6.1. The skeleton system

Human appearance is determined by 3 systems of the body. These systems make up the bony framework, the muscular bulk, and the outer surface covering the body. These 3 systems provide more than appearance. Bones and muscles support the body, protect vital internal organs, and allow for movement. The skin prevents harmful organisms from entering the body, and also protects internal tissues by covering the body.

The human skeleton is a remarkable system. Its materials are strong and light. Bone is as strong as strong as cast iron but several times lighter and considerable more flexible. The skeleton’s design is simple and efficient. Many bones are hollow cylinders, a shape that provides the greatest strength while using the least amount of material.

Functions of the skeleton

The skeleton serves several vital functions. Along with muscles, the skeleton makes possible a wide range of movements. It supports the body and protects internal organs. Bones store calcium and phosphate, which are taken up by the blood when needed. Also, tissue called marrow inside some bones products red and white blood cells.

Structure of the skeleton

The adult human skeleton is an endoskeleton, or internal skeleton, consisting of about 206 bones as well as connective tissues called cartilage and ligaments. The skeleton has 2 main divisions – the axial skeleton and the appendicle skeleton.

The axial skeleton forms the body’s central framework of support and protection. It consists of 80 bones in the skulls, face, vertebral column, and rib cage. The scull protects the brain. In the adult 26 irregularly shaped bone called vertebrae make up the vertebral, or spinal, column, which holds the body upright and protects the spinal cord. The vertebral column has 5 regions: cervical, thoracic, lumbar, sacral, and coccygeal. The rib cage consists of 12 sets of ribs and the sternum, or breastbone. These bones protect the heart, lungs, and other organs in the thoracic cavity. Each of the ribs is attached to the vertebral column. 7 pairs of ribs, called true ribs, are also attached to the sternum by cartilage. Other 5 pairs do not attach to the sternum and are therefore called false ribs.

The appendicular skeleton consists of 126 bones in the pectoral girdle, the pelvic girdle, and the arm and legs. The pectoral girdle – the bones of the shoulder area – provides support for the arms and allows them a wide range of movement. The pelvic girdle - bones of the hip area – attached directly to the lower part of the vertebral column.

Structure of bones

Bones are classified according to their shape. A bone’s shape is closely related to its function. For example, long bones in the arms and legs support weight and are involved in movements such as walking and lifting. Flat bones, such as the sternum and skull, have a large surface area that protects the underlying organs. The short bones of the wrists and ankles allow great flexibility and precise movements.

Bones consist of living and nonliving materials. The living cells that make up the bone are called osteocytes. Osteocytes are embedded in a network of tough protein fibers called collagen. The nonliving part, the mineral portion, consists mainly of compounds containing calcium and phosphorus that surround the osteocytes and make bones hard. A protective fibrous membrane, the periostenum covers all bones and helos commect them to muscles. The middle portion, called the shaft, is composed of a central cavity surrounded by hard bony materials – compact bone. Small channels, known as Haversian canals, run through this compact bone. The central cavity in long bones is filled with yellow marrow, which stores fat. The shaft is separated from the end of the bone by an epiphyseal line, which marks the area where growth formerly took place. Under the thin in flat bones and at the ends of long bones, the hard material is spongy bone, which consists of tough material that resists shearing forces. In certain parts the spongy bone contains red marrow that is soft and spongy.

Development of bones

During early embryonic development, the skeleton consists of only cartilage and layers of membrane. During the second month of development, the cartilage starts to be replaced by bone through the process called ossification. During ossification bone cells replace cartilage cells, and calcium compounds from the blood are deposited around the cells.

Portions of the skull ossify after birth. The spaces between bones, called fontanels, are covered by a tough membrane. This membrane ossifies over 2-year period after a child is born.

A person grows as bones lengthen. In long bones, growth takes place at both ends of the bone in regions called epophyseal plates. An epiphyseal plate is a layer of cartilage that contains cells that undergo mitosis. Divisions of these cells increase the amount of cartilage, and the length of the born increases.

Joints

Because bones do not bend, movement can occur only when bones meet. The point, where 2 or more bones meet, is called a joint. There are 2 kinds of bones: movable and immovable. A joint that permits movement is a movable joint. Some of them are full movable, some are partly movable. Immovable joints exist in bones that are fused together, as in the skull. The body has 4 major types of movable points. Hinge points, such as knees and knuckles, allow forward and backward movement. Pivot joints, such as where the skull joins the vertebral column, permit a rotating movement. Ball-and-socket joints, such as a hip, allow the widest possible movement. Gliding joints in the wrist and ankle allow sliding movement.

Cartilage and a special lubricant called synovial fluid keep joints moving smoothly. Bones are held together at a movable joint by ligaments, which are strong bands of connective tissue.

Athletes and other active people frequently dislocate or sprain joints. Stretching or learning ligaments causes a sprain. Joints may also become swollen and painful in a condition called arthritis. The most painful and crippling type is rheumatoid arthritis, in which cartilage becomes inflamed and enlarged. Eventually it is replaced by bone, which fuses and prevents movement. In osteoarthritis, which is common among elderly people, cartilage wears away, and the bones rub together.