What frame supports the human body and what serves as a reliable support for a person? Body support and movement
90. Consider a drawing depicting a human skeleton. Label the bones.
91. Complete the sentence.
The human musculoskeletal system consists of the bones of the skeleton, their joints and muscles.
92. Answer in writing the question: what is the meaning of the skeleton?
It serves as a support for the body and its organs. The bones of the trunk and limbs are the levers with which the body moves in space. The skeleton also creates the structural shape of the body, determines its size. Parts of the skeleton - such as the skull, chest, pelvis - form receptacles for vital important organs(brain, heart, lungs, stomach, genitals and other organs). Performs a skeleton and other functions, for example, participates in metabolism.
93. Perform practical work « External structure bones."
1. Review the bone specimens or dummies given to you by your teacher.
2. Determine which group of bones (tubular, flat or mixed) these objects belong to. Justify your answer.
3. Draw in a notebook and sign the studied objects.
4. In the absence of specimens or models of bones, use the drawing to complete this work. Determine which groups the bones shown in the figure belong to, and sign them.
94. Complete the sentences.
1. Flexibility and elasticity give the bones organic matter.
2. Hardness is given to the bones by minerals.
95. Why do bones become more fragile in older people, fractures occur more often?
Because with age, organic matter becomes less, therefore, in older people, the bones are more fragile and brittle.
96. Study the section "Structure of the bone" in the textbook. Consider the drawing. Write the names of the structures indicated by numbers.
97. How is bone growth in length and thickness carried out?
The growth of bones in thickness occurs due to cell division of the inner layer of the periosteum. In length, young bones grow due to the cartilage located between the body of the bone and its ends.
98. Look at the picture. Sign the types of bone connections shown in the figure, indicated by numbers.
99. Look at the picture. Write the names of the parts of the joint indicated by numbers.
100. Look at the pictures. Write the names of the bones of the skull. Color the bones of the facial section with a colored pencil.
101. List the sections of the spine and indicate the number of calls in each section.
102. Look at the pictures. Write the names of the bones of the upper and lower limbs marked with numbers.
Upper limb bones
1. Clavicle; 2. Shoulder; 3. Humerus; four. Elbow bone; 5. Radius; 6. Metacarpal bones; 7. Bones of the wrist; 8. Phalanges of fingers.
Bones of the lower limb
1. Pelvic bone; 2. Femur; 3. Tibia; 4. Tibia; 5. Tarsal bones; 6. Metatarsal bones; 7. Phalanges of fingers.
103. Consider a natural vertebra. Sketch it and label the main parts.
104. Underline the names of the bones that make up the chest.
Ribs, cervical vertebrae, sternum, sacrum, thoracic vertebrae, collarbones.
Body support and movement
Skeleton and muscles form support - propulsion system .
Skeleton
The body supports the bony "skeleton", the skeleton. He protects internal organs and serves as an attachment for the muscles. For example, the bones of the head protect the brain, while the bones of the spine protect spinal cord which is located inside the spine. There are over 200 bones in the human skeleton.
muscles
Give shape to the body and allow us to move. There are about 650 muscles in the body, 200 of them are used when walking, 43 - in order to wrinkle the forehead, 17 - in order to smile. Muscles attach to bones and work by contraction, that is, shortening. When a muscle contracts, it sets in motion the bone to which it is attached. Any movement - from running to chewing - depends on the muscles.
Muscles are found throughout the body and vary in shape and size. The longest are in the hips, the largest are in the buttocks, they are necessary to run and climb; the smallest are in the ear.
Muscles make up about half of a person's body weight.
What makes up the musculoskeletal system? Complete the chart.
Consider the drawings. Fill in the circle next to the drawing, which shows the correct posture at the desk.
Write what the person looks like correct posture.
A person with good, correct posture has a straight back, straightened shoulders, and a raised head. Such a person looks slender, beautiful.
Consider the drawings. Think about what this behavior of children can lead to. 
Based on the drawings, think up and write down the rules on how you should not behave.
You can not beat on the head, wring the limbs. You can not push and put steps, especially, it is dangerous to do so on the stairs. All of this can lead to serious injury.
The body of a coral polyp is usually cylindrical and is not subdivided into a trunk and a leg. In colonial forms coral polyps the base is immersed in common body colonies are cenosark, and in single forms it turns into an adhesive sole.
The tentacles of these organisms are always hollow, located in one or more closely spaced corollas.
There are two large groups coral polyps - eight-ray (Octocorallia) and six-ray (Hexacorallia).
The first group always has eight tentacles and they are equipped with small outgrowths at the edges - pinnules, the second group has more tentacles and, as a rule, a multiple of six.
The tentacles of six-pointed corals are almost always smooth, without pinnules. Top part polyp, between the tentacles, is called the oral disc. In its middle is a slit-like mouth opening.
The internal structure of coral polyps is much more complex than that of hydroid and scyphoid polyps. The mouth leads into a laterally compressed pharynx lined with ectoderm. Usually, a groove runs along one of the edges of the pharynx, carrying cells with very long eyelashes- siphonoglyph. Sometimes there are two siphonoglyphs, in such cases they are located on opposite narrow edges of the pharyngeal tube. Cilia constantly move and drive water into the intestinal cavity. The latter is divided by longitudinal partitions (septa) into chambers.
In the upper part of the body of the coral polyp (in the region of the pharynx), the septa are either complete (adhering one edge to the body wall and the other to the pharynx) or incomplete (not reaching the pharynx).
The septa have holes through which all the chambers communicate with each other.
In the lower part of the coral polyp (below the pharynx), the septa adhere only to the body wall. Thereby central part gastric cavity - stomach - remains undivided.
The free edges of the septa are thickened and are called mesenteric filaments. They are playing important role in the digestion of food, since they contain many glandular cells that secrete digestive enzymes.
In corals with one siphonoglyph, two mesenteric filaments located on a pair of opposing septa are not thickened and bear cells with long, strong cilia. Being in constant motion, the cilia drive water out of the gastric cavity of the coral polyp.
The joint work of two mesenteric filaments and a siphonoglyph (or two opposite siphonoglyphs, as in sea anemones) provides permanent shift water in the gastric cavity. As a result, fresh, rich in oxygen water, and with it bacteria, planktonic organisms, detritus particles that feed on coral polyps. With a reverse flow of water, they are taken out carbon dioxide, products of exchange and undigested residues food.
The number of septa and chambers in coral polyps always coincides with the number of tentacles, the cavity of which is a continuation of the corresponding chambers of the gastric cavity. Thus, octagonal corals always have eight septa and chambers, six-armed corals always have six.
Septa are laid gradually and always in pairs.
Like all coelenterates, corals are radially symmetrical. However, in their internal organization there are also features of bilateral symmetry (pharynx compressed from the sides and siphonoglyphs). through the longitudinal axis of the pharynx, only one plane of symmetry can be drawn, which divides the body of the coral polyp into two mirror halves.
The chambers lying against the narrow edges of the pharyngeal tube differ from the rest by the location of the muscular ridges. These chambers and the septa that form them are called directional septa, which are used to conditionally determine the "dorsal" and "ventral" sides of the body of the coral polyp.
The muscle cells of coral polyps separate from the ectoderm and endoderm and pass into the mesoglea, forming a layer of longitudinal and transverse muscles in the body walls.
In addition, in the mesoglea of each septum, on one side, there is thin layer transverse, and on the other - a powerful roller of longitudinal muscles.
coral polyps
The mesoglea is represented in most of the six-rayed corals by a thin supporting plate. On the other hand, in octagonal corals, it reaches a significant development, especially in the trunk and branches of the colony.
The gelatinous substance of the mesoglea is reinforced with collagen elements and filled with huge amount skeletal calcareous needles - spicules, or sclerites.
Thus, the mesoglea constitutes a strong support for the coral colony. At the same time she participates in transport nutrients, as it is permeated with a dense network of endodermal canals connecting separate intestinal cavities of coral polyps into one cavity.
The same channels play an important role in the rhythmic change of the active and passive state of the coral polyp colony.
The skeleton reaches significant development in many coral polyps.
In octocorals, this is an internal, mesogleal skeleton consisting of sclerites that develop into special cages- scleroblasts. Sometimes sclerites fuse with each other or unite with an organic horn-like substance, forming the skeleton of a coral colony. It may also consist of pure horny substance.
Among six-ray corals there are non-skeletal forms (anemones and periantharia).
More often, however, there is a skeleton, and it can be either internal (in the form of a rod of a horn-like substance) or external (calcareous), but always of ectodermal origin.
But neither the calcareous nor the organic skeleton is able to maintain the constancy of the body shape of coral polyps.
This is provided in a different way. All polyps have a peculiar hydroskeleton, which reaches its greatest perfection in coral polyps.
Due to the constant flow of water created by siphonoglyphs, gastric high blood pressure, without which the coral polyp would have the shape of an empty two-layer bag.
The polyp expands under the pressure of the fluid that fills the gastric cavity. In this state, it can be very long and almost without energy expenditure.
Meanwhile, in other animals such a tense state cannot be prolonged, since the muscles get tired, the animal must change the position of the body or move in space.
But one should not think that once straightened out, the coral polyp will retain its shape indefinitely.
Periodically, it is disturbed by the contraction of any muscle group. Contraction of the circular muscles, for example, elongates the body of the polyp and makes it thinner, contraction of the longitudinal muscles of the tentacles leads to their bending, and so on.
In case of danger, all muscles contract at once, water from the gastric cavity is squeezed out and the polyp shrinks or is drawn into the colony.
Colonies of coral polyps, as a rule, are not polymorphic, but dimorphism is observed in some octagonal corals - two types of polyp structure.
All corals are characterized only by the polypoid state. They do not form jellyfish. The gonads develop in the endoderm of the septa of coral polyps.
Related articles:
anemone
Coral polyp with tentacles
Alternative descriptions
. (sea anemone) invertebrate marine animal of the coral polyp class
Coral polyp, sea anemone
Marine animal, coral polyp
The simplest, lowest marine multicellular
Representative of the coelenterates
Judging by its name, this animal should radiate, but in fact it can only hurt
Hermit crab bodyguard
Protozoan, sea anemone
coral polyp
Polyp with tentacles
Sea anemone (polyp)
Sea polyp anemone
Skeletalless polyp "flower"
Coral polyp that looks like a flower
Marine "flower" with "tentacles"
Polyp "flower"
Hermit crab concubine
Coral polyp devoid of skeleton
Marine "flower" with "tentacles"
Marine coelenterate of the coral polyp class
Question: The smallest non-cellular representatives of living nature, consisting of a nucleic acid (DNA or RNA) and a protein coat, 15–350 nm in size or more. Jellyfish, corals, polyps
Do they reproduce not only in living cells, but also on computer hard drives?
Answer: viruses
Question: The pattern of development of wildlife, which determines the adaptability of organisms to changing living conditions, based on the interaction of variability, heredity and survival of organisms
Answer: selection
Question: The doctrine of the origin and development of animal and plant species through natural selection, about the laws of development of living nature
Answer: Darwinism
Question: The evolutionary theory of the development of wildlife by the French naturalist J. B. Lamarck
Answer: Lamarckism
Question: A scientific discipline that studies periodic phenomena in the development of wildlife, due to the change of seasons
Answer: phenology
sea anemones, or sea anemones(lat. Actiniaria) - a detachment of sea cnidarians from the class of coral polyps ( Anthozoa). Representatives are deprived of a mineral skeleton. As a rule, single forms. Most anemones - sessile organisms living on solid marine soil.
Few species (for example, Nematostella vectensis) switched to a burrowing way of life in the thickness of the bottom sediments.
body structure
The cylindrical body of anemones varies in diameter from a few mm to 1.5 meters.
They are attached to solid substrates with the help of a “sole”. In forms that live on soft soils (for example, on sand), special bodies attachment is not formed.
At the pole of the body, facing away from the substrate, there is a slit-like mouth surrounded by a rim of tentacles.
Sea anemones are devoid of a mineral skeleton: their supporting function is taken over by the intestinal cavity, which is isolated from environment when closing the mouth opening. The coordinated work of this hydroskeleton and muscles of the body wall turns out to be quite effective: among the sea anemones there are representatives that are able to move in the thickness of the soil.
Many sea anemones are brightly colored (for example, yellow and red).
Ecology and nutrition
They feed on various small invertebrates, sometimes fish, first killing or paralyzing the prey with "batteries" of stinging cells (cnidocytes), and then pulling them to the mouth with the help of tentacles.
It can cause painful burns in humans.
Some anemones live in symbiosis with hermit crabs or other invertebrates, as well as with some types of fish (for example, clown fish).
Spreading
Distributed widely. Most live in tropical and subtropical waters.
see also
Literature
- Dogel V. A. Zoology of invertebrates, 5th ed. - M., 1959.
- Animal life, vol.
1. - M., 1968, p.
- Ruppert E. E., Fox R. S., Barnes R. D. Protists and lower multicellular organisms // Invertebrate Zoology.
Functional and Evolutionary Aspects = Invertebrate Zoology: A Functional Evolutionary Approach / transl. from English. T. A. Ganf, N. V. Lenzman, E. V. Sabaneeva; ed. A. A. Dobrovolsky and A. I.
Granovich. - 7th edition. - M.: Academy, 2008. - T. 1. - 496 p. - 3000 copies. - ISBN 978-5-7695-3493-5.
- Anemones // encyclopedic Dictionary Brockhaus and Efron: in 86 volumes.
(82 tons and 4 additional). - St. Petersburg, 1890-1907.
CC © wikiredia.ru
The class Coral polyps belongs to the intestinal cavities and includes about 6 thousand species.
In their life cycle there is no medusa stage. Coral polyps, depending on the species, can be either solitary or colonial. The size of single forms can reach a meter or more in diameter, and individual specimens of colonies can be less than a centimeter in size.
Coral polyps mainly live in tropical seas at shallow depths.
A characteristic feature of colonial coral polyps is the presence of a calcareous or horny skeleton.
Polyps with a calcareous skeleton form coral reefs. Single coral polyps do not have such a skeleton; they can move along the bottom, burrow into benthos, and even swim a little while bending.
Corals are called the skeleton of colonial forms. Ancient corals formed huge deposits of limestone, which are now used in construction.
The skeletal structures of the coral polyp are formed in lower parts either ectoderm or mesoglea.
As a result, it turns out that individual individuals of the colony sit in recesses on general skeleton. Communication between polyps is carried out due to a layer of living tissue on the surface of the coral.
There are incomplete radial septa in the intestinal cavity (eight, or a multiple of six).
The cavity has bilateral symmetry, not radial. The mouth opening is surrounded by numerous tentacles. Colonial forms feed on plankton (crustaceans and other arthropods). Solitary coral polyps, such as sea anemones, feed on larger animals (fish, crustaceans).
Coral polyps have muscle cells and a muscular system.
Near the mouth opening there is a denser plexus of nerve cells.
Coral polyps reproduce asexually and sexually.
Asexual reproduction is carried out by budding. In some single polyps, in addition to budding, a longitudinal division of the individual into two parts is possible. During sexual reproduction, germ cells are formed in the endoderm, usually on the partitions of the intestinal cavity.
The spermatozoa leave the male and swim into the intestinal cavity of the female, where fertilization takes place. A floating larva (planula) develops from the zygote, which swims out and after some time settles in a new place, giving rise to a new polyp.
Anemones are a detachment of coral polyps, mostly solitary.
They are distinguished by a bag-shaped body, the absence of a mineral skeleton, numerous tentacles, and a variety of bright colors. Some sea anemones enter into symbiosis with hermit crabs living in shells left over from molluscs.
In this symbiosis, cancer uses anemone as a means of defense against predators (stinging cells of the coelenterates). Anemone moves with the help of cancer, which allows it to capture more food.
Coral polyps are sensitive to water pollution. So the decrease in oxygen in the water leads to their death.
Skeleton is a construction of 212 bones. It serves as a support for the body and protects its sensitive organs: they are covered with bone membranes, put away in boxes of bones, in bone capsules and channels.
A skeleton is a structure made up of 212 bones. (There are 300 of them in a child's body. As he grows, some of the bones fuse together.) Skeleton creates support for the body and protects its sensitive organs: they are covered with bone membranes, put away in boxes of bones, hidden in bone capsules and channels. The basis of the skeleton is connective tissue, reinforced with calcium salts. This material is as hard as concrete. However, the skeleton is distinguished not only by strength, but also by amazing lightness. In an adult, its weight is less than 20% of body weight.
Figuratively speaking, the basis of the skeleton is a tower with 38 floors. It's called the spine. It consists of 7 cervical, 12 thoracic, 5 lumbar, 5 sacral and 4-5 coccygeal vertebrae. It appeared about 500 million years ago. Subsequently, whatever nature invented, it attached to the spine: the giant tail fin of a whale, the legs-columns of an elephant, the massive skull of a bison ... Nevertheless, the spine is flexible, like a steel spring. Maybe you saw a “snake woman” in the circus: amusing the audience, she arches her whole body back and touches her heels with the back of her head.
Inside the spine there is a through cavity - the spinal canal, lined with fibrous connective tissue - the dura mater. Here, in this cavity, the light is poured, clear liquid- liquor, or cerebrospinal fluid. The spinal cord is immersed in it - an appendage of the brain, resembling a pigtail. The spinal cord is connected with countless nerve fibers different bodies body. The lower end of the spine is firmly attached to a ring of three bones - pelvis. He, in turn, rests on thin "columns" - the tubular bones of the legs. These "columns" rest on wide "plates" - the feet.
However, the foot is more like not a plate, but an arch. Its design is as complex as the cross-ribbed vaults of Gothic churches: the entire human leg is composed of 30 bones, and 26 of them went to the creation of the foot.
The top vertebra of the spinal column is called the atlas. Once upon a time, that was the name of a fairy-tale hero who held the vault of heaven on his shoulders. Like this titan, the first cervical vertebra- Atlas holds the head of a man. The skeleton of the head, or skull, consists of 24 bones, mostly of flat bone plates. The cranial box folded from them is very strong: it is difficult to break through it. Its walls are covered with a thick web of fibers connective tissue. Inside it is the control center of the body - the brain, which rests on a liquid pillow. A few "floors" below the spine, the chest is suspended: a movable structure of 24 narrow, arcuately curved plates - the ribs - and one wide plank - the sternum. This bony framework protects the heart and lungs. With each breath, the chest rises; pushing her respiratory muscles. Volume chest cavity every time it increases by several liters.
The skeleton of the arms is attached to the chest. They are very mobile. Each of them is suspended from a flat triangular bone - the scapula, which is deeply pressed into the muscles of the back. The scapula is connected to the chest by a thin tubular bone protruding forward - the clavicle. She rests on her chest. The skeleton of the hand, like the legs, consists of 30 bones, of which 26 form the hand. With its structure, it reminds us that our ancestors lived on trees and they had to cling to branches and climb trunks. Like our distant ancestors, we are very mobile thumb hands, the remaining fingers are rather long and very firmly connected by the bones of the wrist.
What is bone made of?
Bone consists of a compact substance based on calcium and phosphorus, spongy substance, living bone cells - osteocytes, blood vessels and nerves. In the middle of the bone is the medullary cavity containing the bone marrow. In the bones of infants, as well as in some bones of an adult, the bone marrow produces new blood cells - red blood cells(erythrocytes), white blood cells (leukocytes), and platelets (platelets). This function bone marrow It is vital because red blood cells carry oxygen throughout the body, white blood cells protect against infections, and platelets are responsible for blood clotting. In addition, the bones contain minerals necessary for the normal functioning of the body - 99% of all calcium is deposited in the skeleton.
Bones are alive
From birth, the bones gradually stretch and increase, reaching their final size by the age of twenty. Mineral salts, in particular calcium and phosphorus, are essential for bone development. Vitamin D is also necessary for growth, it is he who helps to retain calcium in the bones. A child who does not receive calcium and vitamin D can develop rickets, a disease common in developing countries.
The bones of the hands and feet are similar. Their great flexibility is provided by a complex of joints. The hand consists of 27 bones, which include 8 carpal bones, 5 metacarpal bones located between the carpal bones and the phalanges of the fingers, and 14 phalanges of the fingers. There are 26 bones in the foot: 7 tarsal bones (the bones located above the foot), 5 metatarsal bones (the bones between the tarsus and the phalanges), and 14 phalanges of the fingers.
Rib cage
Rib cage consists of twelve pairs of ribs that are attached to the spinal column: 7 pairs true ribs, 3 pairs of false ribs attached to the sternum (the bone located in front of the chest), and 2 pairs of oscillating ribs that are not attached. The chest contains the vital organs: the heart, lungs and liver.
vertebral column
The vertebral column, or simply the spine, is one of the main parts of the skeleton, because it serves as a support for the head and torso. The spine consists of 33 vertebrae: 7 cervical vertebrae located at the level of the neck, 12 thoracic vertebrae - in the upper and middle sections of the back, 5 lumbar vertebrae - in lower section back, 5 sacral vertebrae (at the level of the sacrum) and 4 (or 5) coccygeal vertebrae located at the level of the coccyx. The vertebrae are separated intervertebral discs, consisting of cartilage tissue and shock absorbing. In order not to damage the vertebrae, one should maintain a straight posture, develop the muscles of the back and abdomen, and avoid lifting objects that are too heavy. Main support human body, spinal column may be subject to deformities, such as scoliosis. Scoliosis - a lateral curvature of the spine - occurs in girls eight times more often than in boys and occurs during the period of growth.
The cranium performs important function protecting the brain and senses. It consists of 8 bones: 1 frontal, 2 parietal, 2 temporal, 1 occipital, 1 sphenoid and 1 ethmoid bone. Facial department includes 14 more bones, including 2 bones of the upper jaw,
fractures
To heal a fracture, the two pieces of bone must be aligned exactly in their original position. The fracture area is then covered with plaster so that the bone can heal. In difficult cases, screws and plates are used to fix parts of the broken bone. Healing happens naturally: bone cells provide proteins and calcium to form new tissues that cover both parts of the broken bone, causing the bone to heal over time.
The longest bone of the skeleton - the femur - corresponds to a quarter of a person's height.
The skeleton serves as a support for the body, and this is its main significance. The human skeleton consists of more than two hundred bones. Some of them, for example, the frontal, two parietal, occipital to other bones of the skull, are connected to each other immovably and very firmly, because they have numerous protrusions of one bone that enter the corresponding recesses of the other. This is how the boards are connected with spikes. Bone spikes are very diverse and bizarre. In ancient times, in India, they were taken for mysterious signs and they thought that their fate was written on the skull of a person with these signs.
The skeleton is not only the support of the body. It also protects some organs from shock and damage. Yes, the brain is well protected. cranium; the spinal cord is in spinal canal, which is formed from individual bone rings-vertebrae; lungs, heart, liver, spleen are covered with ribs, which are connected behind the spine, and in front with the sternum.
SUPPORT IS MOVABLE
If all the bones were connected to each other motionlessly, a person would be like a stone statue: he could not make any movement. But many bones are not fixedly connected, and this makes it possible for a person to move and take various positions. These movable joints of bones are called joints. In the joint, the areas of adjacent bones that are in contact with each other are covered with smooth, slippery cartilage and are, as it were, ground to each other. If the surface of one bone is convex, then the surface of the other has a corresponding depression.
The movements of the bones in different joints are not the same. For example, the leg at the knee only bends and unbends, while other movements are almost impossible for it. We not only bend and unbend the fingers of the hand, but also move them to the side. In some joints, the bone can also rotate.
Features of movements in each joint depend on the shape of the articular surfaces of the bones. The most diverse movements of the bones are where the surface of the joints spherical shape, for example in the shoulder joint.
In each joint, around the adjoining parts of one and the other bone, there is a dense, impermeable even for air shell. Its edges are fused with adjacent parts of the bones. This shell is called the articular bag. Its strength is increased by special ligaments fused with bones.
It is known from physics that if two hollow hemispheres are attached to each other, the edges of which are in close contact at all their points, and air is pumped out of them, then to separate them, you need to apply a huge force that would overcome the pressure of atmospheric air on outer surface hemispheres.
The same can be said about the joints. There is no air inside the joints. It is very difficult to stretch the joint, i.e., push the bones in contact, because this will be prevented Atmosphere pressure outside air, which presses the bones together. It has been established that the strength of a joint decreases sharply if it is pierced with a pin. It is very easy to make movements in the joint. You can raise your hand, stretch it forward or take it to the side. With each such movement, the bones continue to touch each other, but their relative position changes.
It would seem that the mobile joints of the bones should interfere with the supporting function of the skeleton. In fact, try to replace ordinary table legs with ones that, like human legs, would consist of several movably connected links. It is unlikely that anyone will agree to dine or work at such a table. However, in reality, the mobile connection of most of the bones of the skeleton not only does not violate its supporting function, but, on the contrary, makes it more perfect.
AND YET THE SUPPORT IS STABLE
The skeleton serves as a support for the whole body and its individual parts under any conditions: when a person lies or stands, and when he walks or works. This is due to the fact that each joint can be temporarily fixed in one position or another and become immobile. When a person is standing, knee-joint firmly fixed in the unfolded position. It is worth squatting down, as the same joint will be fixed in a bent position.
Temporary fixation of the joint, as well as a change in its position, is carried out by skeletal muscles. Almost all muscles are connected at their ends to two bones, most often neighboring ones.
A nerve comes to each muscle from the spinal cord and brain. Waves of excitement rush through it one after another. Reaching the muscle, they make it work. And the work of the muscle is that it is reduced, in other words, shortened. When contracting, the muscles pull the bones along with them, and this leads to the movement of the body or its individual parts. Consequently, during movements, the muscles play an active role, and the bones - a passive one; they continue to perform their main function, namely to serve as a support for each part of the body. The bones, together with the muscles attached to them, are usually called the organs of movement, or, more correctly, the musculoskeletal system.
LEVERAGE OF OUR BODY
What happens to the bones to which the ends of the contracting muscle are attached?
Suppose a person bends his arm at the elbow while stationary humerus. Then the forearm, together with the hand, can be considered as a lever, which is set in motion by the shoulder muscle and the biceps, or biceps: elbow joint- this is the point of support; the center of gravity of the forearm and hand is the point of application of the force that pulls the lever down; the place of attachment of both muscles is the point of application of the force that raises the lever.
This is a lever of the second kind. In it, the points of application of the acting and opposing forces are on the same side of the fulcrum. We often use such leverage in life. Such, for example, are nutcrackers or waga - a pole that is used to move or turn a thick log, a large stone or other heavy object. In levers of the second kind, the point of application of the acting force is usually located at a greater distance from the fulcrum than the point of application of the opposing force. This makes it possible to overcome a very significant opposition with relatively little effort.
Let's try to figure out what's going on here. Let's take a lever of the second kind 50 cm long. To its middle we will attach two cords with identical weights weighing 20 g each. One cord is thrown over the block and pulls the lever up, and the other pulls it down. The weights precisely balance each other, and the lever does not rise or fall.
Now let's move the attachment points of the cords. We will strengthen the cord thrown over the block at the very end of the lever, and leave the other cord in the old place. In this case, the first cord, which pulls the lever up, will be attached to the lever at a distance twice as large from the fulcrum as the second cord. Will the balance be maintained? Of course not: the lever will immediately rise. To restore balance, you must either halve the load of the far cord, or double the load of the near cord.
Let's do a more complicated experiment. We move the cord that pulls the lever down to the fulcrum so that it is at a distance of 5 cm from it. The weight of the load is still 20 g. We will gradually move the other cord along the lever. First, we will strengthen it at a distance of 10 cm from the fulcrum. Then, to balance on this cord, it is necessary to hang not 20, but only 10 G. Let's move the cord even further so that it is 25 cm from the fulcrum. Now 4 G is enough to achieve equilibrium.
The farther away from the fulcrum is the place of attachment of the cord pulling the lever up, the less weight can balance the lever. If this cord is attached at a distance of 50 cm from the fulcrum, that is, at the very end of the lever, then 2 G is enough to balance a load of 20 G attached to the lever at a distance of 5 cm from the fulcrum.
It is easy to see that the force required to move the lever must be less than more distance from the place of application to the fulcrum. Putting a nut in the recess of the tongs, which is very close to the fulcrum of the lever, we easily split it. The effort required for this is significant. less than that, which is necessary to crack the nut by pressing on it without using a lever.
In the levers of our body, the points of application of force and reaction are almost always located differently: closer to the fulcrum is the place of attachment of the muscle, i.e., the acting force. Therefore, in order to overcome the resistance, the muscles must develop a very great power. However, this results in a significant gain in the range of motion.
Suppose a person is holding an object weighing 1 kg in a bent hand. The point of opposition, i.e., the center of gravity of the forearm, together with the loaded hand, is 8-10 times farther from the fulcrum than the point of application of force, i.e., the place of attachment of the muscle. Therefore, in order to hold a load weighing only 1 kg, the muscle must contract with the force necessary to lift a load of about 10 kg.
JOINTS ARE PARTIALLY SECURED EVEN WHEN MOVING
When a person is standing, the supporting function of the skeleton can only be carried out if each movable connection of the bones of the legs, trunk, neck and head is fixed in a certain position. The work of strengthening the mobile joints of the bones is performed by our muscles. Their coordinated work ensures that the balance is maintained. The force of contraction of individual muscles is constantly changing in full accordance with the forces created in each this moment conditions for maintaining equilibrium. If the bones were fixed to each other, the skeleton could not perform its supporting function with such perfection.
Of particular interest is the supporting function of the skeleton during movement. When you need to reach out, shoulder joint should be fixed in such a way that movement in the desired direction is easy, but that it is impossible to rotate the hand and move it to the side. In other words, there is a partial fixation of the joint, allowing only one specific movement.
During work, the direction of movement in the joints is constantly changing, which means that at any given moment, the partial fixation of the joints occurs in different ways.
THE MOST PERFECT LUBRICANT
The stronger two objects rub against each other, the more their rubbing surfaces heat up. The heat generated can ignite and ignite flammable objects. Already primitive people they knew how to make fire by friction. And until now, we usually use friction to make fire: we strike a match on a box.
However, much more often we have to think not about how to use friction, but about how to deal with it. Friction is a hindrance to movement. The point is not only that the rubbing surfaces in the machines are heated. Friction delays, slows down movement. The rubbing parts are usually gradually erased, destroyed, and therefore from time to time the machine needs to be repaired.
With every movement articular surfaces bones rub against each other. To reduce and prevent harmful effects friction, lubrication is required. Indeed, in every joint there is a lubricant, it is secreted by inner surface joint bag. In addition, the rubbing cartilage pads themselves form the lubricant.
Cartilage is not dead matter. That part of it, which fuses with the bone, grows all the time, replacing the rubbing, collapsing surface of the cartilage. The more movements are made in the joint, the more the surfaces of its cartilaginous layers are erased, the faster the cartilage grows. In this way, there is a kind of constant current repair of the rubbing parts of our body.
When destroyed, the surface layer of cartilage pads turns into a lubricant for the joint. Friction surfaces lubricate themselves, constantly maintaining a uniform thin layer of lubricant. This is why the articular surfaces of bones are always wet and slippery.
There is not a single machine in the world, in which, like a human joint, the rubbing surface, partially collapsing, would turn into a lubricant, and at the same time restore itself.
Each machine must be cleaned from time to time, it is necessary to remove used lubricants. And the human joint does not need to be cleaned. The used lubricant is absorbed into the blood through the wall of the articular sac, just as in the intestinal wall they are absorbed into the blood necessary for the body. nutrients. As a result, excess and unusable lubrication is constantly removed from the joint.
The joints take care of themselves. And this self-service is so perfect that during the whole human life they can work flawlessly. Only in certain diseases articular rheumatism, gout - violated normal work joints, and then the movements become difficult and painful.
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