The Skeleton

I was just going to call this post "bones," as the chapter in the book that I'm reading is also called "bones," but I didn't want to give too many opportunities for the more sick-minded to make jokes about it. Then again, maybe I'm the one who's sick-minded for thinking about this enough to censor it out.

Anyway.

First I'm going to talk about the structure of the bones that make up the skeletal system and talk a bit about what the skeletal system does as a whole before throwing the names of different bones at you.

What's in a Bone?

Bones are made up of several different materials, including connective tissues, cartilage and marrow.

The long shaft of a typical long bone is called the diaphysis. It's made up of a hollow cylinder of compact bone (which is a type of connective tissue) surrounding the yellow bone marrow cavity, which is a fat storage site.

The ends of bones are called the ephiphyses (singular: epiphysis). They have compact bone on the outside, but on the inside they have spongy or cancellous bone, which is more porous than compact bone and thus contains many large spaces. These spaces are filled with marrow, which may or may not be red bone marrow, depending on the bone. Red bone marrow is where many blood cells are produced (see my post titled "Blood" for more info about the different types of blood cells).

The outer surface of the bone is covered by a dense, white, fibrous covering known as the periosteum. The epiphyses are also capped by thin layers of articular cartilage (I'm assuming it's called that because it helps the bones move in the joints or something).

Although you might not think of bones as being alive, bones do carry living cells which carry out various processes, such as growth and repair. These cells are located in a bony matrix (which is known as lamella- plural, lamellae) which is full of non-cellular material, including inorganic salts. These inorganic salts are part of what gives bone its strength.

The lamellae are arranged differently depending on the type of bone. Spongy bone consists of trabeculae, an irregular arrangement of thin, bony plates. The spaces in the trabeculae contain bone cells (osteocytes), nerves and blood vessels. In compact bone, however, there are many structures known as Haversian systems, which are arranged parallel to the long axis of the bone, providing maximum strength. Each Haversian system consists of a Haversian canal, which contains at least one capillary, and possibly nerves and lymph capillaries as well. The canal is surrounded by concentric layers of lamellae. Between the lamellae are small spaces called lacunae (singular: lacuna) which hold the bone cells. Small canals known as canaliculi run between the lacunae, allowing materials and so forth to be passed from cell to cell.

Functions of the Skeleton

The skeleton, aside from providing support and stopping us from going all floppy, also has a variety of different functions:

1. Movement: the muscles can attach to the bones, allowing bones that articulate to move relative to each other. (Articulation is basically the positioning of the bones that allows them to move- I'm assuming this basically means joints and so on.)
2. Protection of vital organs
3. Storage areas for mineral salts and fat. Mineral salts stored here (such as calcium, phosphorus, sodium and potassium) can then be distributed to other regions of the body via the circulatory system.
4. Blood cell production (in bones containing red bone marrow).

Now that's all out of the way, time to throw the names of bones at you! YAAAAAAAAAAYYYYYYYYYYYYYYYY!! not. (Would probably be more fun for everyone involved if I just threw the actual bones at y'all. Never mind.)

There are 206 bones of the skeleton, but I'm not going to give you the names of all of them for two reasons: 1) I'm merciful like that and 2) I can't be bothered looking up the names of all of them. I'm just going to talk about the major bones in the two main parts of the skeleton-

Oh yeah, I should probably provide a quick note on that first. The axial skeleton is the name given to the bones around the central "axis" of the body- that is, the skull, the backbone and the ribs. The appendicular skeleton consists of the bones that make up the limbs as well as the shoulders and hips.

Here we go...!

The Axial Skeleton's Main Bones

• Skull: The cranium of the skull is made up of a number of bones all fused together. In fact pretty much all of the bones of the skull are fused together without moveable joints- the only exception is the mandible, which is the bone that makes up the lower jaw. (For curious people out there- the bone that forms the upper jaw, as well as part of the mouth, eye sockets and nasal cavities, is called the maxilla.)
• Vertebral column: The vertebral column consists of many smaller bones called vertebrae. The top 7 are called the cervical vertebrae, followed by 12 thoracic vertebrae (which are attached to the ribs), 5 lumbar vertebrae, and then the sacrum (which normally consists of 5 vertebrae) and coccyx. There are openings between vertebrae to allow spinal nerves to pass through to various parts of the body.
• Thorax (chest): The thorax is formed by the sternum (breastbone), as well as ribs, costal cartilages and the aforementioned thoracic vertebrae. There are 12 pairs of ribs, which are joined at the back with the corresponding thoracic vertebrae. Some ribs are called "true ribs" because they are directly attached to the sternum by some costal cartilage; some are "false ribs" because they are not directly attached to the sternum by cartilage (instead, the cartilage meets up with another bit of cartilage which then attaches to the ribs); while others are "floating ribs" (don't attach to the sternum at all).

The Appendicular Skeleton's Main Bones

There are a lot here, so I'm going to section this bit up.

Pectoral Girdle (Shoulder Girdle)

The pectoral girdle only has two bones on each side: the scapula (shoulder blade) and the clavicle (collar bone). The clavicle attaches to the sternum and holds the shoulder away from the rib cage, giving somewhat weak support but allowing a range of movement.

The Upper Limb (Arms)

The upper arm bone is called the humerus. The lower arm has two bones- the ulna and the radius. The ulna forms the point of the elbow and joins the wrist on the small-finger side, while the radius joins the wrist on the thumb side and forms the wrist joint. The wrist has 8 carpals arranged in two rows of four, the palm has metacarpals while the finger bones are known as phalanges. There are three phalanges per finger except for the thumb, which only has two.

The Pelvic Girdle (Hip Girdle or Pelvis)

The two main bones of the pelvic girdle are called the pelvic or hip bones. They are joined at the front by a cartilaginous joint called the symphysis pubis and are joined at the rear by the sacrum. Each pelvic bone also has a socket known as the acetabulum, which forms part of the hip joint.

The Lower Limb (Legs)

The upper leg bone is called the femur. The knee joint is protected by a triangular bone called the patella, or kneecap. The lower leg is made up of the tibia and fibula. The tibia is larger, allowing it to bear a greater proportion of weight. The fibula, on the other hand, is quite slender and articulates with the tibia at the hip joint and with one of the ankle bones at its lower end. The ankle consists of seven tarsals. One of these is the talus, which is the only ankle bone to articulate with the fibula and tibia. Another is the calcaneus, which is the heel bone, and is the largest of the ankle bones. The foot contains metatarsals which then lead into the phalanges which make up the toes. Just like in the fingers, there are three phalanges for each toe except for the big toe. (Though given how small and inflexible my little toe is, I seriously doubt there are three bones in there. Um.)

Now, in order to wrap up this post about the skeleton, I'm going to keep up the tradition with the "things that can go wrong with this part of the body" section. Yay!

Stuff That Can Go Wrong

• Bone fractures- occur for a variety of reasons and normally require months to heal properly. Bone-forming cells called osteoblasts help the bones to heal. They are stimulated when the bone is used for support and movement, and thus prolonged immobilisation of a bone may be detrimental to healing (according to this book, anyway. If you break a bone, don't listen to me, listen to your doctor).
• Osteoporosis- A gradual reduction in the rate of bone formation while the rate of bone absorption remains normal. This results in the bones becoming porous, fragile and relatively easily broken. Happens for a variety of reasons, including a decrease in sex hormones, calcium deficiency, vitamin D deficiency, inactivity, and so on.
• Spina bifida- A birth defect where the spine fails to join together at the back, resulting in a gap that membranes and parts of the spinal cord may push through.
• Rickets- A condition in which the bones are soft. Can be caused by lack of vitamin D (which is required to synthesise a protein that transports calcium into the extracellular fluid).

Okay that's it from me, unless I feel like writing a post about joints later tonight (highly unlikely at this stage though). TTFN!