The approximate 206 bones of the human body are the organs of our skeletal system. An organ is a group of tissues that function together to carry out vital activities. Bones are alive and are vital to living.
Our skeletal system give us structure and allows us to function. Bones are joined to each other at joints by ligaments. Ligaments are tough bands of connective tissue which connect bones to each other. Muscles are attached to bones by tendons. Muscle and bones form leverage systems that allow us to move and to do work.
The skeleton also protects vital organs, such as the brain, the spinal cord, the heart, lungs, liver, kidneys and digestive organs. The skeleton serves as a reservoir of minerals which supply our blood with such minerals as calcium and phosphorous. Within bones is the bone marrow which forms blood cells. Our bone marrow forms hundreds of thousands of new red blood cells every minute.
The hardness of bones is due to their high mineral content. Bones are 67% mineral and 33% organic matter. Bones contain such minerals as calcium, phosphorus, sodium, potassium, citrate, magnesium, and chlorine. But despite the hardness of bone tissue, it is living tissue. Blood vessels course through bones in small canals known as Haversian canals. These blood vessels nourish the living bone cells.
However, not all bone is hard. There are two types of bone tissue: spongy and compact. Spongy bone is the soft bone found in the interior of bones and at the ends (epiphysis) of bones. Spongy bone is more porous and contains more blood vessels than does compact bone. Compact bone is hard bone and is found on the surface of bones. The shafts (diaphysis) of long bones, such as the bones of the arms and legs, are composed almost entirely of compact bone.
The medullary canal is located in the otherwise hollow center of bones and
contains the bone marrow. This canal is formed by the action of osteoclasts.
Osteoclasts are cells which destroy bone to form canals and channels for blood
vessels and nerves. The yellow marrow contains blood vessels and fat cells, and
the red marrow forms red blood cells. Lining the medullary canal is a membrane
called the endosteum.
The outside surface of bone is covered by the periosteum, a thick double-layered membrane containing blood vessels, nerves and bone-forming cells. The growth and development of bone occurs here. Neurons in the periosteum mediate the sensation of bone pain and pressure.
Most of the bones of the body are formed in cartilage. Bone forming cells, called osteoblasts, invade the cartilage and slowly replace cartilage with bone tissue in the process called ossification. Cartilage does not turn into bone, but is replaced by bone. Ossification refers to the process of bone formation. Ossification occurs throughout life, but this process is most active in the first 25 years of life. After that, bone tissue remains capable of local growth to repair damage, such as fractures, but there is no further replacement of cartilage with bone tissue beyond this age.
The replacement of cartilage with bone tissue begins in the center of the diaphysis in what is known as a center of ossification. Soon a similar ossification center develops in each epiphysis. These centers will eventually replace all of the cartilage with bone. When this is accomplished, the center in the diaphysis will meet the centers of the epiphyses at what is called the epiphyseal line. This line signifies the end of linear growth for the long bones. The appearance of this line means that one will not get any taller. These centers close at about age 16 for girls and about age 19 for boys. These are average ages and there is variation.
The junction of two or more bones is called a joint. Some joints are free moving, such as the shoulder; slightly movable, such as ribs joined to the spine; and others are immovable, such as the bones of the skull.
There are several types of movable joints. The ball and socket joints provide the freest movement. In the ball and socket joint, one bone with a rounded head moves in a cup-like cavity of the other bone. Examples of ball and socket joints are the joints between the humerus (upper arm bone) and the scapula (shoulder blade), and the femur (upper leg bone) with the pelvis (hip bone).
Hinge joints, such as that of the elbow and knee, allow movement in only one plane. Pivot joints provide rotary movement in which a bone rotates on a ring, or a ring of bone rotates on a central axis. It is a pivotal joint that allows the skull to rotate on our spine. Gliding joints, such as those between vertebrae, permit limited sliding movement. An angular joint, such as that of the wrist, is formed by an oval-shaped surface in a concave cavity.
At joints, the ends of the joined bones are covered with cartilage. In the space between these cartilage ends is found a small amount of lubricating fluid known as synovial fluid. Another type of lubricating and cushioning device is a bursa, which is found between the bones that form the elbow, knee, hip, shoulder and ankle.
The spinal column or back bone is built of a series of block-like bones called vertebrae which are stacked on top of each other. Vertebrae are sort of round with knobs projecting from the rear to which muscles are attached. It is these knobs that you can feel when you run your hand up and down the backbone.
Between each vertebrae are pads of cartilage called intervertebral disks which absorb shock and assist in limited movement of disks. The normal backbone is not straight, but has four curves. The curve in the region of the neck is composed of seven vertebrae and is known as the cervical region. These seven vertebrae are called the cervical vertebrae. Going down the spine, the next curve is composed of 12 vertebrae and is known as the thoracic segment. These 12 vertebrae are called the thoracic vertebrae. To the thoracic vertebrae are connected the ribs. The top ten pairs of ribs curve around the chest area and connect to the breastbone (sternum). The bottom two pairs do not have a frontal connection and are known as floating ribs. Most people have 12 pairs of ribs, but having 11 or 13 pair is not uncommon.
The third curve is called the lumbar area and is composed of the five lumbar vertebrae. This area makes up what is commonly called the "small of the back" and is an area in which back pain frequently occurs from over-exertion. The last curve is composed of the five fused vertebrae of the sacrum. At birth these vertebrae are not fused, but fusion occurs as we grow. The last region of the spine is not a curve, but it too is composed of four fused vertebrae and is known as the coccyx. The coccyx is commonly called our "tail bone."
Babies begin life with 33 vertebrae, adults have 26. This loss of individual bones occurs through fusion. In all, maturation causes us to lose about 60 bones to fusion, so that the new-born has about 266 bones, and the adult about 206 bones.
The skull, which is perched on top of the spinal column contains 22 bones. This includes the six smallest bones of the body, the ossicles, which are in the ears. The brain cage (cranium) is composed of 8 flat bones knitted together to form lines called sutures. In babies, there are several area of the cranium at which true bones have not yet formed. This produces the "soft spots" or fontanelles, of which the largest is at the top of the cranium. The suture lines of the bones of the cranium are the places where bone continues to grow to accommodate the growth of the brain.
The skeletal system has two girdles to which our arms and legs are attached. The shoulder girdle consists of two collarbones (clavicles) and two shoulder blades (scapulas). Where clavicles and scapulas meet they form sockets in which the arms hang. The pelvic girdle is composed of three pairs of fused bones, the ilium, the ischium and the pubis. The ilia can be felt when we put our hands on our hips. The sacrum connects with the ilia at the back at the sacro-iliac joints. The ischia are the bones on which we sit. The pubis bones provide the lower connection of the pelvic girdle.
Where ilia and pubis bones join is the acetabulum. The acetabulum is the cup-shaped cavity in which the rounded head of the femur fits to form the hip joint.
The bone of the upper arm is called the humerus. The bones of the forearm are the ulna and radius. The humerus joins the ulna and radius at the elbow. At this joint there is a nerve which crosses over the end of the humerus. If this nerve is hit, it produces a tingly sensation. For this reason the humerus is known as the "funny bone."
The ulna and radius connect to the bones of the wrist. The bones of the wrist are the carpals and metacarpals. The metacarpals then join to the phalanges, which are the bones of the fingers.
The upper leg bone, the femur, joins to the lower leg bones at the knee. The knee cap is called the patella, and the lower leg bones are the fibula and tibia. The tibia and fibula join the tarsals at the ankle. The bones of the foot include the tarsals, metatarsals and phalanges. The phalanges of the foot are the bones of the toes.
1. Osteomyelitis--an infection of bones resulting from the growth of germs within bones. The germs, usually staphylococcus bacteria, are carried to bone by the blood stream. The symptoms are pain, tenderness near a joint and fever. Osteomyelitis is usually treated with antibiotics.
2. Ruptured disk--also known as a "slipped disk"-- is a protrusion of the cushioning pad (intervertebral disk) between vertebrae through a tare in the surrounding ligament.
3. Spina bifida--a malformation where some of the vertebrae fail to fuse to surround the spinal cord. This creates a condition where the spinal cord may be outside of the vertebrae and protrude under the skin.
4. Scoliosis--a sideways curvature of the spine.
5. Whiplash--an injury that occurs when the head is suddenly thrown forward--then jerked backward, as in the cracking of a whip. This usually results in a sprained neck, but it occasionally causes bone and nerve damage.
6. Bursitis--inflammation of the bursa, usually affecting the shoulder, elbow, knee, or ankle.
7. Tennis elbow--a form of bursitis which is caused by putting severe strain on the elbow joint.
8. Osteoporosis--the enlargement of the canals or spaces in bone causing bones to be porous and thin. This weakens the bone and makes it fragile and easily broken. Osteoporosis is more common in older people, especially in women after menopause. In these women, estrogen replacement therapy appears to make osteoporosis less likely to develop.
9. Compound fracture--a fracture in which the bone breaks through the skin.
10. Simple fracture--a fracture in which bone does not break through the skin.
11. Rheumatoid arthritis--a disease affecting joints, usually the joints of the
hands, wrists, knees and feet. The disease is characterized by pain, swelling and
inflammation of the joints. Rheumatoid arthritis affects movement, and chronic
rheumatoid arthritis may disfigure bones and joints. Rheumatoid arthritis is an
autoimmune disease--an allergy to a part one's self. Rheumatoid arthritis occurs
more often in women than men. Rheumatoid arthritis usually has a psychosomatic
component. A psychosomatic illness is any real physical illness in which
emotional factors play a causal role. Stressful emotional states are almost
always accompanied by an increase in the severity of symptoms.