Chapter 4: Osseous system

Introduction:

  • The human skeleton is the internal framework of the human body. It is composed of around 270 bones at birth – these total decreases to around 206 bones by adulthood after some bones get fused together. The bone mass in the skeleton makes up about 14% of the total body weight. 
  • The skeleton is the bony framework of the body. 
  • It forms the cavities and fossae (Shallow depressions or hollow parts) that protect some important body structures, forms the joints and gives attachment to the skeletal muscles.

Functions of Bones:

  • Protection of soft organs.
  • Support of the body (framework)
  • Storage of minerals and fats (calcium)
  • Blood cell formation.
  • Provide the framework of the body. 
  • Give attachment to muscles and tendons.
  • Permit movement of the body as a whole and of parts of the body, by forming joints that are moved by muscles. 
  • Form the boundaries of the cranial, thoracic and pelvic cavities, protecting the organs they contain.

Divisions of Skeletal System:

  • Skeletal system contains 206 named bones which are divided into two main divisions as follows,
    • The Axial Skeleton (80 bones)
    • The Appendicular Skeleton (126 bones)
  • The Axial Skeleton:
    • Skull.
    • Vertebral column.
    • Sternum or breastbone.
    • Ribs. 
  • The Appendicular Skeleton:
    • The upper limbs and the shoulder girdles.
    • The lower limbs and the innominate bones of the pelvis.

Bone and Types of Bones:

  • Bone is a strong and durable connective tissue.
  • It is made up of water, osteoid, bone cells constituting 50% part and remaining 50% of minerals like Calcium phosphate and Magnesium phosphate.
  • Bones are classified into 5 types as,
    • Long Bones.
    • Short Bones.
    • Irregular Bones.
    • Flat Bones.
    • Sesamoid Bones.
  1. Long Bones:
  • These consist of a shaft and two extremities. 
  • As the name indicates the length is much greater than the width. 
  • Supports weight and facilitates movements.
  • e.g. femur, tibia and fibula. 
  1. Short Bones:
  • Length is almost the same as width. 
  • Located in the wrist and ankle joints, these bones provide stability and some movement.
  • e.g. Carpals in the wrist, tarsals in the ankles.
  1. Irregular Bones:
    • As the name indicates they vary in shape and structure.
    • They have a complex shape and protect the internal organs.
    • e.g. the vertebrae.
  2. Flat Bones:
    • They are flattened bones and also called “Sutural Bones”..
    • Provide protection like shield and a vast area for muscle attachment.
    • e.g. Skull bones, Sternum, Ribs, Pelvis etc.
  3. Sesamoid Bones:
    • These are bones embedded in tendons. 
    • These small, round bones are commonly found in the tendons of the hands, knees, and feet. 
    • Their function is to protect tendons from stress and wear. 
    • e.g. patella (kneecap).

Bone cells:

  • The cells responsible for bone formation are osteoblasts (these later mature into osteocytes). 
  • Osteoblasts: 
    • These are the bone-forming cells that secrete collagen and other constituents of bone tissue. 
    • They are present: 
      • in the deeper layers of periosteum.
      • in the centres of ossification of immature bone.
      • at the ends of the cartilages of long bones.
      • at the site of a fracture. 
    • Osteocytes:
      • As bone develops, osteoblasts become trapped and remain isolated in lacunae. 
      • They stop forming new bone at this stage and are called osteocytes. 
    • Osteoclasts:
      • Their function is resorption of bone to maintain the optimum shape. 
      • This takes place at bone surfaces: 
        • under the periosteum, to maintain the shape of bones during growth and to remove excess callus formed during healing of fractures.
        • round the walls of the medullary canal during growth and to canalise callus during healing. 
  • A fine balance of osteoblast and osteoclast activity is necessary for normal bone structure and functions.