Humans are born with over 270 bones, some of which fuse into a longitudinal axis, the axial skeleton, to which the appendicular skeleton is attached.
Axial skeleton
The axial skeleton (80 bones) is formed by the vertebral column (26), the rib cage (12 pairs of ribs and the sternum), and the skull (22 bones and 7 associated bones). The upright posture of humans is maintained by the axial skeleton, which transmits the weight from the head, the trunk, and the upper extremities down to the lower extremities at the hip joints. The bones of the spine are supported by many ligaments. The erectors spinae muscles are also supporting and are useful for balance.
Your spine is one part of the skeleton that’s easy to check out: Reach around to the center of your back and you’ll feel its bumps under your fingers.
The spine lets you twist and bend, and it holds your body upright. It also protects the spinal cord, a large bundle of nerves that sends information from your brain to the rest of your body. The spine is special because it isn’t made of one or even two bones: It’s made of 26 bones in all! These bones are called vertebrae (say: VER-tuh-bray) and each one is shaped like a ring.
There are different types of vertebrae in the spine and each does a different kind of job:
- The first seven vertebrae at the top are called the cervical(say: SIR-vih-kul) vertebrae. These bones are in the back of your neck, just below your brain, and they support your head and neck. Your head is pretty heavy, so it’s lucky to have help from the cervical vertebrae!
- Below the cervical vertebrae are the thoracic (say: thuh-RAS-ick) vertebrae, and there are 12 in all. These guys anchor your ribs in place. Below the thoracic vertebrae are five lumbar(say: LUM-bar) vertebrae. Beneath the lumbar vertebrae is thesacrum (say: SAY-krum), which is made up of five vertebrae that are fused together to form one single bone.
- Finally, all the way at the bottom of the spine is the coccyx(say: COK-siks), which is one bone made of four fused vertebrae. The bottom sections of the spine are important when it comes to bearing weight and giving you a good center of gravity. So when you pick up a heavy backpack, the lumbar vertebrae, sacrum, and coccyx give you the power. When you dance, skip, and even walk, these parts help keep you balanced.
In between each vertebra (the name for just one of the vertebrae) are small disks made of cartilage. These disks keep the vertebrae from rubbing against one another, and they also act as your spine’s natural shock absorbers. When you jump in the air, or twist while slamming a dunk, the disks give your vertebrae the cushioning they need.
Ribs
Your heart, lungs, and liver are all very important, and luckily you’ve got ribs to keep them safe. Ribs act like a cage of bones around your chest. It’s easy to feel the bottom of this cage by running your fingers along the sides and front of your body, a few inches below your heart. If you breathe in deeply, you can easily feel your ribs right in the front of your body, too. Some thin kids can even see a few of their ribs right through their skin.
Your ribs come in pairs, and the left and right sides of each pair are exactly the same. Most people have 12 pairs of ribs, but some people are born with one or more extra ribs, and some people might have one pair less.
All 12 pairs of ribs attach in the back to the spine, where they are held in place by the thoracic vertebrae. The first seven pairs of ribs attach in the front to the sternum (say: STUR-num), a strong bone in the center of your chest that holds those ribs in place. The remaining sets of ribs don’t attach to the sternum directly. The next three pairs are held on with cartilage to the ribs above them.
The very last two sets of ribs are called floating ribs because they aren’t connected to the sternum or the ribs above them. But don’t worry, these ribs can’t ever float away. Like the rest of the ribs, they are securely attached to the spine in the back.
Skull
Your skull protects the most important part of all, the brain. You can feel your skull by pushing on your head, especially in the back a few inches above your neck. The skull is actually made up of different bones. Some of these bones protect your brain, whereas others make up the structure of your face. If you touch beneath your eyes, you can feel the ridge of the bone that forms the hole where your eye sits.
And although you can’t see it, the smallest bone in your whole body is in your head, too. The stirrup bone behind your eardrum is only .1 to .13 inches (2.5 to 3.3 millimeters) long! Want to know something else? Your lower jawbone is the only bone in your head you can move. It opens and closes to let you talk and chew food.
Your skull is pretty cool, but it’s changed since you were a baby. All babies are born with spaces between the bones in their skulls. This allows the bones to move, close up, and even overlap as the baby goes through the birth canal. As the baby grows, the space between the bones slowly closes up and disappears, and special joints called sutures (say: SOO-churs) connect the bones.
Appendicular skeleton
Hands
As you sit and type at the keyboard, while you swing on a swing, even when you pick up your lunch, you’re using the bones in your fingers, hand, wrist, and arm.
Each arm is attached to a shoulder blade or scapula (say: SKA-pyuh-luh), a large triangular bone on the upper back corner of each side of the ribcage. The arm is made up of three bones: the humerus(say: HYOO-muh-rus), which is above your elbow, and the radius(say: RAY-dee-us) and ulna (say: UL-nuh), which are below the elbow.
Each of these bones is wider at the ends and skinnier in the middle, to help give it strength where it meets another bone. At the end of the radius and ulna are eight smaller bones that make up your wrist. Although these bones are small, they can really move! Twist your wrist around or wave and you’ll see how the wrist can move.
The center part of your hand is made up of five separate bones. Each finger on your hand has three bones, except for your thumb, which has two. So between your wrists, hands, and all your fingers, you’ve got a grand total of 54 bones — all ready to help you grasp things, write your name, pick up the phone, or throw a softball!
Legs
Sure, your arm, wrist, hand, and finger bones are great for picking up the phone, but how are you supposed to run to answer it? Well, with the bones of the legs and feet!
Your legs are attached to a circular group of bones called your pelvis. The pelvis is a bowl-shaped structure that supports the spine. It is made up of the two large hip bones in front, and behind are the sacrum and the coccyx. The pelvis acts as a tough ring of protection around parts of the digestive system, parts of the urinary system, and parts of the reproductive system.
Your leg bones are very large and strong to help support the weight of your body. The bone that goes from your pelvis to your knee is called the femur (say: FEE-mur), and it’s the longest bone in your body. At the knee, there’s a triangular-shaped bone called the patella (say: puh-TEL-luh), or kneecap, that protects the knee joint. Below the knee are two other leg bones: the tibia (say: TIH-bee-uh) and the fibula (say: FIH-byuh-luh). Just like the three bones in the arm, the three bones in the leg are wider at the ends than in the middle to give them strength.
The ankle is a bit different from the wrist; it is where the lower leg bones connect to a large bone in the foot called the talus (say: TAL-iss). Next to the talus are six other bones. But the main part of the foot is similar to the hand, with five bones. Each toe has three tiny bones, except for your big toe, which has just two. This brings the bone total in both feet and ankles to 52!
Most people don’t use their toes and feet for grabbing stuff or writing, but they do use them for two very important things: standing and walking. Without all the bones of the foot working together, it would be impossible to balance properly. The bones in the feet are arranged so the foot is almost flat and a bit wide, to help you stay upright. So the next time you’re walking, be sure to look down and thank those toes!
Joints
The place where two bones meet is called a joint. Some joints move and others don’t.
Fixed joints are fixed in place and don’t move at all. Your skull has some of these joints (called sutures, remember?), which close up the bones of the skull in a young person’s head. One of these joints is called theparieto-temporal (say: par-EYE-ih-toh TEM-puh-rul) suture — it’s the one that runs along the side of the skull.
Moving joints are the ones that let you ride your bike, eat cereal, and play a video game — the ones that allow you to twist, bend, and move different parts of your body. Some moving joints, like the ones in your spine, move only a little. Other joints move a lot. One of the main types of moving joints is called a hinge joint. Your elbows and knees each have hinge joints, which let you bend and then straighten your arms and legs. These joints are like the hinges on a door. Just as most doors can only open one way, you can only bend your arms and legs in one direction. You also have many smaller hinge joints in your fingers and toes.
Another important type of moving joint is the ball and socket joint. You can find these joints at your shoulders and hips. They are made up of the round end of one bone fitting into a small cup-like area of another bone. Ball and socket joints allow for lots of movement in every direction. Make sure you’ve got lots of room, and try swinging your arms all over the place.
Have you ever seen someone put oil on a hinge to make it work easier or stop squeaking? Well, your joints come with their own special fluid called synovial fluid (say: SIH-no-vee-ul) that helps them move freely. Bones are held together at the joints byligaments (say: LIH-guh-mints), which are like very strong rubber bands.
Function
The skeleton serves six major functions.
Support
The skeleton provides the framework which supports the body and maintains its shape. The pelvis, associated ligaments and muscles provide a floor for the pelvic structures. Without the rib cages, costal cartilages, and intercostal muscles, the heart would collapse.
Movement
The joints between bones permit movement, some allowing a wider range of movement than others, e.g. the ball and socket joint allows a greater range of movement than the pivot joint at the neck. Movement is powered by skeletal muscles, which are attached to the skeleton at various sites on bones. Muscles, bones, and joints provide the principal mechanics for movement, all coordinated by the nervous system.
Protection
The skeleton protects many vital organs:
- The skull protects the brain, the eyes, and the middle and inner ears.
- The vertebrae protect the spinal cord.
- The rib cage, spine, and sternum protect the human lungs, human heart and major blood vessels.
- The clavicle and scapula protect the shoulder.
- The ilium and spine protect the digestive and urogenital systems and the hip.
- The patella and the ulna protect the knee and the elbow respectively.
- The carpals and tarsals protect the wrist and ankle respectively.
Blood cell production
The skeleton is the site of haematopoiesis, the development of blood cells that takes place in the bone marrow.
Storage
Bone matrix can store calcium and is involved in calcium metabolism, and bone marrow can store iron in ferrotin and is involved in iron metabolism. However, bones are not entirely made of calcium, but a mixture of chondroitin sulfate and hydroxyapatite, the latter making up 70% of a bone.
Endocrine regulation
Bone cells release a hormone called osteocalcin, which contributes to the regulation of blood sugar (glucose) and fat deposition. Osteocalcin increases both the insulin secretion and sensitivity, in addition to boosting the number of insulin-producing cells and reducing stores of fat.[5]
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