7.2 Divisions of Human Skeleton

The human skeletal system is composed of 206 bones, which are categorized into two primary divisions: the Axial Skeleton and the Appendicular Skeleton.

Axial Skeleton

The axial skeleton forms the central axis of the body. It consists of the bones of the skull, vertebral column, and rib cage.

Head Bones (Skull)

The head contains a total of 29 bones, organized into four groups.

Vertebral Column

The vertebral column consists of 33 vertebrae organized into five regions. It protects the spinal cord and supports the head and body.

Neuron→

Note: The sacral and coccygeal vertebrae are sometimes referred to as the pelvic vertebrae.

Rib Cage

The rib cage consists of 12 pairs of ribs that articulate posteriorly with the thoracic vertebrae and the sternum (breastbone) anteriorly. It protects the heart and lungs.

Mechanism of Ventilation→

Appendicular Skeleton

The appendicular skeleton includes the bones of the limbs (appendages) and the girdles that attach them to the axial skeleton. It consists of 126 bones.

Pectoral Girdle and Upper Limb

• Pectoral Girdle: Connects the upper limb to the axial skeleton.

  • Clavicle (collar bone): Articulates with the sternum and scapula.
  • Scapula (shoulder blade): A flat, triangular bone.

• Upper Limb (Forelimb):

  • Humerus: Long bone of the upper arm.
  • Radius and Ulna: Bones of the forearm. The radius is on the thumb side.
  • Carpals: 8 short bones forming the wrist.
  • Metacarpals: 5 bones forming the palm.
  • Phalanges: 14 bones of the fingers (3 in each finger, 2 in the thumb).

Pelvic Girdle and Lower Limb

• Pelvic Girdle: Connects the lower limb to the axial skeleton.

  • Composed of two hip bones (coxa), each formed by the fusion of three bones: ileum, ischium, and pubis.
  • The two halves join at the pubic symphysis.
  • The acetabulum is the socket that articulates with the head of the femur.

• Lower Limb (Hind Limb):

  • Femur: Thigh bone; the longest and strongest bone in the body.
  • Patella: Kneecap.
  • Tibia and Fibula: Bones of the lower leg. The tibia (shin bone) is the larger bone.
  • Tarsals: 7 bones forming the ankle.
  • Metatarsals: 5 bones forming the sole of the foot.
  • Phalanges: 14 bones of the toes (3 in each toe, 2 in the big toe).

Possible Questions and Answers

Q: What are the two main divisions of the human skeleton? A: The axial skeleton (central axis: skull, vertebral column, rib cage) and the appendicular skeleton (limbs and girdles).

Q: Which bone is unique because it does not articulate with any other bone? A: The hyoid bone, located at the base of the skull below the tongue.

Q: What is the difference between true, false, and floating ribs? A:

• True ribs (1-7): Attach directly to the sternum.
• False ribs (8-10): Attach indirectly to the sternum via shared cartilage.
• Floating ribs (11-12): Do not attach to the sternum.

Q: What three bones fuse to form the coxa (hip bone) of the pelvic girdle? A: The ileum, ischium, and pubis.

Summary

• The human skeleton consists of 206 bones, divided into the axial and appendicular skeletons.
• The axial skeleton forms the body's central axis and includes the skull, vertebral column, and rib cage.
• The appendicular skeleton consists of the pectoral and pelvic girdles, and the bones of the upper and lower limbs, facilitating movement.

Biological Significance: The skeletal system provides a rigid framework for support, protects internal organs (such as the brain, heart, and lungs), allows for movement through articulation with muscles, stores minerals, and is the site of blood cell formation (hematopoiesis).