Human Nutrition

Human Nutrition, study of how food affects the health and survival of the human body. Human beings require food to grow, reproduce, and maintain good health. Without food, our bodies could not stay warm, build or repair tissue, or maintain a heartbeat. Eating the right foods can help us avoid certain diseases or recover faster when illness occurs. These and other important functions are fueled by chemical substances in our food called nutrients. Nutrients are classified as carbohydrates, proteins, fats, vitamins, minerals, and water.When we eat a meal, nutrients are released from food through digestion. Digestion begins in the mouth by the action of chewing and the chemical activity of saliva, a watery fluid that contains enzymes, certain proteins that help break down food. Further digestion occurs as food travels through the stomach and the small intestine, where digestive enzymes and acids liquefy food and muscle contractions push it along the digestive tract. Nutrients are absorbed from the inside of the small intestine into the bloodstream and carried to the sites in the body where they are needed. At these sites, several chemical reactions occur that ensure the growth and function of body tissues. The parts of foods that are not absorbed continue to move down the intestinal tract and are eliminated from the body as feces.Once digested, carbohydrates, proteins, and fats provide the body with the energy it needs to maintain its many functions. Scientists measure this energy in kilocalories, the amount of energy needed to raise 1 kilogram of water 1 degree Celsius. In nutrition discussions, scientists use the term calorie instead of kilocalorie as the standard unit of measure in nutrition.
ESSENTIAL NUTRIENTS

Nutrients are classified as essential or nonessential. Nonessential nutrients are manufactured in the body and do not need to be obtained from food. Examples include cholesterol, a fatlike substance present in all animal cells. Essential nutrients must be obtained from food sources, because the body either does not produce them or produces them in amounts too small to maintain growth and health. Essential nutrients include water, carbohydrates, proteins, fats, vitamins, and minerals.

An individual needs varying amounts of each essential nutrient, depending upon such factors as gender and age. Specific health conditions, such as pregnancy, breast-feeding, illness, or drug use, make unusual demands on the body and increase its need for nutrients. Dietary guidelines, which take many of these factors into account, provide general guidance in meeting daily nutritional needs.

WATER

If the importance of a nutrient is judged by how long we can do without it, water ranks as the most important. A person can survive only eight to ten days without water, whereas it takes weeks or even months to die from a lack of food. Water circulates through our blood and lymphatic system, transporting oxygen and nutrients to cells and removing wastes through urine and sweat. Water also maintains the natural balance between dissolved salts and water inside and outside of cells. Our joints and soft tissues depend on the cushioning that water provides for them. While water has no caloric value and therefore is not an energy source, without it in our diets we could not digest or absorb the foods we eat or eliminate the body’s digestive waste.

The human body is 65 percent water, and it takes an average of eight to ten cups to replenish the water our bodies lose each day. How much water a person needs depends largely on the volume of urine and sweat lost daily, and water needs are increased if a person suffers from diarrhea or vomiting or undergoes heavy physical exercise. Water is replenished by drinking liquids, preferably those without caffeine or alcohol, both of which increase the output of urine and thus dehydrate the body. Many foods are also a good source of water—fruits and vegetables, for instance, are 80 to 95 percent water; meats are made up of 50 percent water; and grains, such as oats and rice, can have as much as 35 percent water.

Carbohydrates

Carbohydrates are the human body’s key source of energy, providing 4 calories of energy per gram. When carbohydrates are broken down by the body, the sugar glucose is produced; glucose is critical to help maintain tissue protein, metabolize fat, and fuel the central nervous system.

Glucose is absorbed into the bloodstream through the intestinal wall. Some of this glucose goes straight to work in our brain cells and red blood cells, while the rest makes its way to the liver and muscles, where it is stored as glycogen (animal starch), and to fat cells, where it is stored as fat. Glycogen is the body’s auxiliary energy source, tapped and converted back into glucose when we need more energy. Although stored fat can also serve as a backup source of energy, it is never converted into glucose. Fructose and galactose, other sugar products resulting from the breakdown of carbohydrates, go straight to the liver, where they are converted into glucose.

Starches and sugars are the major carbohydrates. Common starch foods include whole-grain breads and cereals, pasta, corn, beans, peas, and potatoes. Naturally occurring sugars are found in fruits and many vegetables; milk products; and honey, maple sugar, and sugar cane. Foods that contain starches and naturally occurring sugars are referred to as complex carbohydrates, because their molecular complexity requires our bodies to break them down into a simpler form to obtain the much-needed fuel, glucose. Our bodies digest and absorb complex carbohydrates at a rate that helps maintain the healthful levels of glucose already in the blood.

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Common Chemistry Equations

Density:

where m is the mass of a substance and V is its volume

Charles’s Law:

where V is volume, k is Boltzmann’s constant, and T is the temperature in Kelvin

Ideal Gas Equation:

where P is pressure, V is volume, n is the number of moles of gas, R is the gas constant, and T is the temperature in Kelvin

Molarity:

where n is the number of moles of solute and V is the volume of solution in liters

Molarity equation:

where M₁ is the initial molarity of a solution, V₁ is its initial volume, M₂ is its diluted molarity, and V₂ is its diluted volume

Molality:

where n is the number of moles of solute and m_s is the mass of solvent in kilograms

pH:

where H⁺ is the concentration of hydronium ions (H₃O⁺) in the solution

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CAT

Cat, small, mainly carnivorous animal, Felis silvestris catus, member of the family Felidae, popular as a household pet, and valuable for killing mice and rats. Like other members of the cat family, the domestic cat has retractile claws; keen hearing and smell; remarkable night vision; and a compact, muscular, and highly supple body. Cats possess excellent memory and exhibit considerable aptitude for learning by observation and experience. The natural life span of a domestic cat is about 15 years. There are an estimated 600 million house cats in the world.
ORIGIN OF SPECIES
Debate has surrounded the origin of the domestic cat. A common theory held that cats were first domesticated by ancient Egyptians perhaps as early as 2500 bc from the African or Near Eastern wildcat Felis silvestris libyca, also called the Caffre cat. Crusaders then transported the cat to Europe, where it interbred with the indigenous smaller wildcats Felis silvestris silvestris. The idea that domestic cats in different parts of the world had originated from, or interbred with, populations of local wildcats and other small cat species was proposed by a number of experts. For example, the longhaired breeds of domestic cats were said to come from the Asian Pallas’s cat, Felis manul.

However, a study published in 2007 compared the mitochondrial DNA of domestic cats and wildcats and concluded that the domestic cat derives only from Felis silvestris libyca. Members of this particular subspecies of wildcat were domesticated in the Middle East, likely around the time that farming villages first developed in the Fertile Crescent region between 10,000 and 12,000 years ago. Wildcats probably began associating with human settlements to prey on the rodents and other pests attracted by stored grains and cereals. Some of the wildcats then gave up their more aggressive wild behaviors to adapt to life with people.

The DNA study indicates that at least five individual female cats from the Middle East served as founders for all the domestic cats that were later carried around the world by humans. This new DNA evidence appears to contradict theories that domestic cats carry genes that come from other types of small cats and from wildcats found in different parts of the world. Some interbreeding between domestic cats and local wildcats probably took place, however. Over the centuries, cats have remained virtually the same in size, weighing about 3.6 kg (about 8 lb) when full-grown, and have preserved their instinct for solitary hunting.

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Heliophysics

       We live in the extended atmosphere of an active star. While sunlight enables and sustains life, the Sun's variability produces streams of high energy particles and radiation that can harm life or alter its evolution.
Under the protective shield of a magnetic field and atmosphere, the Earth is an island in the Universe where life has developed and flourished. The origins and fate of life on Earth are intimately connected to the way the Earth responds to the Sun's variations.
Understanding the Sun, Heliosphere, and Planetary Environments as a single connected system is the goal of the Science Mission Directorate's Heliophysics Research Program. In addition to solar processes, our domain of study includes the interaction of solar plasma and radiation with Earth, the other planets, and the Galaxy. By analyzing the connections between the Sun, solar wind, planetary space environments, and our place in the Galaxy, we are uncovering the fundamental physical processes that occur throughout the Universe. Understanding the connections between the Sun and its planets will allow us to predict the impacts of solar variability on humans, technological systems, and even the presence of life itself.
We have already discovered ways to peer into the internal workings of the Sun and understand how the Earth's magnetosphere responds to solar activity. Our challenge now is to explore the full system of complex interactions that characterize the relationship of the Sun with the solar system. Understanding these connections is especially critical as we contemplate our destiny in the third millennium. Heliophysics is needed to facilitate the accelerated expansion of human experience beyond the confines of our Earthly home. Recent advances in technology allow us, for the first time, to realistically contemplate voyages beyond the solar system.

There are three primary objectives that define the multi-decadal studies needed:

• To understand the changing flow of energy and matter throughout the Sun, Heliosphere, and Planetary Environments.
• To explore the fundamental physical processes of space plasma systems.
• To define the origins and societal impacts of variability in the Earth-Sun System.

A combination of interrelated elements is used to achieve these objectives. They include complementary missions of various sizes; timely development of enabling and enhancing technologies; and acquisition of knowledge through research, analysis, theory, and modeling.
sorces    

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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. The muscle rests only for a fraction of a second between beats. Over a typical life span of 76 years, the heart will beat nearly 2.8 billion times and move 169 million liters (179 million quarts) of blood.

Since prehistoric times people have had a sense of the heart’s vital importance. Cave paintings from 20,000 years ago depict a stylized heart inside the outline of hunted animals such as bison and elephant. The ancient Greeks believed the heart was the seat of intelligence. Others believed the heart to be the source of the soul or of the emotions—an idea that persists in popular culture and various verbal expressions, such as heartbreak, to the present day.

STRUCTURE OF THE HEART

The human heart has four chambers. The upper two chambers, the right and left atria, are receiving chambers for blood. The atria are sometimes known as auricles. They collect blood that pours in from veins, blood vessels that return blood to the heart. The heart’s lower two chambers, the right and left ventricles, are the powerful pumping chambers. The ventricles propel blood into arteries, blood vessels that carry blood away from the heart.

A wall of tissue separates the right and left sides of the heart. Each side pumps blood through a different circuit of blood vessels: The right side of the heart pumps oxygen-poor blood to the lungs, while the left side of the heart pumps oxygen-rich blood to the body. Blood returning from a trip around the body has given up most of its oxygen and picked up carbon dioxide in the body’s tissues. This oxygen-poor blood feeds into two large veins, the superior vena cava and inferior vena cava, which empty into the right atrium of the heart.The right atrium conducts blood to the right ventricle, and the right ventricle pumps blood into the pulmonary artery. The pulmonary artery carries the blood to the lungs, where it picks up a fresh supply of oxygen and eliminates carbon dioxide. The blood, now oxygen-rich, returns to the heart through the pulmonary veins, which empty into the left atrium. Blood passes from the left atrium into the left ventricle, from where it is pumped out of the heart into the aorta, the body’s largest artery. Smaller arteries that branch off the aorta distribute blood to various parts of the body.
Heart Valves
Four valves within the heart prevent blood from flowing backward in the heart. The valves open easily in the direction of blood flow, but when blood pushes against the valves in the opposite direction, the valves close. Two valves, known as atrioventricular valves, are located between the atria and ventricles. The right atrioventricular valve is formed from three flaps of tissue and is called the tricuspid valve. The left atrioventricular valve has two flaps and is called the bicuspid or mitral valve. The other two heart valves are located between the ventricles and arteries. They are called semilunar valves because they each consist of three half-moon-shaped flaps of tissue. The right semilunar valve, between the right ventricle and pulmonary artery, is also called the pulmonary valve. The left semilunar valve, between the left ventricle and aorta, is also called the aortic valve.

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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. The muscle rests only for a fraction of a second between beats. Over a typical life span of 76 years, the heart will beat nearly 2.8 billion times and move 169 million liters (179 million quarts) of blood.

Since prehistoric times people have had a sense of the heart’s vital importance. Cave paintings from 20,000 years ago depict a stylized heart inside the outline of hunted animals such as bison and elephant. The ancient Greeks believed the heart was the seat of intelligence. Others believed the heart to be the source of the soul or of the emotions—an idea that persists in popular culture and various verbal expressions, such as heartbreak, to the present day.

STRUCTURE OF THE HEART

The human heart has four chambers. The upper two chambers, the right and left atria, are receiving chambers for blood. The atria are sometimes known as auricles. They collect blood that pours in from veins, blood vessels that return blood to the heart. The heart’s lower two chambers, the right and left ventricles, are the powerful pumping chambers. The ventricles propel blood into arteries, blood vessels that carry blood away from the heart.

A wall of tissue separates the right and left sides of the heart. Each side pumps blood through a different circuit of blood vessels: The right side of the heart pumps oxygen-poor blood to the lungs, while the left side of the heart pumps oxygen-rich blood to the body. Blood returning from a trip around the body has given up most of its oxygen and picked up carbon dioxide in the body’s tissues. This oxygen-poor blood feeds into two large veins, the superior vena cava and inferior vena cava, which empty into the right atrium of the heart.The right atrium conducts blood to the right ventricle, and the right ventricle pumps blood into the pulmonary artery. The pulmonary artery carries the blood to the lungs, where it picks up a fresh supply of oxygen and eliminates carbon dioxide. The blood, now oxygen-rich, returns to the heart through the pulmonary veins, which empty into the left atrium. Blood passes from the left atrium into the left ventricle, from where it is pumped out of the heart into the aorta, the body’s largest artery. Smaller arteries that branch off the aorta distribute blood to various parts of the body.
Heart Valves
Four valves within the heart prevent blood from flowing backward in the heart. The valves open easily in the direction of blood flow, but when blood pushes against the valves in the opposite direction, the valves close. Two valves, known as atrioventricular valves, are located between the atria and ventricles. The right atrioventricular valve is formed from three flaps of tissue and is called the tricuspid valve. The left atrioventricular valve has two flaps and is called the bicuspid or mitral valve. The other two heart valves are located between the ventricles and arteries. They are called semilunar valves because they each consist of three half-moon-shaped flaps of tissue. The right semilunar valve, between the right ventricle and pulmonary artery, is also called the pulmonary valve. The left semilunar valve, between the left ventricle and aorta, is also called the aortic valve.

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nervous system

The nervous system has two divisions: the central nervous system and the peripheral nervous system. The central nervous system includes the brain and spinal cord. It processes incoming sensory information and sends outgoing motor commands. The peripheral nervous system includes all neural tissue outside the central nervous system. It is divided into motor and sensory systems. Impulses go to the central nervous system through sensory nerves and are carried away from it by the motor nerves. The motor system is further divided into the somatic (or skeletal) nervous system and the autonomic nervous system. The somatic, or skeletal, motor system allows voluntary control over skeletal muscle with a few exceptions. The autonomic nervous system is largely involuntary and controls cardiac and smooth muscles and glands.The autonomic nervous system has three divisions: the enteric, the sympathetic, and the parasympathetic. The enteric nervous system is a system of nerves in the gastrointestinal tract, pancreas, and gallbladder that influences all digestive processes. The enteric system operates without input from the brain or spinal cord.
The sympathetic and parasympathetic divisions may operate together or in opposition. Many, but not all, of the muscles and glands that distribute nerve impulses to the larger interior organs have both sympathetic and parasympathetic nerve systems. 
In such cases the two divisions may exert opposing effects. Thus, the sympathetic system increases heartbeat, and the parasympathetic system decreases heartbeat. The two nervous systems are not always in opposition, however. For example, both nerve supplies to the salivary glands excite the cells of secretion. Furthermore, a single division of the autonomic nervous system may both stimulate and inhibit, as in the sympathetic supply to the blood vessels of skeletal muscle. Finally, the sweat glands, the muscles that cause involuntary erection or bristling of the hair, the smooth muscle of the spleen, and the blood vessels of the skin and skeletal muscle are actuated only by the sympathetic division.

Voluntary movement of head, limbs, and body is caused by nerve impulses arising in the motor area of the cortex of the brain and carried by cranial nerves or by nerves that emerge from the spinal cord to connect with skeletal muscles. The reaction involves both excitation of nerve cells stimulating the muscles involved and inhibition of the cells that stimulate opposing muscles. A nerve impulse is an electrical change within a nerve cell or fiber; measured in millivolts, it lasts a few milliseconds and can be recorded by electrodes.
Movement may occur also in direct response to an outside stimulus; thus, a tap on the knee causes a jerk, and a light shone into the eye makes the pupil contract. These involuntary responses are called reflexes. Various nerve terminals called receptors constantly send impulses into the central nervous system. These are of three classes: exteroceptors, which are sensitive to pain, temperature, touch, and pressure; interoceptors, which react to changes in the internal environment; and proprioceptors, which respond to variations in movement, position, and tension. These impulses terminate in special areas of the brain, as do those of special receptors concerned with sight, hearing, smell, and taste.
Muscular contractions do not always cause actual movement. A small fraction of the total number of fibers in most muscles are usually contracting. This serves to maintain the posture of a limb and enables the limb to resist passive elongation or stretch. This slight continuous contraction is called muscle tone.

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HUMAN EMBRYOLOGY

The human ovum, fertilized high in a fallopian tube, is brushed by the hairlike cilia in the tube toward the uterus, where it becomes implanted, that is, attached to and enclosed by decidual tissue of the uterine lining. Studies of primate embryos indicate that, in humans as well as in apes, cell multiplication begins during the journey of the ovum through the tube. The implanted embryo consists of a hollow sphere, the blastocyst, containing a mass of cells, called the embryonic mass, attached by a stalk to one side of the encircling membrane. In a blastocyst less than two weeks old and measuring 1 mm (0.04 in) in diameter, the microscope reveals the amnion (a sac surrounding the embryo), chorion (a membrane that develops around the amnion and lines the uterine wall), yolk sac, and distinct germ layers.

In the third week a closed tube appears in which the brain and spinal cord are to develop. Another tube, folding on itself, is developing into the heart, and at about this stage a portion of the minute yolk sac is enclosed in the body of the embryo to form a part of the embryonic alimentary canal. At the beginning of its fourth week the embryo, now about 4 to 5 mm (about 0.16 to 0.2 in) long, has the rudiments of eyes and ears, and each side of the neck shows four gill clefts. A tail is also present.

Early in the second month the buds of the arms and legs appear. The major internal organs begin to take shape, and in about the sixth week bones and muscles begin to form. By the third month the embryo is recognizable as that of a primate, and is now called a fetus. It has a definite face, with the mouth and nostrils distinct, and the external ears are forming. By the end of the eighth week the tail has usually been incorporated in the body, and in the 11th or 12th week the external genitals become evident. The human embryo is especially vulnerable to the damaging effects of X rays, of disease viruses such as measles, and of certain drugs during the fourth to the eighth week of gestation. These agents can result in the death of the embryo or in the birth of a child with deformed limbs or other abnormalities. By the fourth month an embryo has developed obvious human features. For development in the fetal stage, see Fetus. For abnormalities due to anomalous development, see Birth Defects. See alsoDevelopment; Multiple Birth; Obstetrics.

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HUMAN EMBRYOLOGY

The human ovum, fertilized high in a fallopian tube, is brushed by the hairlike cilia in the tube toward the uterus, where it becomes implanted, that is, attached to and enclosed by decidual tissue of the uterine lining. Studies of primate embryos indicate that, in humans as well as in apes, cell multiplication begins during the journey of the ovum through the tube. The implanted embryo consists of a hollow sphere, the blastocyst, containing a mass of cells, called the embryonic mass, attached by a stalk to one side of the encircling membrane. In a blastocyst less than two weeks old and measuring 1 mm (0.04 in) in diameter, the microscope reveals the amnion (a sac surrounding the embryo), chorion (a membrane that develops around the amnion and lines the uterine wall), yolk sac, and distinct germ layers.

In the third week a closed tube appears in which the brain and spinal cord are to develop. Another tube, folding on itself, is developing into the heart, and at about this stage a portion of the minute yolk sac is enclosed in the body of the embryo to form a part of the embryonic alimentary canal. At the beginning of its fourth week the embryo, now about 4 to 5 mm (about 0.16 to 0.2 in) long, has the rudiments of eyes and ears, and each side of the neck shows four gill clefts. A tail is also present.

Early in the second month the buds of the arms and legs appear. The major internal organs begin to take shape, and in about the sixth week bones and muscles begin to form. By the third month the embryo is recognizable as that of a primate, and is now called a fetus. It has a definite face, with the mouth and nostrils distinct, and the external ears are forming. By the end of the eighth week the tail has usually been incorporated in the body, and in the 11th or 12th week the external genitals become evident. The human embryo is especially vulnerable to the damaging effects of X rays, of disease viruses such as measles, and of certain drugs during the fourth to the eighth week of gestation. These agents can result in the death of the embryo or in the birth of a child with deformed limbs or other abnormalities. By the fourth month an embryo has developed obvious human features. For development in the fetal stage, see Fetus. For abnormalities due to anomalous development, see Birth Defects. See alsoDevelopment; Multiple Birth; Obstetrics.

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HUMAN EMBRYOLOGY

The human ovum, fertilized high in a fallopian tube, is brushed by the hairlike cilia in the tube toward the uterus, where it becomes implanted, that is, attached to and enclosed by decidual tissue of the uterine lining. Studies of primate embryos indicate that, in humans as well as in apes, cell multiplication begins during the journey of the ovum through the tube. The implanted embryo consists of a hollow sphere, the blastocyst, containing a mass of cells, called the embryonic mass, attached by a stalk to one side of the encircling membrane. In a blastocyst less than two weeks old and measuring 1 mm (0.04 in) in diameter, the microscope reveals the amnion (a sac surrounding the embryo), chorion (a membrane that develops around the amnion and lines the uterine wall), yolk sac, and distinct germ layers.

	Developing Embryo's First Month

	Developing Embryo's First Month Thirty hours after conception, the fertilized egg undergoes its first cell division. The embryo, as it is now called, continues to divide as it travels down the fallopian tube. It implants in the uterine lining approximately six days after fertilization, a ball of cells with a disk-shaped embryonic mass. In the second week, the placenta begins to form, nourishing an embryo now composed of the three primary types of tissue: endoderm, ectoderm, and mesoderm. The third week sees the formation of the neural tube, precursor to the central nervous system. Blocks of muscle tissue called somites, from which major organs and glands will arise, form along the embryo’s dorsal surface. Blood vessels and the beginnings of the digestive cavity appear by the end of the week. At the close of the first month, all major organs have begun their development. The eyes are visible, the arms and legs begin to bud, and the four-chambered heart beats for the first time. Encarta Encyclopedia © Microsoft Corporation. All Rights Reserved.
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In the third week a closed tube appears in which the brain and spinal cord are to develop. Another tube, folding on itself, is developing into the heart, and at about this stage a portion of the minute yolk sac is enclosed in the body of the embryo to form a part of the embryonic alimentary canal. At the beginning of its fourth week the embryo, now about 4 to 5 mm (about 0.16 to 0.2 in) long, has the rudiments of eyes and ears, and each side of the neck shows four gill clefts. A tail is also present.

Early in the second month the buds of the arms and legs appear. The major internal organs begin to take shape, and in about the sixth week bones and muscles begin to form. By the third month the embryo is recognizable as that of a primate, and is now called a fetus. It has a definite face, with the mouth and nostrils distinct, and the external ears are forming. By the end of the eighth week the tail has usually been incorporated in the body, and in the 11th or 12th week the external genitals become evident. The human embryo is especially vulnerable to the damaging effects of X rays, of disease viruses such as measles, and of certain drugs during the fourth to the eighth week of gestation. These agents can result in the death of the embryo or in the birth of a child with deformed limbs or other abnormalities. By the fourth month an embryo has developed obvious human features. For development in the fetal stage, see Fetus. For abnormalities due to anomalous development, see Birth Defects. See also Development; Multiple Birth; Obstetrics.

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