14 DAY TRIAL // 1.The scaffold of our body is made of 206 bones, joined through articulations. The largest human bone is the femur (about 50 cm or 20 in long), while the smallest is the stapes (2.6 mm or 0.1 in) from the middle ear.
2.There are 4 categories of bones: long (cylindrical) which are stretched and slightly curbed, having the role of absorbing the shocks; short (cubic) are rugged, thick (like the bones of the wrist); irregular bones with various shapes and sizes (like face bones or the vertebrae) and wide bones (ribs, skull, or shoulder blade), acting like shields for the vital organs.
3.Bones are moved by about 500 skeletal muscles. The skeleton is flexible due to the articulations uniting the bones. But some articulations are stiff, the bones being welded. Such is the coccis bone of the pelvis, made in fact of three pairs of bones (ilium, ischium and pubis). On newborn, these bones can move a little, but till the adult state, they are completely ossified.
Body's largest mobile articulation is the knee, which functions like a hinge: it allows the backward movement, but not on the sideways or forward. The same way function the digits' articulations.
The coxo-femural articulation of the hip is spherical (free): the spherical surface of the articular head of the femur fits into the articular cavity of the pelvis. This structure confers high mobility to the articulation, allowing ample movements forward, backward and on the side. On the same way functions the shoulder's articulation (between humerus and shoulder's blade).
4.The spine is made of 26 vertebrae, joined through articulations. Vertebrae move very little one from another, but the cumulative effect gives a large flexibility to the spine. If not, we could not bend forward, backward or on the sideways.
The head is articulated to the spine through two bony bumps called occipital condyles, which fit into articular cavities of the first vertebra (called atlas), so that we bend the head forward and backward. The ring shaped atlas fits with the second vertebra called axis. Their type of articulation is revolving, allowing us to move the head to the right and to the left. The same way functions the elbow articulation, which allows us to rotate the forearm.
The simplest articulations are those in which an articular surface slips over another. Such articulations are found between the kneepan and the distal head of the femur, or between the wrist's bones.
In saddle type articulations, the bones can move in different directions, but none can move apart from each other. Such is the articulation between the metacarpal bone of the thumb and the wrist. This is how we manage to turn the thumb to the hand and grab objects.
5.Inside an articulation the bones touch and even rub each other. That's why their heads are covered by a smooth cartilage, against wearing. The articulation is also wrapped in a sack containing viscous synovial liquid, with the role of lubricating the articulation. The cartilage and the synovial liquid prevent grinding and the usage of the bone heads during their use.
The articulations between vertebrae (except atlas and axis) have cartilaginous intervertebral disks. When one disk is dislocated, pressing a nerve, it produces a painful condition called disk hernia.
Each mobile articulation is secured by elastic ligaments which fix on the heads of the bones forming the articulation and protects the articulation from exaggerated displacements of the bones, that would force the articulation causing an abarticulation.
6.The skull is made of 29 bones. 8 protect the brain and are strongly welded. This bones are mobile in the newborn, to ease its passing through the vaginal duct. 14 bones makes the face, while inside each ear there are tree auditory bones. To these adds the mandible or jaw bone. Cavities in the skull bones decrease its weight.
7.The spine contains 7 cervical (neck) vertebrae, 12 dorsal (thorax) vertebrae, and 5 lumbar (loin) vertebrae, which are wide and strong. The sacrum bone, located inside the pelvis bone, is made of 5 welded sacral vertebrae. The last vertebra is the coccis, made of 4 welded tail vertebrae.
The thorax is made of 12 pairs of ribs and the sternum in front. Backwards, the ribs articulate to the dorsal vertebrae, while frontwards the upper 10 join the sternum through cartilages.
64 bones make the upper limbs and their girdle. The scapular girdle, connecting the arm to the shoulder, is made of 2 scapulas (shoulder's blade) and 2 clavicles. The arm is made of humerus, and the forearm by radius and cubitus. The wrist is made of 8 carpal bones; the hand by 5 metacarpal bones and the fingers by phalanxes (3 for each finger, except the thumb, which has 2).
The pelvis makes the pelvic girdle connecting the lower limb to the trunk. The pelvis of the man is longer and narrower, because men are heavier. The pelvis of a woman is wider, allowing the passage of the head and shoulders of the newborn through it (pelvis articulations in pregnant women also turn more elastic, to ease the birth).
Downwards follows the femur, patella (in the knee), tibia and fibula. The ankle contains 7 tarsal bones, the sole 5 metatarsal, and the number of toe phalanxes is like in fingers.
8.Bones in the living body are not white, dry and rigid, as seen in the museums, but gray, covered by a resistant membrane called periosteum, through which blood vessels and nerves enter the bone.
The bone tissue is not compact, but the bone is crossed transversally by canals called Havers. They harbor vessels and nerves. Around them, there are small lacunae containing bone cells (osteocytes). Under the "compact" external layer of bony tissue, there is a layer of spongy bony tissue. In the core of the bone, there is the bone marrow which produces most of the blood cells.
9.The bone is continuously wearing, dissolving and regenerating. Various activities boost the development of specific bone areas. Riding, for example, stimulates the development of the thigh and buttock bones.
Like muscles, not solicited bones degenerate. Sedentary people have also weak bones, not only weak muscles.
Bones can be compressed, bend a little and absorb the impact of smaller shocks or blows. When we jump, for example, great forces are exerted on our bones. The femur can resist even to pressures of 2 tonnes per square cm! But in some situations the bone can easily crack or break.
When a person is very flexible, this is not due to the bones, but to the lax articulations.
10.Soon after fracturing, the bone starts its healing. That's why the bones must be fixed, and not solicited (being placed in a gypsum scaffold).
First, a large hematoma forms at the fractured heads of the bones, then the minerals are removed from the fracture's place through the blood. The blood clot will be included into a fibrous web fixing the fractured heads. Osteocytes will migrate into the fibrous web, forming a new, strong bone. In about 3 weeks a new calcium rich bony tissue is formed, joining the fractured heads. This tissues, called callus, turn gradually into a real bone tissue. During the next months and years, the edges of the fracture get smooth, the bone turning like before the accident.