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Two things about the human skeleton set it off from most or all other skeletons: it is built erect, as opposed to walking on four legs, and the hand has an opposable thumb. The skeleton, made up of 206 bones in adulthood, is what gives us the shape we have, and the power to move. The skeleton is divided into two groups: the axial skeleton and the appendicular skeleton, each with it's own purpose.
The axial skeleton, making up 80 of your 206 bones, encompasses all your upper body bones. It itself is subdivided into three groups: the skull, the vertebral column, and the bony thorax. It's main purposes are to protect your vital organs, such as the brain, heart, and lungs, and to provide an efficient structure to preform a variety of work.
The skull, or cranium, could be thought of as the most important structure in your skeleton, especially considering that it houses your brain. Your skull provides the framework for most of your sensory organs, such as eyes, ears, tongue, nose, and some skin.
Your skull is made up of 22 cranial or facial bones, plus the three in each ear. As a baby you have more. Most are fixed joints (all but the mandible) separated by cartilage as a baby, but fuse together a later as you grow. Once fused, they are locked together, forming an immovable joints, called a suture.
The backbone, or vertebral or spinal column, though called a "bone", is really a flexible structure made of 26 bones. As a baby, you have 33 vertebrae, or back bones, but the lower four fuse to form the coccyx, and the next lower five fuse to become the sacrum. The backbone serves several important functions itself. It provides structure from which all other upper body structures branch, and it protects the spinal nerve, which is the "highway" that all the information your brain sends to your body travels. If the spinal column were to be damaging, also damaging the spinal nerve, it would be like the highway collapsed, and all landmarks past that point could no longer be visited, causing paralyzation from that point in your back and down.
The backbone is approximately 28 inches, or 70 cm, long, and is separated into five regions. The last two, the coccyx and sacrum, are separated by the fused vertebrae. The remaining three are distinguished mostly by concavity with respect to your front. The remaining regions are: the cervical curvature made of 7 vertebrae and concave; the thoracic region made of 12 vertebrae and convex, and the lumbar curvature made of 5 vertebrae and concave.
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The Bony Thorax
The bony thorax is basically your chest, comprising your breast bone and ribs. Your breast bone, or sternum, is around 6 inches (15 cm) tall, spanning about half the length of your ribs.
You have twelve ribs, forming the structure for your chest. One primary purpose of your ribs is to protect your lungs are heart. Except for your floating ribs, each rib connects to the sternum by cartilage on the tips. Your top seven ribs are called true ribs because they connect directly to the sternum. The next four ribs are called false ribs because they attach to the sternum so indirectly if at all. If they do not connect to the sternum, they do connect to upper cartilage for support. The last two ribs are called floating ribs because they do not connect to the sternum or any other support on the end.
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The Appendicular Skeleton
The appendicular skeleton refers to your arms and legs. They are called appendicular (from "append") because they are attached by girdles, which bridge each with the main body, as if they had been appended after the main body was formed. These girdles give these appendages a remarkable range of movement unique from anywhere else in the body. Obviously the arms are the same allowing symmetry, and the legs are too. But ignoring size and shape, and instead focusing on joints and relative placement, your arms and legs are the same, too.
The pectoral, or shoulder, girdle connects the arm to the axial body. The scapula, or shoulder blade, and the clavicle, or collarbone, make up the girdle.
The Upper Appendages
The main purpose of the arms is to do work. They are lighter and are made to focus on detail.
In your upper arm is your humerus which connects to the girdle as a ball-and-socket joint. It connects to your forearm with a hinge joint. The forearm is really two bones, the radius and the ulna. The ulna is the bone that joins with the humerus in the elbow joint. Having two bones instead of one allows for a wide range of twisting of the wrists.
The carpus, or wrist of the hand, is made of eight small bones in two irregular rows connect with gliding joints. These eight bones give your wrist the flexibility it has.
Five metacarples extend from the carpus, covered with skin, form the palm. Looking at just the skeleton, they look like the base of really long fingers. The tips of these bones are the knuckles you see when you make a fist. At the beginning of this, the skeletal, topic, one of the characteristics that make the human skeleton unique is the opposable thumb. The first metacarpal, the thumb base, is jointed differently that the rest of the metacarples. They lay in a single plane, while the thumb metacarpal is connected with a saddle joint, giving it a range of movement. This is what makes grasping things as easy as it is.
The rest of the hand comprises the phalanges, what we see as the fingers. Each finger has three phalanges, except for the thumb, which has two.
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The Lower Appendages
As said earlier, the upper and lower appendages are structurally similar. One difference is that the lower appendages are thicker and stronger to support the incredible stress put on them when running and jumping. They are designed mostly for movement
The pelvic, or hip, girdle does the same thing as the upper body girdles. One major difference is that the two girdle in the upper body are replaced by one for the lower body.
The femur is the lower appendage equivalent of the humerus. It's the longest, largest and strongest bone of the body, must be so because of the incredible forces placed upon it. Jumping hard can put as much as two tons of pressure per square inch on it!
The tibia and fibula are the equivalents of the ulna and radius. In the leg, however, the range of movement has been restricted to almost none. The tarsals are like the carpals, the metatarsus like the metacarples, and the phalanges have the same name.
In the leg, the foot meets the leg a right angles, unlike the hand. This is to support our weight and move us in a direction. Though a single bone could replace the foot, have many bones make it adaptable to irregular terrain. The foot is also arched to support the weight it has on it. When it needs to, the curve gives away a little, and then bends back when the weight is taken away. Like the thumb, the big toe has a fairly unique function. Much larger and stronger that the other toes, it supports the weight of our body, especially when we walk. Notice that it is the last part of your foot to leave the ground when we step.
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The bone has five main functions:
Composition of the Bone
The inside of the bone is made of three primal tissue layers: periosteum, compact bone, and spongy bone.
The periosteum is a glistening double layerd tissue which covers the hard bone are called the compaact bone. The periosteum can not be seen by the naked eye. The periosteum is exceedingly important since it's lower layer houses bone forming cells called osteoblast. Also, the double layer serves as a place where tendons and ligaments can insert and anchor into the bone.
Compact bone seems very hard when seen by the normal eye, but at a microscopic level, the compact bone is very hollow. Canals called canaliculi, which are filled with blood vessels, channel through the bone and all join to many major canals called Haversian canals. These canals make the bone hollow. The compact bone is also rich with nerves. The hardness of the bone is made by spider like ostecytes, mature bone cells, which surround the caniculli and Heversian canals.
Spongy bone is not exactly spongy. Towards the center of the bone, the bone gets more hollow. This is why it is called spongy. Located within the spongy bone is red bone marrow nad yellow bone marrow. Red bone marrow is what makes red blood cells. The majority of the red boone marrow for an adult is located in the head of the femeur and hemerus. Yellow bone marrow is stored fat. This marrow can sometimes turn in to red bone maarow when a person is very anemic.
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