Human skeleton

Human skeleton

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A human skeleton

The human skeleton consists of both fused and individual bones supported and supplemented by ligaments, tendons, muscles and cartilage. Fused bones include those of the pelvis and the cranium. Osteocytes are present in the bone matrix. Osteoblasts and osteoclasts are present at the edges of bone matrix and are involved with bone matrix production and absorption respectively. Bones all have an arterial blood supply, venous drainage and nerves. The non-articular surfaces of bones are covered with a tough fibrous layer called the periosteum, and the articular surfaces of bones are covered with a smooth layer of articular cartilage. Spaces within some bones, generally the larger bones, are filled with bone marrow responsible for generation of blood cells, known as haematopoiesis.

Early in gestation, a foetus has a cartilaginous skeleton from which the long bones and most other bones gradually form throughout the remaining gestation period and for years after birth in a process called endochondral ossification. The flat bones of the skull and the clavicles are formed from connective tissue in a process known as intramembranous ossification, and ossification of the mandible occurs in the fibrous membrane covering the outer surfaces of Meckel's cartilages. At birth a newborn baby has approximately 300 bones, whereas on average an adult human has 206 bones (these numbers can vary slightly from individual to individual). The difference comes from a number of small bones that fuse together during growth, such as the sacrum and coccyx of the vertebral column. The sacrum (the bone at the base of the spine) consists of five bones which are separate at birth but fuse together into a solid structure in later years. An infant is born with zones of cartilage, called epiphyseal plates, between segments of bone to allow further growth. Growing is usually completed between ages 13 and 18, at which time the epiphyseal plates of long bones close allowing no further growth.

Not all bones are interconnected directly: There are 6 bones in the middle ear called the ossicles (three on each side) that articulate only with each other. The hyoid bone which is located in the neck and serves as the point of attachment for the tongue, does not articulate with any other bones in the body, being supported by muscles and ligaments. The longest and heaviest bone in the body is the femur and the smallest is the stapes bone in the middle ear. In an adult, the skeleton comprises around 20% of the total body weight.

Parts of the human skeleton maintain the ancient segmental pattern that is present in all vertebrates (mammals, birds, fishes, reptiles and amphibians) with basic units being repeated. This segmental pattern is particularly obvious in the spinal column and in the ribcage.

Contents [hide] 1 Function 1.1 Provide shape and support 1.2 Attachment 1.3 Movement 1.4 Protection 1.5 Blood cell production 1.6 Storage 2 Organization 3 Gender differences 4 Diseases 5 See also 6 External links 7 Gallery

[edit] Function

List of bones of the human skeleton

The skeleton has six main functions:

[edit] Provide shape and support

The skeleton provides the framework which supports the body, and maintains its shape. The joints between bones permit movement.

[edit] Attachment

The bones of the skeleton provide an attachment surface for muscles and tendons which together enable movement of the body. Ligaments often connect bones across a joint to provide stability. Microscopic fibres called Sharpey's fibres connect teeth to the bone of their sockets.

[edit] Movement

Movement in vertebrates is dependent on the skeletal muscles, which are attached to the skeleton by tendons. Without the skeleton to give leverage, movement would be greatly restricted.

[edit] Protection

The skeleton protects many vital organs:

• The skull protects the brain, the eyes, and the middle and inner ears.
• The spine protects the spinal cord.
• The ribs, spine, and sternum protect the lungs, heart and major blood vessels.
• The clavicle and scapula protect the shoulder.
• The ilium and spine protect the digestive and urogenital systems and the hip.
• The patella and the ulna protect the knee and the elbow respectively.
• The carpals and tarsals protect the wrist and ankle respectively.

[edit] Blood cell production

The skeleton is the site of haematopoiesis – the generation of blood cells, which takes place in red bone marrow.

[edit] Storage

Bone also serves as a mineral storage deposit in which nutrients can be stored and retrieved. Calcium, especially, can be released by dissolution of bone tissue under the control of 1,25-dihydroxyvitamin D₃ (a form of vitamin D) during periods of low calcium intake.

[edit] Organization

The human skeleton can be divided into the axial skeleton and the appendicular skeleton.

The axial skeleton has five areas and consists of 80 bones in a typical adult (number of bones):

• Skull (22)
• Ossicles (bones of the middle ear) (6)
• Hyoid bone (bone in the throat) (1)
• Vertebral column (26)
• Chest (25)

The appendicular skeleton has six areas and consists of 126 bones in a typical adult:

• Shoulder girdle (4)
• Arms (6)
• Hands (54)
• Pelvic girdle (2)
• Legs (8)
• Feet (52)

[edit] Gender differences

An articulated human skeleton, as used in biology education

There are many differences between the male and female human skeletons. Most prominent is the difference in the pelvis, owing to characteristics required for the processes of gestation and parturition (childbirth). The shape of a female pelvis is flatter, more rounded and proportionally larger to allow the head of the foetus to pass. Men tend to have slightly thicker and longer limbs and digit bones (phalanges), while women tend to have narrower rib cages, smaller teeth, less angular mandibles, less pronounced cranial features such as the brow ridges and occipital protuberance (the small bump at the back of the skull), and the carrying angle of the forearm is more pronounced in females.

[edit] Diseases

The skeleton can be affected by many diseases ranging from minor to serious that can compromise its strength and growth, as well as the mobility of joints. Bone diseases can be congenital or acquired, and bone tumours can be benign or malignant.

Bone cancer can be serious and sometimes treatment may include radical surgery such as amputation of the affected limb.

Various forms of arthritis affect the joints, which can result in joint pain, joint stiffness, joint deformity and joint swelling.

A fracture occurs when a bone is subjected to too much stress, usually by acute trauma. Fractures are divided into "simple", "complex" and "compound". A "simple" or a "complex" fracture indicates that the fractured bone has not broken through the skin. A "compound fracture" indicates that at least one part of a fractured bone has broken through the skin, but this does not necessarily imply that the bone has broken in more than one place.

Osteoporosis, a condition in which there is a reduced content of bone matrix and distorted bone architecture, increases the likelihood of fractures in the elderly in general and in particular post-menopausal women.

Osteomalacia, leading to softening of bones, has many causes including insufficient nutrition or faulty metabolism of vitamin D or calcium.

Osteopetrosis, is a genetic disorder characterised by bone fragility.