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Divisions of the Skeletal System

The adult human skeleton usually consists of 206 bones which can be grouped into two main divisions: the axial and the appendicular skeleton. The axial skeleton consists of the bones of the ribs, the skull and the backbone or vertebral column. The appendicular division contains the bones of the appendages and the girdles which connect the free appendages to the axial skeleton. The axial skeleton consist of 80 bones and the appendicular consists of 106 bones.
(A) (B)

(A) Human Skeleton (Front View)
(B) The Vertebral Column Viewed from the Left Side

The Skull

The skull contains 22 bones and it rests on the superior end of the vertebral column. It consists of two sets of bones: the cranial bones and the facial bones. The cranial bones enclose and protect the brain and the organs of sight, hearing and balance. There are 14 facial bones consisting of nasal bones, inferior nasal conchae and vomer.

Human Skull (Side View)

The Vertebral Column

The vertebral column consists of 33 individual bones called vertebrae. Twenty-four of these are free. The last 9 vertebrae are fused. The first five of these fused vertebrae form the sacrum and the last four are fused to form what is known as coccyx. Five regions can be recognised in the vertebral column, namely
  1. Neck or cervical region,
  2. Thoracic region,
  3. Lumbar region,
  4. Sacral region, and
  5. Coccygeal region.
The cervical region consists of seven cervical vertebrae. The first and second cervical vertebrae are called atlas and axis respectively.

There are 12 thoracic vertebrae in the thoracic region. The lumbar or abdominal region consists of five lumbar vertebrae which are large and heavy.

The pelvic or hip region (sacral region) consists of five sacral vertebrae which are fused to form the sacrum.

The coccygeal region or tail region consists of four small vertebrae which are fused to form the tail bone.

The vertebrae of each of these five regions have characteristic features of their own. However, a typical vertebra has the following structures: 
  1. A ring-like hole (neural canal) through which passes the spinal cord, 
  2. Paired processes (post and pre-zygapophyses), 
  3. Transverse processes and
  4. The body of the vertebrae is called the centrum. In between the two vetebrae is the intervertebral disc. A displacement of this disc from its position results in the slip disc.

Bones of the Chest and Thorax

Bones of the chest region are so arranged as to form a thoracic basket. The thoracic basket encloses and protects the heart and lungs. In the middle of the front portion of the thorax is the sternum or the breast bone. The sides of the thoracic cavity are formed of twelve pairs of ribs. The first seven pairs of ribs are attached to the thoracic vertebrae at the back and to the sternum in the front. The next three pairs are connected to the rib above them. The last two pairs are free and are called floating ribs. This arrangement of ribs allows free movement and expansion and contraction of the chest cavity during breathing movements.

Human Ribs and Sternum

The Girdles

The bones of the limbs are attached to the girdles. The bones of the forelimbs are attached to the pectoral girdle and the bones of the hindlimbs are attached to the pelvic girdle.

The pectoral girdle is made up of two bones
The clavicle (collar bones) and the scapula (shoulder blades). The clavicle is a curved bone, one end of which is attached to the sternum with the other end forming a joint with the scapula. The scapula is a flat triangular bone. It has a circular depression called the glenoid cavity into which fits the head of the humerus.

The Pectoral Girdle of Man with Bones of the Hand

Human Pelvic Girdle

The pelvic girdle consists of two large hip bones. Each hip bone is formed by the union of three bones: the ilium, ischium and pubis. The head of femur fits into the cavity in the pelvic girdle. This cavity is the acetabulum.

Bones of the Hand and the Leg

The forelimb consists of the upper arm, forearm, wrist and hand.

The upper arm is formed by a strong bone called the humerus. The head of humerus articulates with the pectoral girdle and fits in the glenoid cavity. The lower end of humerus articulates with the bones of the forearm.

The forearm is formed of two bones: the radius and ulna. The radius is situated on the outer side and the ulna on the inner side of the forearm.

Bones of the Legs

The wrist of carpus consists of eight small bones called the carpals. These are arranged in two rows. The palm is formed of five long bones called the meta carpals. The fingers are supported by small bones called phalanges. Each finger is supported by three phalanges except the thumb which has only two phalanges.

The thigh is supported by the thigh bone, the femur which is the largest bone of the leg. Its upper end is ball shaped and fits into the socket of the pelvic girdle.

The knee cap is protected by a flat triangular bone called patella.

The lower leg or shank is formed of two bones: the tibia and fibula. The ankle bones are called the tarsals and the foot bones are called meta-tarsals. The toes are supported by the phalanges.

Articulation of Bones
Joints are places of contact between two or more bones. Joints permit differing degrees of movements depending on their structure. There are various types of joints in the human body. Some of them allow free movements whereas some others allow only partial or restricted movements.

Synovial Joints

They are freely movable joints and are present at the elbow, ankle, hip, wrist and knee.

In synovial joints the articular surfaces of the bones are covered with a smooth piece of hyaline cartilage. The space between the ends of the bones is filled with a slippery fluid called synovial fluid. The cavity or space is lined by a membrane, the synovial membrane. The synovial fluid and the cartilage allow free movement of bones at the joints. Synovial joints are classified into various types according to the nature of articulation and the degree of movement they allow.

  Synovial Joint

  1. Ball and Socket JointsIn this type of joint, one of the bones forms a cup-like or socket-like depression into which fit the ball-like head of the other bone. This sort of joint makes movements possible from side to side and rotation. Examples of such joints are shoulder and hip joints.
  2. Hinge jointsThese joints are found in the elbow, knee and fingers. The ends of the bones fit into each other in such a way that movement is possible in one direction only e.g., flexion and extension.
  3. Annular JointsThese are found in the wrist and the metacarpophalangeal joints. In this case movements are possible around two axes.
  4. Pivot JointsExamples of such joints are the joints between the skull and the vertebral column and the joint between radius and ulna. In this type of joints turning or rotatory movements are possible around one axis only.
  5. Gliding JointsJoints between tarsal bones in the ankle, carpal bones in the wrist and between sternum and clavicle, are all examples of gliding joints. In this type the end of one bone glides across a certain portion of the surface of the other bone.
Fibrous Joints
These are immovable or fixed joints. The bones articulating at such joints cannot change position relative to each other. The bones are held firmly and closely together by dense bundles of tough and strong white collagenous fibres. The examples of fibrous joints are joints in tooth sockets and between skull bones.

Cartilaginous Joints
These are slightly movable joints. At these joints the bones are held together by discs of white fibro-cartilage. These cartilages are strong, but more elastic and compressible than the white fibrous tissue. Examples of cartilaginous joints are joints between the vertebrae, at the symphysis pubis and between sternum and ribs. The bones make some movements at these joints through compression of the discs of cartilage.

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