Skeletal System

The muscular system generates force, and the skeletal system is the mass against which the muscles contract to produce motion. The endoskeleton serves as a framework within all vertebrate organisms. In this framework, skeletal muscles are attached to bones, permitting movement when a muscle contracts by bringing two bones together. The endoskeleton also provides protection, since bones surround delicate vital organs. For example, the rib cage protects the thoracic organs (heart and lungs), while the skull and vertebral column protect the brain and spinal cord.

Cartilage

Cartilage, although firm, is not as hard or as brittle as bone. It makes up the skeletons of lower vertebrates, such as sharks and rays. In higher animals, cartilage is the principle component of embryonic skeletons, and is replaced during development by bone.

Bone

Bone makes up the skeleton of mature higher vertebrates, including humans. Bone arises through the replacement of cartilage or through direct ossification. Bone produced through ossification is called dermal bone. The bones of the skull are examples of this. In replacement bone, on the other hand, osteoblasts replace the cartilage that has already formed. As a result, a hollow cavity is formed within each bone. This cavity is subsequently filled with bone marrow (the site of formation of blood cells).

While the division between dermal and replacement bone is based on embryologic origin, the division between spongy and compact bone is based on function and internal structure. Spongy bone is located in the central body portions of bone. It consists of a network of hard spicules separated by marrow-filled spaces. The low density and ability to withstand lateral stress that are characteristic of bone may be attributed to this type of spongy bone.

Compact bone, located on the outer surfaces and articular surfaces, is responsible for the hardness of bone and its ability to withstand longitudinal stress. It consists of cylindrical units called Haversian systems. Each unit consists of cells embedded in a matrix of inorganic material (cacium phosphate) which directly leads to the hardness of bone. These cells radiate around a central capillary within a Haversian canal.

This animation (Audio - Important) describes the structure of bone.

Bones are held together by sutures or immovable joints, such as those in the skull, or by movable joints like the hip joint. In the latter type, ligaments serve as bone-to-bone connectors, while tendons attach skeletal muscle to bones and bend the skeleton at the movable joints.

In the vertebrate skeleton, the axial skeleton is the midline basic framework of the body, consisting of the skull, vertebral column, and the rib cage. The appendicular skeleton, on the other hand, includes the bones of the appendages and the pectoral and pelvic girdles.

This animation (Audio - Important) describes the structure of the human skeletal system.

REVIEW: The human axial skeleton includes all of the following EXCEPT
a. skull.
b. ribs.
c. pectoral girdle.
d. sternum.
e. vertebral column.
Answer pectoral girdle.