cell. Nine chemical compounds—belonging to three chemical families—are widely recognized as neurotransmitters. In addition, certain other body chemicals, including adenosine, histamine , enkephalins, endorphins, and epinephrine , have neurotransmitterlike properties. Experts believe that there are many more neurotransmitters as yet undiscovered. The first of the three families is composed of amines, a group of compounds containing molecules of carbon, hydrogen, and nitrogen. Among the...

Insulin, hormone , produced in the islets of Langerhans of the pancreas , that regulates the metabolism of carbohydrates, fats, and starches in the body. Like other proteins, insulin is partially digested if administered orally and hence must be injected into a muscle when used clinically. In the treatment of diabetes mellitus , which is caused by a deficiency of insulin production or by inhibition of its action on cells, insulin is often combined with protamine, which prolongs the period of absorption of the hormone. Insulin crystallized from the pancreas contains zinc, which also lengthens absorption. A preparation called protamine zinc insulin extends the hormone's action still further. Insulin was first extracted from the pancreatic tissue of dogs in 1921 by the Canadian physiologists Sir Frederick Grant Banting and Charles Herbert Best and the British physiologist John James Rickard Macleod. The Canadian biochemist James Bertram Collip then produced it in sufficiently pure for...

Hormone, chemical that transfers information and instructions between cells in animals and plants. Often described as the body’s chemical messengers, hormones regulate growth and development, control the function of various tissues, support reproductive functions, and regulate metabolism (the process used to break down food to create energy). Unlike information sent by the nervous system , which is transmitted via electronic impulses that travel quickly and have an almost immediate and short-term effect, hormones act more slowly, and their effects typically are maintained over a longer period of time. Hormones are made by specialized glands or tissues that manufacture and secrete these chemicals as the body needs them. The majority of hormones are produced by the glands of the endocrine system , such as the pituitary , thyroid , adrenal glands , and the ovaries or testes . These endocrine glands produce and secrete hormones directly into the bloodstream. However, not all hormones are...

Steroids, large group of naturally occurring and synthetic lipids , or fat-soluble chemicals, with a great diversity of physiological activity. Included among the steroids are certain alcohols (sterols), bile acids, many important hormones, some natural drugs, and the poisons found in the skin of some toads (see Digitalis ; Hormone ). Various sterols found in the skin of human beings are transformed into vitamin D when they are exposed to the ultraviolet rays of the sun (see Vitamin : Vitamin D ). Cholesterol , a major contributor to arteriosclerosis, is a sterol. Steroid hormones, which are similar to but not identical with sterols, include the adrenal cortical steroids hydrocortisone , cortisone, aldosterone, and progesterone; and the female and male sex hormones (see Estrogen ; Testosterone ). Most oral contraceptives are synthetic steroids consisting of female sex hormones that inhibit ovulation (see Birth Control ). Perhaps the most widely used steroids in medicine are cortisone ...

Unlike the epidermis, the dermis or lower layer of the skin is richly supplied with blood vessels and sensory nerve endings. The dermis also contains relatively few cells compared to the epidermis—instead, it is made up mainly of fibrous proteins and other large molecules. The main structural component of the dermis is a protein called collagen. Bundles of collagen molecules pack together throughout the dermis, accounting for three-fourths of the dry weight of skin. Collagen is also responsible for the skin’s strength. Another protein in the dermis, elastin, is the main component of elastic fibers. These protein bundles give skin its elasticity—the ability to return to its original shape after stretching. Collagen and elastin are produced by cells called fibroblasts, which are found scattered throughout the dermis. The upper part of the dermis is known as the papillary layer. It is characterized by dermal papillae, tiny, fingerlike projections of tissue that indent into the epidermis a...

About 90 percent of the cells in the epidermis are keratinocytes, named because they produce a tough, fibrous protein called keratin. This protein is the main structural protein of the epidermis, and it provides many of the skin’s protective properties. Keratinocytes in the epidermis are arranged in layers, with the youngest cells in the lower layers and the oldest cells in the upper layers. The old keratinocytes at the surface of the skin constantly slough off. Meanwhile, cells in the lower layers of the epidermis divide continually, producing new keratinocytes to replace those that have sloughed off. As keratinocytes push up through the layers of the epidermis, they age and, in the process, produce keratin. By the time the cells reach the uppermost layer of the epidermis, they are dead and completely filled with the tough protein. Healthy epidermis replaces itself in a neatly orchestrated way every month. Scattered among the keratinocytes in the epidermis are melanocytes, cells that ...

Hair, collective term for slender, threadlike outgrowths of the epidermis of mammals, forming a characteristic body covering. No animals other than mammals have true hair, and all mammals have hair. Even such apparently hairless mammals as the rhinoceros, elephant, and armadillo have hairs around the snout, at the tip of the tail, and behind each scale, respectively. (Whales and manatees have hair only in the embryonic state.) When the individual hairs are fine and closely spaced, the coat of hair is called fur; when soft, kinked, and matted together, the coat is called wool. Coarse, stiff hairs are called bristles. When bristles are also pointed, as in the hedgehog and porcupine, they are called spines or quills. In humans the development of the hair begins in the embryo, and by the sixth month the fetus is covered by a growth of fine hair, the lanugo. In the first few months of infancy the lanugo is shed and is replaced by hair, relatively coarse over the cranium and the eyebrows, bu...

Skin, outer body covering of an animal. The term skin is commonly used to describe the body covering of any animal but technically refers only to the body covering of vertebrates (animals that have a backbone). The skin has the same basic structure in all vertebrates, including fish, reptiles, birds, and humans and other mammals. This article focuses primarily on human skin. The skin is made up of two layers, the epidermis and the dermis . The epidermis, the upper or outer layer of the skin, is a tough, waterproof, protective layer. The dermis, or inner layer, is thicker than the epidermis and gives the skin its strength and elasticity. The two layers of the skin are anchored to one another by a thin but complex layer of tissue, known as the basement membrane. This tissue is composed of a series of elaborately interconnecting molecules that act as ropes and grappling hooks to hold the skin together. Below the dermis is the subcutaneous layer, a layer of tissue composed of protein fib...

Reproduction, process whereby all living organisms produce offspring. Reproduction is one of the essential functions of plants, animals, and single celled organisms, as necessary for the preservation of the species as eating is for the preservation of the individual. In almost all animal organisms, reproduction occurs during or after the period of maximum growth. In plants, which continue to grow throughout their lifetime, the relationship between growth and reproduction is more complex. Individual plants have growth limitations imposed by inherited characteristics and environmental conditions; if the plant grows excessively, any of a number of reproductive processes may be stimulated (see Plant Propagation ). Environmental conditions also play some part in the reproduction of higher animals, but hormonal elements are more important.

Human Sexuality, general term referring to various sexually related aspects of human life, including physical and psychological development, and behaviors, attitudes, and social customs associated with the individual's sense of gender, relationships, sexual activity, mate selection, and reproduction. Sexuality permeates many areas of human life and culture, thereby setting humans apart from other members of the animal kingdom, in which the objective of sexuality is more often confined to reproduction. This article discusses the sexual anatomy, development, physiology, and behavior of human beings. Topics: Human Sexual Characteristics Sexual Development Physiology of Sex Sexual Risks Sexual Dysfunctions

Estrogen is any of a group of female sex hormones that stimulate the appearance of secondary female sex characteristics in girls at puberty . Estrogens control growth of the lining of the uterus during the first part of the menstrual cycle, cause changes in the breast during pregnancy, and regulate various metabolic processes. Among the better known estrogens are estrone, ethinyl estradiol, and estriol, all produced primarily in the ovaries. Stilbestrol and ethinyl estradiol, two synthetic estrogens, are respectively five and ten times as potent as estrone; their activity is similar to that of natural estrogens. They are used to treat various conditions, including estrogen deficiencies in women (most commonly after menopause ) and inflammation of the vagina. They may be used to stimulate lactation following childbirth and in the treatment, but not cure, of advanced and even disseminated cancer of the prostate gland in men.

Menstruation, periodic vaginal discharge in humans and other mammals, consisting of blood and cells shed from the endometrium, or lining of the uterus (see Reproductive System ). Menstruation accompanies a woman's childbearing years, usually beginning between the ages of 10 and 16, at puberty , and most often ceasing between the ages of 45 and 50, at menopause . Menstruation is part of the process that prepares a woman for pregnancy. Each month the lining of the uterus thickens; if pregnancy does not occur, this lining breaks down and is discharged through the vagina. The three to seven days that menstruation lasts is called the menstrual period. In most women the menstrual cycle is about 28 days, but it can vary considerably even from one month to another. The cycle is initiated by hormones in the blood that stimulate the ovaries (the two female organs that produce ova, or eggs). Each month, hormones cause an egg in one of the two ovaries to mature (to become capable of being fert...

. Adult humans typically have 32 teeth—16 in the upper jaw and 16 in the lower jaw—that fit together and work in concert to chew food. Teeth on the right side of each jaw are usually identical to the teeth on the left side and matching teeth on opposite sides are referred to as sets, or pairs. Humans are heterodonts—that is, they have teeth of different sizes and shapes that serve different functions, such as tearing and grinding. In contrast, the homodont teeth found in many animals are all the same size and shape, and perform the same function. Humans have four types of teeth, each with a specific size, shape, and function. Adult humans have eight incisors, located at the front of the mouth—four in the upper jaw and four in the lower jaw. Incisors have a sharp edge that is used to cut food. On either side of the incisors are the canines, named for their resemblance to the pointy fangs of dogs. The upper canines are sometimes called eyeteeth. There are two canines in each jaw, and the...

. Teeth, hard, bony structures in the mouths of humans and animals used primarily to chew food, but also for gnawing, digging, fighting, and catching and killing prey. Teeth are the body’s hardest, most durable organ—long after bones and flesh have dissolved, archaeologists find well-preserved teeth from humans and other animals that lived thousands of years ago. Structure of a Tooth The sensitive nerves and blood vessels at the center of each tooth are protected by several layers of tissue, the outermost (the enamel) being the hardest substance in the body. Under the enamel, surrounding the pulp from crown to root, lies a layer of bonelike dentin. A hard tissue called cementum separates the root from the periodontal ligament, which in turn holds the root in place and cushions the tooth against the gum and jaw during the grinding, jarring activity of chewing. Human teeth are made of four distinct types of tissue: enamel, dentin, pulp, and cementum. Enamel, the clear outer layer of the ...

. Salivary Glands, glands that secrete saliva, a somewhat alkaline fluid that moistens the mouth, softens food, and aids in digestion. The submaxillary glands are located around the mouth under the lower jaw, the sublingual glands are located beneath the tongue, and the parotid glands are found in front of each ear. The buccal glands, in the cheeks near the front of the mouth, also secrete saliva. The saliva of the parotid gland contains enzymes called amylases, one of which, known as ptyalin, aids in the digestion of carbohydrates. In human beings the salivary glands, especially the parotid, are affected by the disease called mumps , a viral disease most common among children, although adults can also be infected.

. Palate, roof of the mouth, separating the mouth from the nasal cavities. The palate consists of two portions: the hard palate in front and the soft palate behind. The hard palate is formed of periosteum, a bony plate covered by mucous membrane, and arches over to meet the gums in front and on either side. The soft palate is a movable fold of mucous membrane enclosing muscular fibers. Its sides blend with the pharynx (throat), but its lower border is free. It is suspended from the rear of the hard palate so as to form a wall or division between the mouth and the pharynx. During swallowing, this wall is raised to close the entrance to the nasal passages. A small cone-shaped structure, the uvula, hangs from the lower border of the soft palate. The condition called cleft palate is a birth defect that results from incomplete development of the palate. It is characterized by a hole or gap in the palate that may extend from behind the teeth to the nasal cavity.

. 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. 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. Inside the mouth is the large, muscular tongue . This extremely flexible muscle is used for eating and ...

. Epiglottis, thin, lidlike flap of cartilage attached to the base of the tongue of terrestrial vertebrates. The epiglottis is normally pointed upward, but during the passage of solids and liquids from the mouth into the esophagus, the epiglottis is folded down over the glottis, the opening between the vocal cords, to prevent food from passing into the trachea.

Intestine, also bowels, in higher animals, the portion of the digestive tract between the stomach and anus. In humans the intestine is divided into two major sections: the small intestine, which is about 6 m (20 ft) long, where the most extensive part of digestion occurs and where most food products are absorbed; and the large intestine, which has a larger diameter and is about 1.5 m (5 ft) long, where water is absorbed and from which solid waste material is excreted (see Digestive System ; Feces ). The small intestine, which is coiled in the center of the abdominal cavity (see Abdomen ), is divided into three sections. The upper portion includes the pylorus, the opening at the lower part of the stomach, through which the contents of the stomach pass into the duodenum. The duodenum is a horseshoe-shaped section surrounding part of the pancreas and the pancreatic duct, as well as ducts from the liver and gall bladder that open into it. The middle part of the small intestine, extending...

. Liver, largest internal organ of the human body. The liver, which is part of the digestive system , performs more than 500 different functions, all of which are essential to life. Its essential functions include helping the body to digest fats, storing reserves of nutrients, filtering poisons and wastes from the blood, synthesizing a variety of proteins, and regulating the levels of many chemicals found in the bloodstream. The liver is unique among the body’s vital organs in that it can regenerate, or grow back, cells that have been destroyed by some short-term injury or disease. But if the liver is damaged repeatedly over a long period of time, it may undergo irreversible changes that permanently interfere with function. The human liver is a dark red-brown organ with a soft, spongy texture. It is located at the top of the abdomen, on the right side of the body just below the diaphragm—a sheet of muscle tissue that separates the lungs from the abdominal organs. The lower part of the ...

Genetics, study of the function and behavior of genes . Genes are bits of biochemical instructions found inside the cells of every organism from bacteria to humans. Offspring receive a mixture of genetic information from both parents. This process contributes to the great variation of traits that we see in nature, such as the color of a flower’s petals, the markings on a butterfly’s wings, or such human behavioral traits as personality or musical talent. Geneticists seek to understand how the information encoded in genes is used and controlled by cells and how it is transmitted from one generation to the next. Geneticists also study how tiny variations in genes can disrupt an organism’s development or cause disease. Increasingly, modern genetics involves genetic engineering, a technique used by scientists to manipulate genes. Genetic engineering has produced many advances in medicine and industry, but the potential for abuse of this technique has also presented society with many ethica...

Sex is a physical and behavioral difference that distinguishes individual organisms according to their functions in the reproductive process. For information on issues of sexual health, sexual behavior, and sexual activity, see Human Sexuality . Sex occurs at all levels of biological organization, with the exception of viruses. At the lowest level, bacteria conjugate and a length of the single chromosome is passed from the male, or donor cell, to the female, or recipient cell. At more advanced levels, multicellular individuals have specialized organs (gonads) that produce specialized sex cells (gametes). Upon fertilization, genetic information is transferred from the small, motile spermatozoa (male gametes) to the much larger ova (female gametes). Many organisms, including most plants, many protozoans and invertebrates, and some fishes, have both male and female gonads and are called hermaphroditic (see Hermaphroditism ). Hermaphroditic organisms, however, are rarely self-fertilizing; ...