The skeletal system of the body is divided into two main sections: the axial skeleton and the appendicular skeletion. Both of these sections form the structural system of the body. This system also provides protection for the inner organs of the body, storage for fat, and formation of new red and white blood cells.There are four types of bones in the skeletal system. These types are classified by the shape of the bone: long, short, irregular, or flat. All bones are made of living tissue, and on the microscopic level, the structure of bones is always changing.
When a long bone first starts to develop, it is formed of cartilage which is hardened into bone by a process called ossification. The ossification process is divided into two main phases. In the first stage of ossification, the cartilage is covered with a layer of cells called osteoblasts, which form other bone cells. Once this encasement of osteoblasts has formed, the cartilage is slowly eaten away and the bone cells replace the cartilage. These bone cells are arranged in concentric circles, which causes the bone to be very hard. The mature cells, called osteocytes, store the calcium of the body which can be released or extracted from the bloodstream as needed. After the bone completes its formation process, the mature bone is encased in a membrane of connective tissue called the periosteum.
Also, within the structure of a long bone is a series of canals called Haversian Canals and Volkmann's Canals. Haversian canals run vertiacally, while the Volkmann's canals run horizontally. Inside these canals are blood vessels and nerves. These blood vessels provide osteocytes with nourishment while at the same time providing transportation of newly formed blood cells into the bloodstream. The red and yellow marrow in the bones store these new blood cells and fats. The red marrow that contains the new blood cells is contained within the spongy bone that is found in the epiphyses (rounded heads) of the bone. The yellow marrow which contains fats is housed in the medullary cavity found at the heart of the diaphysis, or shaft, of a long bone.
The Axial Skeleton
The axial skeleton is composed of three major parts or regions. These parts are the skull, the vertebral column, and the thoracic region.
The skull:
The outside structures of the skull are composed of a series of flat bones that are fused together at infancy. The major bones of the skull are the frontal bone, the left and right parietal bones, the occipital bone, the sphenoid bone, and the left and right temporal bones. these bones make up most of the skull which encloses and protects the brain. the major bones that make up the structure of the face are the nasal bone, the ethmoid bone, the zygomatic bone and arch, the maxilla, and the mandible.
The vertebral column:
The vertebral column is divided into five main sections. The first seven vertebrae (starting from the top of the spine) are called the cervical curvature. More specifically, the first two vertebrae are called the atlas and the axis because of the pivot joint they provide for the skull. The next section of vertebrae is called the thoracis curvature, which is made of 12 vertebrae. The third section in the area of the lower back is called teh lumbar curvature which is made of five lumbar vertebrae. The fourth section is composed of 5 fused vertebrae called the sacrum. The last section is the coccyx, or tailbone, which ios formed by the fusing of four vertebrae.
The thoracic region:
The thoracic region, more commonly known as the rib cage, is the structure that protects the vital organs of the body. The ribs of the rib cage are divided into three types. The upper series of ribs that extends from the spaces between the thoracic vertebrae of the vertebral column and connect directly to the sternum are called true ribs. The series of three ribs that share a connection of costal cartilage with the sternum are calle false ribs. The two ribs in the lower portion of the rib cage that do not connect to the sternum at all are called floating ribs.
The Appendicular Skeleton:
The shoulder girdle:
The choulder girdle is composed of two main bones on each side of the body. The clavicle is the bone more commonly referred to as the collar bone. The clavicle is an irregularly shaped bone that connects to the sternum and scapula. The scapula is the irregular bone known more commanly as the shoulder blade. The bones of the shoulder girdle allow the arms of the body to have extreme amounts of flexibility and range of movement, but this is achieved only at the sacrifice of strength and stability in the structure.
The upper extremities (arms):
The upper arm is composed of only one bone, the humerus. The humerus has a ball-and-socket joint with the scapula and a hinge joint with the bones of the forearm. There are two bones that compose the structure of the forearm, teh radius and the ulna. The radius forms a radial joint with the humerus which allows the forearm to rotate. The bases of the radius and ulna connect with many short bones called the carpals. The carpals are the bones which make up the wrist and allow us the many unique movements of which the wrist is capable. The bones in the palm of the hand are the metacarpals which attach to the actual bones of the fingers, the phalanges.
The pelvic girdle:
The pelvic girdle is composed of two bones. These bones are called coxal bones. The coxal bones are formed from three separate bones that have been fused together: the ilium, ischium, and pubis. These coxal bones are attached to the sacrum of the vertebral column, and are attached to each other by a joint of cartilage called the pubic symphysis. The angle atwhich these bones are joined depends on the gender of the person. For males the pubic arch is generally less than 90 degrees, while females usually have a pubic arch of 90 degrees or more.
Lower extremities (legs):
The portion of the leg commonly referred to as the thigh is composed of one very large long bone called the femur. The top of the femur joins with the coxal bones in a socket joint, while the lower end of the femur connects with one of the bones of the lower leg, the tibia, in a hinge joint. The two bones of the lower leg are called the tibia and fibula. The lower end of the tibia joins with the talus, which is actually just a fusion of seven tarsal bones. The bone of the heel is called the calcaneous, which actually carries most of the body's weight. The rest of the foot is composed of metatarsals and the bones of the toes, which are called phalanges.
written by Matt Richards
