АНГЛЗАЧЕТ. Reveal striated поперечнополосатый
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3. The human digestive tract is a long, muscular tube (up to 25 feet in length) that begins at the mouth and ends at the anus. This tube consists of the oral cavity, pharynx, esophagus, small intestine, and large intestine. 4. Several glands, located outside the digestive tract, are also important in the digestive process. Our tast is to describe them in detail. These glands, known as accessory glands, are connected by ducts to the digestive tube. These accesory glands include the salivary glands,liver, gall-bladder and the pancreas. Each gland produces secretions that function in the digestive process, and each is therefore part of the digestive system. 5. The process of digestion is due to the activity of many enzymes, chemicals, and physical processes within the digestive tract. According to the area in which digestion is carried on, these digestive processes may be classified as salivary digestion, when occurring in the mouth; gastric digestion in the stomach; and intestinal digestion in the small intestine. In the large intestine (the last section of the digestive tube) no digestion takes place. Here water is absorbed, bacteria grow, and the unabsorbed solid-residue wastes of digestion collect and are excreted as feces. 6. The absorption means the passage of digested foods through the lining of the intestines into the blood or lymph. Practically all absorption takes place in the small intestine. A few drugs and alcohol are absorbed through the walls of the stomach, but no foods. Glucose is an exception, but it must be present in such high concentrations as to cause vomiting. Furthermore, we eat very little glucose, which is formed mainly in the small intestine due to the action of the disaccharide-splitting enzymes. Therefore, absorption of food does not normally occur through the stomach walls. 7. Water is absorbed throughout the length of the small intestine and also, as has been noted, in the ascending limb of the colon. With normal digestion, between 95 and 100 per cent of all carbohydrates, fats, and animal proteins are absorbed. Plant proteins, such as beans or peas, are protected by the plant cell membrane, so that only 60 to 70 per cent are absorbed. The remaining 30 to 40 per cent undergo bacterial decomposition in the intestine, which results in the formation of large amounts of intestinal gas («flatus»). To study the pathway of food through digestive tract is very important for explanation of the process of digestion. The Movements of the Stomach It is advisable to study the movements of the stomach by direct observation by means of the X-rays. In order to make the shape of the stomach visible the food - bread and milk - is mixed with a quantity of barium sulphate. The presence of this substance does not interfere with the processes of digestion, but renders the gastric contents to the Rontgen rays. In the human stomach the term fundus is limited to that part of the stomach situated above the cardiac orifice (in the erect position). The body of the stomach is marked off from the pyloric part by the incisura angularis on the lesser curvature represented in many animals by a strong «transverse band». The pyloric portion consists of the pyloric vestibule (or antrum) and the pyloric canal, the latter being a tubular portion with thick muscular walls about 3 cm in length, especially well marked in children. When food has been swallowed (in the erect position) its weight is sufficient to overcome the resistance of the contracted gastric wall and some of it rapidly passes to the pyloric part. The remainder stays in the body of the stomach. It is due to constant pressure on its contents, that is forced them towards the pylorus. Peristalsis begins almost at once, each constriction starting near the middle of the stomach, and deepening as it slowly progresses towards the pylorus. These waves succeed one another, so that the pyloric part may present a series of constrictions. Their effect is to force towards the pylorus the food which has been mixed with gastric juice. The longer the pylorus remains closed the longer the food cannot escape and therefore is squeezed back, forming an axial reflux stream towards the body. These contractions last throughout the whole period of gastric digestion, and become more marked as it proceeds. Due to their action a thorough mixture of food and gastric juice results. Movements of the stomach may be observed even on a stomach which has been excised and placed in warm water-salt solution. They must therefore have their origin in the walls of the stomach itself. Foods Foods are substances which when taken into the body yield energy on oxidation, build new tissue, repair old tissue and play an essential role in growth and nutrition. We know the oxidation of foods produces heat thus maintaining the body temperature and providing kinetic energy for work. Supplying bodily heat and energy and leaving waste materials behind the food is «burned up» in combination with the oxygen to be furnished by the air we breathe. Scientists have studied the problem of food classification for many years. Foods are to be divided into two general classes. These are inorganic and organic foods. The former class includes inorganic salts and water. The latter class includes carbohydrates, fats and proteins. There are accessory foodstuffs called vitamins which are essential to growth and freedom from deficiency diseases. The overall composition of the body is about 59 per cent water, 18 per cent protein, 18 per cent fat and 4.3 per cent minerals. At any time there is less than 1 per cent carbohydrate in the make-up of the body. These substances which make up the body are not distributed equally in all organs. For example, the percentage of water varies from 90-92 per cent in blood plasma to 72-78 per cent in muscles, 45 per cent in bone, and only 5 per cent in tooth enamel. Proteins are found most abundantly in muscles. Fat is concentrated in the adipose (fat) cells under the skin and around the intestines. Carbohydrates are found mainly in the liver, muscles and blood. Carbohydrates are known as the chief source of energy. The absence of carbohydrates upsets the fat and protein metabolism. As for the minerals, high levels of calcium and phosphorus form part of the bones and teeth, sodium and chloride are found mainly in the body fluids (blood plasma and lymph), potassium is the main mineral in mus- cles, iron is essential to red blood cells, and magnesium is found throughout the body. These are the main minerals to be supplied to the body as food but many other minerals are essential to the human body in proportionally smaller amounts. They too must be ingested with our food. Other types of food (vitamins) needed in very small amounts for various functions of the body are essential. You determine how you will feel throughout each day by the type of breakfast you eat. Your breakfast establishes how readily your body can produce energy that day or, more specifically, the amount of sugar in your blood. Your energy production, which corresponds to the quantity of sugar available, determines how you think, act and feel. Energy is produced in your body when sugar alone or sugar and fat together are burned (oxidized). It should be noted, sixty more nutrients are needed to build health. For example, cheese is an excellent source of protein but is largely lacking in carbohydrate. Black currants provide a rich source of ascorbic acid though they make little contribution to the calorie intake of the body. Milk we usually use is regarded as the most excellent food, for it contains much protein but little sugar. Therefore, it is neccessary to select a well balanced diet containing all the essential nutritional substances to maintain health and to prevent illness. text B Vitamin means life. The story of vitamins actually begins in 1911, when a Polish chemist by the name of Kazimir Punk extracted from rice polishings a crystalline substance. This substance was capable to cure beri-beri. Analyses of these crystals revealed the presence of nitrogen in basic combination, i.e. the «amino»-nitrogen; Punk therefore called this substance «vita-mine». The root «vita» indicates that the substance is essential to life and health. In this way, the word vitamin was born. For four years before Punk's discovery a series of studies had begun in the USA to determine the value of cereals such as wheat, corn and oats as a cattle diet. Eventually it was found necessary to resort to rats to solve the problem of cereal differences. Today the successful isolation and synthesis of many of the substances has proved that vitamins are organic chemical compounds to be present in the diet for the maintenance of growth and health. Vitamins are substances to be found in certain foods which are necessary for the growth, development and general health of the body. There are several different kinds of these protective substances to be provided in the diet. To make sure our bodies get all the vitamins they need, it is best to include several different vitamin-containing foods in the diet. Such foods include milk and many of the products made from it, all the green leafy vegetables like spinach, cabbage, lettuce, other fresh vegetables, fruit and fruit juices, whole-grained cereals, eggs and a number of others. When we plan a nutrition program for any person, young and old, well and ill, we must know certain foods are the best sources each body requires. 1. Vitamin A: fruits and vegetables, cream, butter or margarine, eggs and liver. 2. The В vitamins: yeast, liver, whole-grained breads and cereals, milk, meat. 3. Vitamin C: orange or grapefruit juice, any fresh raw fruit or vegetable, ascorbic acid tablets if needed. 4. Vitamin D: fish-liver oil or vitamin-D capsule. 5. Vitamin E: soy-bean oil, vegetables oils. 6. Vitamin K: is produced by intestinal bacteria. The diet must be adequate in milk and unsaturated fatty acids and low in refined carbohydrates; intestinal bacteria are increased by eating yogurt. 7. Vitamin P (rutin): citrus fruits, especially lemons. 8. Calcium: milk, yogurt. 9. Phosphorus: milk, eggs, cheese, meat. 10. Iron: liver, yeast, meat, bread and cereals. 11. Proteins: yeast, milk, yogurt, cheese, meat, fish, eggs. 12. Liquids: milk, fruit, juices, soup, water. Experts in the study of foods are constantly conducting experiments. They are making their discoveries public from time to time for such knowledge enables us to select the proper foods in order to protect us against the diseases. Nervous System. The Brain and Nerves 1. Nerves lead from the spinal cord or from the brain to each part of the body. Then they lead from each part of the body back to the brain or spinal cord. The brain and spinal cord are the centres of this system of nerves. 2. All parts of your body are connected by nerves. The nerve cells with their fibres make up the nervous system. When we study one nerve cell, we see that it has a long fibre at one end and short fibres at the other. The nerve cells send impulses to each other by means of the fibres at their ends. These fibres do not actually touch but are so close to each other that an impulse can travel from one fibre to another. Physical agents become stimuli for nerve terminals by transferring energy from the external world to the nerve terminals. 3. Thus all nerve cells connect with each other. There are millions of these connecting nerve cells. Thus a stimulus from any part of the body can reach any other part of it. In the spinal cord and brain, the nerve cells connect with each other by their connecting fibres. Outside the spinal cord and brain, certain long fibres are grouped together forming nerves. Each nerve is made up of thousands of nerve fibres together in a bundle, as a cable is made up of separate wires. The Brain Centre of the Nervous System 4. We know the nerves to carry impulses to the brain. We know that the brain sends these impulses along so that they go to the right place. The brain is made up of three parts. The cerebrum sits like a cap on the cerebellum. And the medulla is that long portion connecting the brain with the spinal cord. The cerebrum has certain parts that do certain work. Studying human beings with accidential injuries of brains helped scientists to get information about these areas. For instance they have discovered that the part for thought, memory, and feeling is found in the front of the cerebrum. The part for hearing is found at the side of the cerebrum, and the part for sight in the back of the cerebrum. 5. Many experiments have shown that the brain is the centre of feeling and understanding. The nerve cells in the brain can be «put to sleep» with ether or other anesthetics. Then the brain does not feel any impulses from the part being operated on. Sometimes the nerve cells near the part of our body being treated may be deadened by novocaine, as when the dentist pulls a tooth. What the novocaine does is preventing the impulses from getting to the brain from the nerve in the tooth. 6. The cerebellum is the centre for making your muscles work as a team. The medulla is the centre of certain of our most important acts: breathing and heartbeat, on which life itself depends. The medulla is also capable of controlling acts such as swallowing and yawning. Classification of the Senses Sense organs are specialized endings of the sensory division of the peripheral nerves. We are commonly thought to possess five senses. Actually, there are many more. We may classify them as follows: 1) the cutaneous senses — touch, heat, cold and pain; 2) the deeper senses — pressure and muscle sense; 3) the internal senses, or senses from the internal organs of the body; 4) the special senses, or those in which the receptors lie in special organs - sight, hearing, equilibrium, taste and smell; and finally 5) the general body senses — hunger, thirst, fatigue, sexual sensation, etc. The cutaneous senses. There are said to be 500,000 touch receptors in the skin. They are unevenly distributed, being most numerous in the finger tips, lips and tongue, and least numerous on the back. Their receptors are specialized structures called Meissner's corpuscles. The sense-organs for cold constitute 150,000 receptors; they are the endorgans of Krause. Warmth has about 16,000 receptors, the end-organs of Ruffini; and pain has some 3,000,000 receptors. Pain receptors, however, are not specialized; they are simply the naked ends of the pain nerves, ^soW^ftat branched at their terminals. The Pacinian corpuscles are the receptors for pressure, and the muscle spindles for muscle sense. Proprioception. Everyone knows what pain and touch are, but proprioception («muscle sense») may be less familiar. It is a very important sense since it is the sensory link of a reflex controlling muscle tone and contraction; and it also gives the brain important information about the location or position of the limbs. Muscle spindles are tiny, spindle-shaped structures scattered throughout muscles, and they are most numerous around the tendons and joints. The stimulus exciting then* is muscle contraction and joint movement. Since muscles are never completely at rest — one portion or another is contracting all the time — there is a constant flow of nerve impulses into the spinal cord over the muscle sense fibres. Any activity of muscles, such as walking, augments the flow. Let us analyze the fact of walking. One foot is lifted from the ground, moved forward, and, as it descends, the weight of the body is shifted to this foot. The other foot is then lifted, moved, etc. Once a child has learned to walk, he accomplishes this action not notising it; it is done reflexly, and this reflex is one in which the sensory information comes over the fibres of proprioception. At any instant of time, the spinal cord is receiving information as to the immediate, present location of the feet and legs, and it is sending out, over motor fibres, impulses which continue the activity. At any time, a person knows, without looking, approximately where his feet are and where his legs are, since this information is also being sent to the brain. Viruses 1. For three-quarters of a century, scientists have known that many diseases of man, animals, plants and even of microorganisms are caused by transmissible agents which cannot be seen under the light microscope, they are so small that they can pass through filters fine enough to hold back the most minute bacteria. These mysterious invisible agents were given the generic name of viruses. In the 1930's, two great discoveries were made which provided concrete informa- tion concerning the nature of viruses. It was found that some of them would be crystallized almost as readily as if they were ordinary chemical substances. 2. Chemically, the active virus particles were found to behave like giant molecules. At about the same time, the electron microscope became available and permitted pictures to be obtained of these crystals as well as of particles present in fluids and other materials having virus activity. Viruses would now be seen as concrete objects instead of being merely imagined. 3. The first unexpected fact revealed by electron microscopy was that the various viruses differ among themselves in shape and in size, as various types of bacteria. The virologist can differentiate between several types of viruses on the basis of their size and shape as revealed by electron micrographs. For example, the vaccinia virus is rather large. In contrast, the polioviruses are much smaller and yield very characteristic crystals. As to the tobacco mosaic virus, it can be crystallized in the form of thin needles having different lengths. The viruses that attack bacteria, which are called bacteriophages, are more complex, at least in shape. Many of them have a thin tail and a large round or cylindrical head. Each active virus particle consists of at least two very different types of structural components. One structure made up of nucleic acid, carries the genetic hereditary characteristics of the virus. Another, protein in nature, is thought to protect this genetic apparatus and to facilitate its transfer from one infected cell to another. For example, electron micrographs revealed that virus of tobacco mosaic consisted of an inner constituent of nucleic acid lodged within an outer coat, cylindrical in shape and made up of protein. The central structure, the core, should be compared to the nucleus of ordinary cells in higher organisms, which also contains large amounts of nucleic acid and also carries the genetic endowment. In fact, the nucleic acid core of this virus is its most essential constituent. However, proteins and |