Structure Of The Human Skeleton

Description of the structure of the human skeleton

The skeleton determines your form. Its bony framework affords a scaffold upon which the muscles work, providing many levers for movement. It houses and protects important organs. It serves as a storage place for minerals, and within its narrow cavities are factories for making red and white corpuscles. By weight the skeleton makes up about l8 per cent of your body.

The skeleton makes up about 20 per cent of the weight of your body. It includes some 200 bones held together by ligaments and acted upon by your skeletal or voluntary muscles. Without your skeleton there would be little movement, since your bones act as levers on which the muscles pull. A baby is born with more bones. Some of the bones that are separate in a child become fused, or joined as they become an adult. Bones will range in size from the tiny bones of the middle ear to the body's largest and strongest bones, the thighbones. Some bones are long and cylindrical and others are short, flat, or knobby. Movements will frequently mean survival as we have to move to adapt to our surroundings. There are many movements caused by skeletal muscle varying in complexity from the blink of an eye to the movement of a runner.

Important organs like the skull, thorax, and spine are protected by bones. Bones have cavities of red marrow wherein red blood corpuscles are manufactured, and many white corpuscles, as well. Bone is of the most highly specialized forms of connective tissue. Connective tissue is the most abundant and widely distributed tissue in the body. It also exists as strong and tough cords, rigid bones and even in the form of blood. The matrix of bone is hard and calcified. It forms numerous structural building blocks called osteons (AHS-tee-onz) or Haversian (ha-VER-shan) systems. When bone is viewed under a microscope, we can see these circular arrangements of calcified matrix and cells that give bone its characteristic appearance. Bones are a storage area for calcium and provide support and protection for the body.

The bones that make up your skeleton are made of living tissue in the form of modified connective tissue, and contain both organic and inorganic substances. As you age the relative proportions of organic to inorganic substances will vary. As you age you have relatively more organic material in your bones, therefore they are softer and can be bent some without breaking. As you age your bones take in more minerals and become stiff and brittle. More of your bone cells are unable to obtain nourishment by the surrounding minerals and die.

The outer part of a bone is hard and is called compact bone. The inside of the bone is called spongy bone as there are bony plates that are arranged in a sort of network. The spaces within the spongy bone are filled with marrow. Marrow during prenatal and infancy is all red then later it is replaced by yellow marrow that is mostly fat. Since only the red marrow forms red corpuscles the production of the latter is restricted.

Bone is living tissue so it must be nourished. Each bone is covered with a rough, fibrous membrane called the periosteum that carries a supply of blood vessels and nerves. As the bone is growing the inner layer of the periostium has many bone forming cells. This causes the capacity of the membrane to produce new bone for replacement and for knitting broken bones. The blood vessels are carried into the interior of bone by extensions of connective tissue thorough small canals. As you age, the blood vessels become smaller and less numerous. When you are young you need special nourishment for your growing bones and when older you need to have a healthy diet to keep your bones healthy.

The axial skeleton is made of bones of the skull, the chest, and the spine. The bones of the axial skeleton protect the body's vital organs, the brain, spinal cord, heart, and lungs. The skull consists of 28 bones, 22 of them form the head. The other six bones, three in each ear will carry sound waves from the eardrums to the brain. The cranium is a part of the skull that protects the brain. There are 8 bones that fit together at immovable joints to form the cranium. The shape of the joints is zigzagged to make them stronger. At birth the bones in the cranium are not joint as there are spaces between them to allow the baby to fit through the birth canal. The cranium does not completely fuse until about two years of age. There are sinuses that are hollow air passages in the skull and are connected to the throat and nose. The sinuses will lighten the weight of the skull, act as echo chambers and will help to warm and moisten inhaled air.

The backbone, or the spinal column, is composed of 33 small bones called the vertebrae. The spinal column protects the spinal cord. Between the vertebrae, cartilage discs act as cushions to absorb shock. The spinal column is divided into regions. There are seven cervical vertebrae found in the neck area to hold up the head. These certival vertebrae give the neck greater flexibility and allow the head to move in different directions. Below the neck there are twelve thoracic vertebrae to help support the frame of the chest. The five lumbar vertebrae are in the lower back, which is the major weight-bearing area of the body. The lower back has the biggest and strongest vertebrae of the spinal column.

The sacrum consists of five vertebrae fused in the hip area that are joined to the hip bones on either side. The sacrum and the hip bones form the pelvis. The sacrum connects the spine to the pelvis. The sacrum is the coccyx, also called the tailbone. The coccyx has three to five fused vertebrae.

A cage is formed by the bones of the chest to protect the heart and lungs. It expands and contracts as you breathe. The sternum, or breastbone, is a flat bone in the middle and front of the chest and serves as an anchor for the ribs and muscles that help in breathing. It also protects the heart. There are twelve pairs of ribs that are all not the same. In the back, the ribs connect to the thoracic vertebrae, but in the front their attachments vary. The top seven ribs have cartilage at their ends that connect directly to the sternum. The three pairs of ribs below them are not directly connected to the sternum. These three pairs are connected to cartilage that extends up to the sternum. The bottom two pairs of ribs are attached only in the back and not to the sternum.

The appendicular skeleton consists of the appendages or limbs that connect to the axial skeleton. The 126 bones of the appendicular skeleton include the bones of the hips, leges, and feet plus the bones of the shoulders, arms, and hands. This skeleton gives the body a wide range of movement. On each side of the body a collarbone extends from the top to the sternum to the shoulder blade. The pectoral girdle is formed by the collarbones and the shoulder blades. The pectoral girdle provides the connecting points for the arms to the axial skeleton.

There is only one bone in the upper arm and it fits directly into the shoulder blade. At the other end two bones meet forming the forearm at the elbow joint. There are eight small bones of the wrist that are angled and arranged in two rows. This gives the wrist flexibility. These wrist bones connect with five bones in the hand that form the frame of the palm. In each finger there are three bones, except the thumb that has only two bones. The joints in the fingers and thumb are called knuckles.

The pelvis or bones of the hip, form a strong ring that balances the weight of the body on the legs. It serves as protection for most of the abdominal organs, especially the reproductive organs. These bones are tightly fused so they are usually considered just one bone. The hip bones fuse soon after birth. The structure of the pelvis will vary in men and women. In the male the pelvis is narrow and deep with a small opening. In the female the pelvis is broad and shallow with a large opening that allows a baby to pass through during birth.

The biggest, strongest and heaviest bones in your body are the thighbones as they must support the body. Each of the thighbones extend from the pelvis to the knee. Below the knee, two smaller bones share the body's weight. The larger of the two is located on the inside of the leg, while the smaller is on the outside. The larger bone carries most of the weight coming from the thighbone and transfers it to the foot. The smaller bone allows the ankle a wide range of movement. The kneecap is held in place by tendons from the muscles around it. The structure of the foot is somewhat like that of the hand, but is stronger and more rigid. Five bones form a frame for the top of the food connecting with the toe bones. The big toe, like the thumb, has only two bones. The other toes have three bones each, just like the fingers. The large bones of the foot and the smaller bones of the toes absorb the shock of walking.

Joints are the place at which two bones meet with varied construction to move or to prevent movement. These joints are crossed by ligaments and tendons. Ligaments are thick cords of white fibers that bind bones of one other. Tendons are bands of white fibers that connect muscles to bones. They are both made of collagen

There are three kinds of joints: freely movable joints, partially movable joints, and immovable joints. Most joints are freely movable. Of the freely movable joints there are four kinds. The free moving joints consist of the hinger joint, the pivot joint, the gliding joint, and the ball-and-socket joint. The elbow and the knee joints are examples of hinge joints. The two bones in the forearm meet to form a pivot joint. Gliding joints allow bones to slide over one another. A ball-and-socket joint is formed when the ball-shaped end of one bone fits into a cup-shaped section of the bone joining it.

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