  One necessary means of establishing proper relations between the body and its surroundings is motion.77 Not only can the body move itself from place to place, but it is able to move surrounding objects as well. In the production of motion three important systems are employed—the muscular system, the nervous system, and a system of mechanical devices which are found mainly in the skeleton. The muscular system supplies the energy for operating the mechanical devices, while the nervous system controls the movements.78 Although the skeleton serves other purposes, such as giving shape to the body and protecting certain organs, its main use is that of an aid in the production of motion. [pg 217] The Bones.—The separate units, or parts, of which the skeleton is constructed are called bones. They are the hard structures that can be felt in all parts of the body, and they comprise nearly the entire amount of material found in the prepared skeleton. As usually estimated, the bones are 208 in number. They vary greatly in size and shape in different parts of the body. Composition and Properties of Bones.—The most noticeable and important properties of the bones are those of hardness, stiffness, and toughness. Upon these properties the uses of the bones depend. These properties may, in turn, be shown to depend upon the presence in osseous tissue of two essentially different kinds of substance, known as the animal matter and the mineral matter. If a bone is soaked in an acid, the mineral matter is dissolved out, and as a result it loses its properties of hardness and stiffness. (See Practical Work.) This is because the mineral matter supplies these properties, being composed of substances which are hard and closely resemble certain kinds of rock. The chief materials forming the mineral matter are calcium phosphate and calcium carbonate. [pg 218]
Fig. 93 Fig. 93—Section of a long bone (tibia), showing the gross structure. If a dry bone from a full-grown, but not old, animal be weighed before and after being burned, it is found to lose about one third of its weight. From this we may conclude that about one third of the bone by weight is animal matter and two thirds is mineral matter. This proportion, however, varies with age, the mineral matter increasing with advance of years. Gross Structure of Bones.—The gross structure of the bones is best learned by studying both dry and fresh specimens. (See Practical Work.) The ends of the bones are capped by a layer of smooth, elastic cartilage, while all the remaining surface is covered by a rather dense sheath of connective tissue, called the periosteum. Usually the central part[pg 219] The arrangement of the compact and spongy substance varies with the different bones. In the short bones (wrist and ankle bones, vertebrÆ, etc.) and also in the flat bones (skull bones, ribs, shoulder blades, etc.) there is no cavity for the yellow marrow, all of the interior space being filled with the spongy substance. The red marrow, relations of which to the red corpuscles of the blood have already been noted (page 27), occupies the minute spaces in the spongy substance.
Fig. 94 Fig. 94—Cross section of bone showing minute structure. Magnified. 1. Surface layer of bone. 2. Deeper portion. 3. Haversian canals from which pass the canaliculi. 4. A lacuna. Observe arrangement of lacunÆ at surface and in deeper portion. Minute Structure of Bone.—A microscopic examination of a thin slice of bone taken from the compact substance shows this to be porous as well as the spongy substance. Two kinds of small channels are found running through it in different directions, known as the Haversian canals and the canaliculi (Fig. 94). These serve the general purpose of distributing nourishment through the bone. The Haversian canals are larger [pg 220]
Fig. 95 Fig. 95—Section showing Haversian canal and contents, highly magnified (after SchÄfer). 1. Arterial capillary. 2. Venous capillary. 3. Nerve fibers. 4. Lymph vessel. The Bone Cells.—Surrounding the Haversian canals are thin layers of bone substance called the laminÆ, and within these are great numbers of irregular bodies, known as the lacunÆ. The walls of the lacunÆ are hard and dense, but within each is an open space. In this lies a flattened body, having a nucleus, which is recognized as the bone cell, or the bone corpuscle (Fig. 96). It appears to be the work of the bone cells to deposit mineral matter in the walls surrounding them and in this way to supply the properties of hardness and stiffness to the bones. The canaliculi connect with the lacunÆ in all parts of the bone, causing them to appear under the microscope like so many burs fastened together by their projecting spines (Fig. 94).
Fig. 96 Fig. 96—Bone cell removed from the lacuna and very highly magnified. (From Quain's Anatomy.) How the Bone Cells are Nourished.—The bone cells, like all the other cells of the body, are nourished by the lymph that escapes from the blood. This passes through the canaliculi to the cells in the different parts of the bone, as follows: [pg 221] 2. The cells within the interior of the bone receive their nourishment from the small blood vessels in the Haversian canals. Lymph from these vessels is conveyed to the cells through the canaliculi that connect with the Haversian canals. Plan and Purpose of the Skeleton.—The framework of the body is such as to adapt it to a movable structure. Obviously the different parts of the body cannot be secured to a foundation, as are those of a stationary building, but must be arranged after a plan that is conducive to motion. A moving structure, as a wagon or a bicycle, has within it some strong central part to which the remainder is joined. The same is true of the skeleton. That part to which the others are attached is a long, bony axis, known as the spinal column. Certain parts, as the ribs and the skull, are attached directly to the spinal column, while others are attached indirectly to it. The arrangement of all the parts is such that the spinal column is made the central, cohering portion of the skeleton and also of the whole body. Besides the general arrangement of the parts of the skeleton, there is such a grouping of the bones in each of its main divisions as will enable them to serve definite purposes. In most places they form mechanical devices for supplying special movements, and in certain places they provide for the support or protection of important organs. In most cases there is a definite combination of different bones, forming what is called the bone group.
Fig. 97 Fig. 97—The human skeleton. [pg 223] 1. The Spinal Column.—This group consists of twenty-four similarly shaped bones, placed one above the other, called the vertebrÆ, and two bones found below the vertebrÆ, known as the sacrum and the coccyx (Fig. 98). These twenty-six bones supply the central axis of the body, support the head and upper extremities, and inclose and protect the spinal cord.
Fig. 98 Fig. 98—The spinal column. The upper seven vertebrÆ form the neck and are called the cervical vertebrÆ. They are smaller and have greater freedom of motion than the others. The first and second cervical vertebrÆ, known as the atlas and the axis, are specially modified to form a support for the head and provide for its movements. The head rests upon the atlas, forming with it a hinge joint (used in nodding to indicate "yes"); and the atlas turns upon an upward projection of the axis forming a pivot joint (used in shaking the head to indicate "no"). [pg 224]
Fig. 99 Fig. 99—Two views of a lumbar vertebra. A. From above. B. From the side. 1. Body. 2, 3, 4, 5. Projections from the neural arch. The Joining of the VertebrÆ.—A typical vertebra consists of a heavy, disk-shaped portion in front, called the body, which is connected with a ring-like portion behind, called the neural arch. The body and the neural arch together encircle a round opening which is a part of the canal that contains the spinal cord (Fig. 99). From the neural arch are seven bony projections, or processes, three of which serve for the attachment of muscles and ligaments, while the other four, two above and two below, are for the interlocking of the vertebrÆ with each other. The separate vertebrÆ are joined together in the spinal column, as follows: a. Between the bodies of adjacent vertebrÆ are disks of elastic cartilage. Each disk is about one fourth of an inch thick and is grown [pg 225] b. On the back of the column, the downward projections from the neural arch of each vertebra above fit into depressions found in the neural arch of the vertebra below. This interlocking of the vertebrÆ, which is most marked in the lumbar region, strengthens greatly the back portion of the column. c. To further secure one bone upon the other, numerous ligaments pass from vertebra to vertebra on all sides of the column. 2. The Skull.—The skull is formed by the close union of twenty-two irregular bones. These fall naturally into two subgroups—the cranium and the face (Fig. 100). The cranium consists of eight thin, curved bones which inclose the space, called the cranial cavity, that holds the brain. The face group, consisting of fourteen bones, provides cavities and supports for the different organs of the face, and supplies a movable part (the inferior maxillary) which, with the bones above (superior maxillary), forms the machine for masticating the food.
Fig. 100 Fig. 100—The skull (Huxley). The illustration shows most of the bones of the skull. 3. The Thorax.—This group contains twenty-four bones of similar form, called ribs, and a straight flat bone, called the sternum, or breastbone (Fig. 101). The ribs connect with the spinal column behind, and all but the two lowest ones connect with the sternum in front, and, by so doing, inclose the thoracic cavity. As already stated (page 85),[pg 226]
Fig. 101 Fig. 101—Bone groups of trunk. 4. The Shoulder and Pelvic Girdles.—These groups form two bony supports—one at the upper and the other at the lower portion of the trunk—which serve for the attachment of the arms and legs (Fig. 101). The shoulder girdle is formed by four bones—two clavicles, or collar bones, and two scapulÆ, or shoulder blades. The clavicle on either side connects with the upper end of the sternum and serves as a brace for the shoulder, while the scapula forms a socket for the humerus (the large bone of the arm) and supplies many places for the attachment of muscles. The pelvic girdle consists of two large bones of irregular shape, called the innominate bones. They connect behind with the sacrum and in front they connect, through a small pad of cartilage, with each other. On the inside of the girdle is a smooth, basin-shaped support for the contents of the abdomen, but on the outside the bones are rough[pg 227] 5. The Arm and Hand Groups.—A long bone, the humerus, connects the arm with the shoulder and gives form to the upper arm. In the forearm are two bones, the radius and the ulna, which connect at one end with the humerus and at the other with the bones of the wrist (Fig. 102).
Fig. 102 Fig. 102—Bone groups of arm and leg. A group of eight small, round bones is found in the wrist, known as the carpal bones. These are arranged in two rows and are movable upon one another. Five straight bones, the metacarpals, connect with the wrist bones and form the framework for the palm of the hand. Attached to the metacarpals are the bones of the fingers and thumb. These form an interesting group of fourteen bones, called the phalanges of the fingers (Fig. 102). The bones of the hand provide a mechanical device, or machine, for grasping, and the arm serves as a device for moving this grasping machine from place to place. The work of the arm, in this respect, is not unlike that of a revolving crane upon the end of which is a grab-hook. The hand without the arm to move it about would be of little use. [pg 228] The legs are mechanical devices (walking machines) for moving the body from place to place. The feet serve both as supports for the body and as levers for pushing the body forward. By their attachment to the legs they may be placed in all necessary positions for supporting and moving the body. The different bone groups are shown in Fig. 97 and named in Table IV. Adaptation to Special Needs.—When any single bone is studied in its relation to the other members of the group to which it belongs or with particular reference to its purpose in the body, its adaptation to some special place or use is at once apparent. Each bone serves some special purpose, and to this purpose it is adapted by its form and structure. Long bones, like the humerus and femur, are suited to giving strength, form, and stiffness to certain parts, while irregular bones, like the vertebrÆ and the pelvic bones, are fitted for supporting and protecting organs. Others, like the wrist and ear bones, make possible a peculiar kind of motion, and still others, like the ribs, are adapted to more than one purpose. The vast differences in shape, size, structure, and surface among the various bones are but the conditions that adapt them to particular forms of service in the body. [pg 229]
[pg 230] ARTICULATIONSAny place in the body where two or more bones meet is called an articulation, or joint. At the place of meeting the bones are firmly attached to each other, thereby securing the necessary coherence of the skeleton. The large number of bones, and consequently of articulations, are necessary for the different movements of the body and also on account of the manner in which the skeleton develops, or grows. Articulations are classed with reference to their freedom of motion, as movable, slightly movable, and immovable articulations. Most of the immovable articulations are found in the skull. Here irregular, tooth-like projections from the different bones enable them to interlock with one another, while they are held firmly together by a thin layer of connective tissue. The wavy lines formed by articulations of this kind are called sutures (Fig. 100). The best examples of joints that are slightly, but not freely, movable are found in the front of the spinal column. The cartilaginous pads between the vertebrÆ permit, by their elasticity, of a slight bending of the column in different directions. These movements are caused, not by one bone gliding over another, but by compressions and extensions of the cartilage. Between the vertebrÆ in the back of the spinal column, however, there is a slight movement of the bone surfaces upon one another. Structure of the Movable Joints.—By far the most numerous and important of the joints are those that are freely movable. Such joints are strongly constructed and endure great strain without dislocation, and yet their parts move over each other easily and without friction. The ends of the bones are usually enlarged and have specially formed[pg 231]
Fig. 103 Fig. 103—Outside and inside view of knee joint. 1. Tendons. 2. Ligaments. 3. Cartilage. 4. Space containing synovial fluid. This space is lined, except upon the articular surfaces, by the synovial membrane. The interior of the joint, except where the bone surfaces rub upon each other, is covered with a serous lining, called the synovial membrane (B, Fig. 103). This secretes a thick, viscid liquid, the synovial fluid, which prevents friction. The synovial membrane does not cover the ends of the bones, but passes around the joint and connects with the bones at their edges so as to form a closed sac in which the fluid is retained. Kinds of Movable Joints.—The different kinds of movable joints are the ball and socket joint, the hinge joint, the pivot joint, the condyloid joint, and the gliding joint. These are constructed and admit of motion, as follows: 1. In the ball and socket joint the ball-shaped end of one bone fits into a cup-shaped cavity in another bone, called the socket. The best examples of such joints are[pg 232] 2. In the hinge joint the bones are grooved and fit together after the manner of a hinge. Hinge joints are found at the elbows and knees and also in the fingers. The hinge joint gives motion in but two directions—forward and backward. 3. A pivot joint is formed by the fitting of a pivot-like projection of one bone into a ring-like receptacle of a second bone, so that one, or the other, is free to turn. A good example of the pivot joint is found at the elbow, where the radius turns upon the humerus. Another example is the articulation of the atlas with the axis vertebra as already noted. The pivot joint admits of motion around an axis. 4. The condyloid joint is formed by the fitting of the ovoid (egg-shaped) end of one bone into an elliptical cavity of a second bone. Examples of condyloid joints are found at the knuckles and where the wrist bones articulate with the radius and ulna. They move easily in two directions, like hinge joints, and slightly in other directions. 5. Gliding joints are formed by the articulation of plain (almost flat) surfaces. Examples of gliding joints are found in the articulations between the bones of the wrist and those of the ankle. They are the simplest of the movable joints and are formed by one bone gliding, or slipping, upon the surface of another. The Machinery of the Body.—A machine is a contrivance for directing energy in doing work. A sewing machine, for example, so directs the energy of the foot that it is made to sew. Through its construction the machine is able to produce just that form of motion needed for its work, and no other forms, so that energy is not wasted in the production of useless motion. The places in machines where parts rub or[pg 233] The body cannot properly be compared to any single machine, but must be looked upon as a complex organization which employs a number of different kinds of machines in carrying on its work. The majority of these machines are found in the skeleton. The bones are the parts that are moved, and the joints serve as bearings. Connected with the bones are the muscles that supply energy, and attached to the muscles are the nerves that control the motion. Other parts also are required for rendering the machines of the body effective in doing work. These are supplied by the tissues connected with the bones and the muscles. HYGIENE OF THE SKELETONOf chief concern in the hygiene of the skeleton is the proper adjustment of its parts. The efficiency of any of the body machines is impaired by lack of proper adjustment. Not only this, but because of the fact that the skeleton forms the groundwork of the whole body—muscles, blood vessels, nerves, everything in fact, being arranged with reference to it—any lack of proper adjustment of the bones interferes generally with the arrangement and work of tissues and organs. The displaced bones may even compress blood vessels and nerves and interfere, in this way, with the nourishment and control of organs remote from the places where the displacements occur. For these reasons the proper adjustment of the different parts of the skeleton supplies one of the essential conditions for preserving the health. Hygienic Importance of the Spinal Column.—What has been said about the adjustment of the skeleton in general applies with particular force to the spinal column. The spinal column serves both as the central axis of the body and as the container of the spinal cord. Thirty-one pairs[pg 234]
Fig. 104 Fig. 104—A tendency toward spinal curvature (after Mosher)
Fig. 105 Fig. 105—Effect on spinal column of improper position in writing. (From Pyle's Personal Hygiene.) How the Skeleton becomes Deformed—We are accustomed to look upon the skeleton as a rigid framework which can get out of its natural form only through severe strain or by violence. This view is far from being correct. On account of their necessary freedom of motion, the bones, especially those of the spinal column, are easily slipped from their normal positions; and where improper attitudes are frequently[pg 235] Prevention of Skeletal Deformities.—Those deformities of the skeleton that are acquired through improper positions are prevented by giving sufficient attention to the positions assumed in sitting, standing, and sleeping, and also to the posture in various kinds of work. In sitting the trunk should be erect and the hips should touch the back of the chair. One should not lounge in the ordinary chair. In standing the body should be erect, the shoulders back and down, the chest pushed slightly up and forward, and the chin slightly depressed, while the weight should, as a rule, rest about equally on the two feet. The habit of leaning against some object when standing (the pupil in[pg 236] Where one is compelled by his work to assume harmful positions, these should be corrected by proper exercises, and by cultivating opposing positions during the leisure hours. Much is to be accomplished through those forms of physical exercise which develop the muscles whose work it is to keep the body in an upright position. School Furniture.—It has long been observed that school children are more subject to curvature of the spine and other deformities of the skeleton than the children who do not attend school. While this is due largely to faulty positions assumed by the pupils at their work, it has been suggested that the school furniture may be in part to blame for these positions. Investigations of this problem have shown that most of the school desks and seats in use in our public schools are unhygienically constructed, in that they force pupils into unnatural positions. School seats should support the pupil in a natural position, both in the use of his books and in writing, and there are many arguments in favor of the so-called "adjustable" school furniture. Fig. 106 shows the seat and desk designed by the Boston, Mass., Schoolhouse Commission after much study and experimenting and used in the Boston schools. This furniture, which provides a seat adjustable for height, having a back rest also adjustable for height, and a desk which is likewise provided with a vertical adjustment, supplies all essential hygienic requirements. It is to be hoped that school furniture of this character may in the near future come into general use.
Fig. 106 Fig. 106—Adjustable seat and desk used in schools of Boston, Mass. [pg 237] "Setting Up" Exercises.—The splendid carriage of students from military schools shows what may be accomplished in securing erectness of form where proper attention is given to this matter. The military student gets his fine form partly through his exercises in handling arms, but mainly through his so-called "setting up" drill. As a suggestion to one desiring to improve the form of his body, a modification of the usual "setting up" drill is here given: 1. Standing erect, with the heels together, the feet at an angle of 45°, and hands at the sides, bring the arms to a horizontal position in front, little fingers touching and nails down. From this position raise the hands straight over the head, bringing the palms gradually together. Then with a backward sweeping movement, return the hands again to the sides. Repeat several times. 2. With the feet as in the above exercise, bring the hands and the arms to a level with the shoulders, palms down, elbows bent, middle fingers of the two hands touching, and the extended thumbs touching the chest. Keeping the palms down and the arms on a level with the[pg 238] 3. With the arms at the sides and the feet side by side and touching, bring the hands in a circular movement to a vertical position over the head, and lock the thumbs. Keeping the knees straight and the thumbs locked, bend forward, letting the hands touch the ground if possible, and then bring the body and hands again to the vertical position. Then by a backward sweeping movement, return the hands again to the sides. Repeat. While these exercises may be practiced whenever convenient, it is best to set apart some special time each day for them, as on retiring at night or on rising in the morning. Hygienic Footwear.—A necessary aid to erectness of position in standing and walking is a properly fitting shoe. Heels that are too high tilt the body unnaturally forward, and shoes that cause any kind of discomfort in walking lead to unnatural positions in order to protect the feet. Shoes should fit snugly, being neither too large nor too small. Many shoes, however, are unhygienically constructed, and no attempt should be made to wear them. Certainly is this true of styles that approach the "French heel" or the "toothpick toe" (Fig. 107). However, many styles of shoes are manufactured that are both hygienic and neat fitting. Rubber heels, on account of their elasticity, are to be preferred to those made of leather.
Fig. 107 Fig. 107—Heels and toes of unhygienic and of hygienic footwear. The Skeleton in Childhood and Old Age.—Certain peculiarities are found to exist in the bones of children and of old people which call for special care of the skeleton during the first and last periods of life. The bones of children are soft, lacking mineral matter, and are liable to become bent[pg 239] On account of the accumulation of mineral matter, the bones of elderly people become brittle and are easily broken, and from lack of vigor of the bone cells they heal slowly after such injuries occur. This makes the breaking of a bone by an aged person a serious matter. Old people should, as far as possible, avoid liabilities to falls, such as going rapidly up and down stairs, or walking on icy sidewalks, and should use the utmost care in getting about. In old people also the cartilage between the bones softens, increasing the liability of getting misshaped. Special attention, therefore, should be given to erectness of form, and to such exercises as tend to preserve the natural shape of the body. Treatment of Fractures.—A fractured bone always requires the aid of a surgeon, and no time should be lost in securing his services. In the meantime the patient should be put in a comfortable position, and the broken limb supported above the rest of the body. Though the breaking of a bone is not, as a rule, a serious mishap, it is necessary that the very best skill be employed in setting it. Any failure to bring the ends of the broken bone into their normal[pg 240] Dislocations and Sprains.—Dislocations, if they be of the larger joints, also require the aid of the surgeon in their reduction and sometimes in their subsequent treatment. Simple dislocations of the finger joints, however, may be reduced by pulling the parts until the bones can be slipped into position. A sprain, which is an overstrained condition of the ligaments surrounding a joint, frequently requires very careful treatment. When the sprain is at all serious, a physician should be called. Because of the limited supply of blood to the ligaments, they are slow to heal, and the temptation to use the joint before it is fully recovered is always great. Massage82 judiciously applied to a sprained joint, by bringing about a more rapid change in the blood and the lymph, is beneficial both in relieving the pain, and in hastening recovery. Summary.—The skeleton, or framework of the body, is a structure which is movable as a whole and in most of its parts. It preserves the form of the body, protects important organs, and supplies the mechanical devices, or machines, upon which the muscles act in the production of motion. The skeleton is adapted to its purposes through the number and properties of the bones, and through the cartilage and connective tissue associated with the bones. The places where the different bones connect one with another are known as joints, and most of these admit of motion. The preservation of the natural form of the skeleton is necessary, both for its proper action and for the health of the body. [pg 241] 2. How may the per cent of animal and of mineral matter in a bone be determined? 3. What properties are given the bones by the animal matter? What by the mineral matter? 4. Locate the bone cells. What is their special function? 5. State the plan by which nourishment is supplied to the bone cells in different parts of the bone. 6. Give the uses of the periosteum. 7. State the purpose of the Haversian canals. Of the canaliculi. 8. Give functions of the spinal column. 9. Name the different materials used in the construction of a joint and the purpose served by each. 10. Name four mechanical devices, or machines, found in the skeleton and state the purpose served by each. 11. Name one or more of the body machines not located in the skeleton. 12. Of what advantage is the peculiar shape of the lower jaw? Of the ribs? Of the bones of the pelvic girdle? 13. State the importance of preserving the natural form of the skeleton. How are unnatural curves produced in the spinal column? 14. How may slight deformities of the skeleton be corrected? 15. What different systems are employed in the body in the production of motion? What is the special function of each? PRACTICAL WORKTo obtain clear ideas of the form and functions of the bones, a careful examination of a prepared and mounted skeleton is necessary. Many of the bones, however, may be located and their general form made out from the living body. Bones of the lower animals may also be studied to advantage. Experiments to show the Composition of Bone.—1. Examine a slender bone, like that in a chicken's leg. Note that it resists bending and is difficult to break. Note also that it is elastic—that, when slightly bent, it will spring back. 2. Soak such a bone over night in a mixture of one part hydrochloric acid and four parts water. Then ascertain by bending, stretching, and[pg 242] 3. Burn a small piece of bone in a clear gas flame, or on a bed of coals, until it ceases to blaze and turns a white color. Can the bone now be bent or twisted? What properties has it lost and what retained? What substance has been removed from the bone by burning? Observation on the Gross Structure of Bone.—1. Procure a long, dry bone. (One that has lain out in the field until it has bleached will answer the purpose excellently.) Test its hardness, strength, and stiffness. Saw it in two a third of the distance from one end, and saw the shorter piece in two lengthwise. Compare the structure at different places. Find rough elevations on the outside for the attachment of muscles, and small openings into the bone for the entrance of blood vessels and nerves. Make drawings to represent the sections. 2. Procure a fresh bone from the butcher shop. Note the difference between it and the dry bone. Examine the materials surrounding the sides and covering the ends of the bone. Saw through the enlarged portion at the end and examine the red marrow. Saw through the middle of the bone and observe the yellow marrow. To show the Minute Structure of the Bone.—Prepare a section of bone for microscopic study as follows: With a jeweler's saw cut as thin a slice as possible. Place this upon a good-sized whetstone, not having too much grit, and keeping it wet rub it under the finger, or a piece of leather, until it is thin enough to let the light shine through. The section may then be washed and examined with the microscope. If the specimen is to be preserved for future study, it may be mounted in the usual way, but with hard balsam. Prepare and study both transverse and longitudinal sections, making drawings. The sections should be prepared from bones that are thoroughly dry but which have not begun to decay. To show the Structure of a Joint.—Procure from a butcher the joint of some small animal (hog or sheep). Cut it open and locate the cartilage, synovial membrane, and ligaments. Observe the shape and surface of the rubbing parts and the strength of the ligaments. |
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