The Skeletal System
The human skeleton is a magnificent feat of engineering. The primary purpose of the skeleton is to carry the weight of the body and to support and protect the internal organs. The skeleton must be strong and be able to absorb reasonable amounts of shock without fracturing. At the same time, the body framework must be flexible and light enough in weight to permit movement. Skeletal bones move in response to muscles that work like levers, allowing a variety of movements such as walking, running, hopping, sitting, bending, lifting, and stooping.
Parts of the Skeleton:
The human skeleton is divided into two major parts:
a) The Axial Skeleton: The skull, the thorax (rib cage) and the vertebral, or spiral, column are the three regions of the axial skeleton.
Skull:
All the bones of the head compose the skull. The two regions of the skull - the cranium and the face - are made up of 22 flat and irregularly shaped bones. Eight bones form the cranium, which functions in the protection of the brain. The facial region, designed to protect the eyes, nose, mouth, and ears, is composed of 14 bones. The sinuses are air spaces in the facial bones which aid in reducing the weight of the skull. The bones of the middle ear that function in transmitting sound to the inner ear are the smallest bones in the body - namely, the hammer, anvil, and stirrups.
Vertebral Column:
The vertebral column is composed of 26 bones known as vertebrae. At birth the vertebral column consists of 33 bones: seven cervical (neck) vertebrae, twelve thoracic vertebrae, five lumbar vertebrae, five sacral vertebrae, and four coccygeal (tail) vertebrae, but the five sacral bones fuse into one large triangular bone - the sacrum - at the back of the pelvis and the four coccygeal bones fuse into a single coccyx.
The vertebral column is the backbone made flexible by the cartilage and ligaments that join the individual vertebrae. Such a flexible backbone permits movement of the head and bending of the trunk. Of major importance is the backbone's function in the protection of the spinal cord, which extends downward from the brain through the opening in each vertebra. Nerves branching from the spinal cord radiate to all parts of the body through openings in the sides of the vertebrae. The discs of the cartilage that separate the individual vertebrae. These discs prevent friction due to the rubbing of the bones and serve as shock absorbers.
Thorax:
Just below the neck are 12 pairs of ribs that are attached to the vertebral column. The general shape of the thoracic basket, or rib cage. The first ten pairs of the ribs are attached to the breastbone, known also as the sternum, by cartilage strips, forming a structure that is smaller on top. The loose connections of the ribs to the vertebral column and the flexible cartilage connections at the sternum allow the ribs to move when the lungs are inflated. The 11th and 12th pairs of the ribs are often referred to as "floating ribs" because they are attached to the vertebral column but not to the breastbone.
b) The Appendicular Skeleton:
The arms and hands, the legs and feet, the bones of the shoulder and the pelvis make up the appendicular skeleton. You probably realize that "appendicular" is the adjective of the word appendage. An appendage is an attachment to a main body or structure. Legs and arms are attachments to the axial skeleton. The sites where arms and legs are attached to the axial skeleton are bones referred to as girdles.
The pectoral girdle where the arms are attached is composed of the scapula (shoulder blade), a large triangular bone, and the clavicle (collarbone), a smaller served bone. The legs are attached to the pelvic girdle, which is formed by the fusion of three bones: the ilium, the ischium, and the pubis on each side of the middle of the body.
The bones of the legs and arms are appropriately called the long bones. The femur, the long bone between the hip and the knee is the longest and strongest bone in the body, supporting the weight of the body. The long bones in the lower leg are the thinner fibula and the thicker tibia.
Between the shoulder blade and the elbow is the bone of the upper arm, the humerus, a thinner version of the femur. The two long bones of the lower arm are the radius and the ulna. Finger and toe bones are known as phalanges; the bones of the foot, as metatarsals, the bones of the hand, as metacarpals.
In general, long bones are shaped like tubes with rounded processes at the ends which are designed to the fit into other bones to from joints. The ends of the long bones are filled with spongy bone, a structural device which makes the bones light in weight but strong. The open spaces in spongy bone are filled with red marrow, while yellow marrow fills the shaft of the long bone.
Bone marrow is an important substance. Red marrow in the spongy areas of the long bones and in the ribs and vertebrate is the site where red blood cells are produced at the rate of millions per minute. The yellow marrow in the shaft of the long bones contains mostly fat. However, when the blood-making capability of the red marrow is low, yellow marrow is somehow converted into red marrow.
