The Skeleton

If a baby has a fall, it is much less likely to break the bones of its skeleton than an adult. This is partly because a newborn baby's skeleton is very different from the skeleton of an adult. The newborn baby has more than 300 parts that we call bones, yet some of these contain very little true bone at all. They are made mainly of the slightly softer, more flexible cartilage (gristle). In a fall, cartilage bends rather than breaking. During childhood, true bone gradually replaces nearly all of the cartilage. Some of the separate bones fuse together, as in the skull, reducing the total number of parts. The skeleton continues to get larger and harder in the teenage years. By the age of 20 to 25 years it has the final number of 206 fully mature, hardened bones.




















 





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Know Your Body
 





Bones




 




 
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Bones are rigid organs that form part of the endoskeleton of vertebrates. They function to move, support, and protect the various organs of the body, produce red and white blood cells and store minerals. Because bones come in a variety of shapes and have a complex internal and external structure, they are lightweight, yet strong and hard, in addition to fulfilling their many other functions. One of the types of tissues that make up bone is the mineralized osseous tissue, also called bone tissue that gives it rigidity and honeycomb-like three-dimensional internal structure. Other types of tissue found in bones include marrow, endosteum and periosteum, nerves, blood vessels and cartilage. There are two hundred six bones in the adult body.

FUNCTIONS
CHARACTERISTICS
TYPES OF BONES
QUESTIONS AND ANSWERS ON BONE

FUNCTIONS

Bones have eight main functions:

1. Protection — Bones can serve to protect internal organs, such as the skull protecting the brain or the ribs protecting the heart and lungs.

2. Shape — Bones provide a frame to keep the body supported.

3. Blood production — The marrow, located within the medullary cavity of long bones and the interstices of cancellous bone, produces blood cells in a process called haematopoiesis.

4. Mineral storage — Bones act as reserves of minerals important for the body, most notably calcium and phosphorus.

5. Movement — Bones, skeletal muscles, tendons, ligaments and joints function together to generate and transfer forces so that individual body parts or the whole body can be manipulated in three-dimensional space. The interaction between bone and muscle is studied in biomechanics.

6. Acid-base balance — Bone buffers the blood against excessive pH changes by absorbing or releasing alkaline salts.

7. Detoxification — Bone tissues can also store heavy metals and other foreign elements, removing them from the blood and reducing their effects on other tissues. These can later be gradually released for excretion.[citation needed]

8. Sound transduction — Bones are important in the mechanical aspect of hearing.


CHARACTERISTICS

The primary tissue of bone, osseous tissue, is a relatively hard and lightweight composite material, formed mostly of calcium phosphate in the chemical arrangement termed calcium hydroxylapatite (this is the osseous tissue that gives bones their rigidity). It has relatively high compressive strength but poor tensile strength, meaning it resists pushing forces well, but not pulling forces. While bone is essentially brittle, it does have a significant degree of elasticity contributed chiefly by collagen. All bones consist of living cells embedded in the mineralised organic matrix that makes up the osseous tissue.


TYPES OF BONES

There are five types of bones in the human body: long, short, flat, irregular and sesamoid.

1. Long bones are longer than they are wide, consisting of a long shaft (the diaphysis) plus two articular (joint) surfaces, called epiphyses. They are comprised mostly of compact bone, but are generally thick enough to contain considerable spongy bone and marrow in the hollow centre (the medullary cavity). Most bones of the limbs (including the three bones of the fingers) are long bones, except for the kneecap (patella), and the carpal, metacarpal, tarsal and metatarsal bones of the wrist and ankle. The classification refers to shape rather than the size.

2. Short bones are roughly cube-shaped, and have only a thin layer of compact bone surrounding a spongy interior. The bones of the wrist and ankle are short bones, as are the sesamoid bones.

3. Flat bones are thin and generally curved, with two parallel layers of compact bones sandwiching a layer of spongy bone. Most of the bones of the skull are flat bones, as is the sternum.

4. Irregular bones do not fit into the above categories. They consist of thin layers of compact bone surrounding a spongy interior. As implied by the name, their shapes are irregular and complicated. The bones of the spine and hips are irregular bones.

5. Sesamoid bones are bones embedded in tendons. Since they act to hold the tendon further away from the joint, the angle of the tendon is increased and thus the force of the muscle is increased. Examples of sesamoid bones are the patella and the pisiform.

QUESTIONS AND ANSWERS ON BONE

1. How many bones are in the human body?
An average adult has 206 bones. Some people have an extra rib or two and may have more bones in their hands and feet.

2. How many bones are in the spine?
There are 33. The bones in the spine are called vertebrae (ver-tea-bray). Beginning at the neck, there are 7 vertebrae in cervical (sir-vick-all) spine - 12 thoracic (thor-ah-sick) - 5 or 6 lumbar (lum-bar) - 5 sacrum (say-crumb) - 3 coccyx (cock-six). The coccyx is the tailbone.

3. How much do all the bones in the body weigh?
Total bone weight depends on how much the person's entire body weighs. Bones make up about 15% of a person's total body weight. For example, the bones of a person weighing 100 pounds would weigh about 15 pounds.

4. Are bones alive?
Yes! In fact bones contain thousands of living cells that are replaced as they die. Like other organs in the body, blood brings food and oxygen to bones. The blood also removes waste - you might say that blood takes out the garbage!

5. What is bone?
Bone is dense and hard yet, slightly elastic. It is made up of different layers of bone tissue, cartilage, blood vessels, and other tissue. The outer layer is compact bone. It is very hard and strong. The inside layer is called spongy bone and it does look something like a kitchen sponge! In the center of the bone is marrow. Within the bone marrow the body makes red blood cells.

6. What is bone marrow?
In the center of bone is a jelly like matter called bone marrow. Bone marrow is the heart of the body's blood factory. Not only does bone marrow make red blood cells, but it also makes different types of white blood cells. How do bones get blood? The outer layer of bone cells form rings around spaces called Haversian canals. The canals are connected to each other. Each canal is tiny and long. A blood vessel runs through each little canal.

7. Do bones have skin?
Bones have an outer layer that is like a thin skin or membrane (mem-brain). This membrane is called the periosteum (per-eye-os-tee-um). The periosteum can be thick and is filled with blood vessels that help feed the bone. Many nerves also fill the periosteum.

 




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