Skeleton, Movement, and Locomotion Notes

Skeleton, Movement, and Locomotion

12. SKELETON

Syllabus:
  • Skeleton: Movement and Locomotion
    • (i) Functions of the human skeleton
    • (ii) Axial and Appendicular Skeleton
    • (iii) Types of joints with reference to their location
      • immovable joints
      • slightly movable joints
      • freely movable (hinge joint, ball and socket joint, gliding joint, pivot joint.)

Most animals have a definite shape and the capacity for movement of different body parts, aiding in locomotion (movement from place to place). These functions are performed by the skeleton and muscles together.

12.1 Functions of the Human Skeleton

The skeleton serves six main purposes:

  1. Support and Shape: Provides a framework to all soft parts, giving the body and its parts a definite shape. For example, phalanges (small bones) in the fingers enable holding a pen.

    • Cartilage provides support and shape (e.g., in the ear).

  2. Protection: Bones protect delicate organs. For example:

    • The skull protects the brain.
    • The vertebral column protects the spinal cord.
    • The ribs protect the heart and lungs.

  3. Movement: Bones are joined to each other allowing movement, facilitated by muscles originating on one bone and inserting on another.

  4. Leverage: Bones and joints act as levers, increasing the speed and distance of movement generated by a muscle. For example:

    • Contraction of the biceps muscle swings the hand swiftly through an arc.

  5. Formation of Blood Cells: Certain blood cells, including red and white blood cells, are formed in the marrow of some long bones.

  6. Storage: Bones act as a storehouse of calcium and phosphorus.

Constituents of Skeleton

Our skeleton consists of bones, cartilages, and ligaments.

  • Bones: Form the hard framework of the body.
  • Cartilages: Supporting and connecting structures (e.g., in external ears and the tip of the nose).
  • Ligaments: Bind bones together.

12.2 Bone

Bone is the chief component of our skeleton, consisting of organic and inorganic material.

  • Inorganic Part: Constitutes approximately two-thirds of the bone substance, including compounds of calcium and phosphorus.
    • If a bone is placed in weak hydrochloric acid, the mineral part is removed, resulting in a decalcified bone, which is soft and flexible.
  • Organic Matter: If a bone is strongly heated, the organic matter is destroyed, leaving only the mineral part (ash), which makes the bone brittle.
  • Age: With age, the organic part of the bone reduces, making bones more fragile and slower to heal after a fracture.
Classification of bone on the basis of shape:

Bones are classified based on shape into long, short, flat, and irregular bones.

The structure of a typical long bone (e.g., thigh bone) is highly calcified, hard, and rigid connective tissue.

  • It consists of bone cells (Osteocytes) arranged in concentric rings embedded in a ground matrix.

  • The matrix contains collagen fibers and mineral salts (calcium and phosphorus).

  • Periosteum: The external surface of the bone is covered by a membrane richly supplied with blood vessels.

  • Bone Marrow: A long bone has a hollow cavity filled with bone marrow.

    • Yellow marrow: Made up of adipose tissue and blood vessels, gives rise to white blood cells.
    • Red marrow: Present at the ends and produces red blood cells.

  • Long bones: Consist of a shaft with a knob at each end. Shaft is thick walled, tubular, filled with yellow marrow, ends are spongy e.g., bones of arms, legs, and chest (ribs)

  • Short bones: Box like, spongy bone, show little movement, e.g. ankle, wrist.

  • Flat bones: Composed of two or more parallel plates of compact bone enclosing spongy bone e.g. shoulder blade, skull, sternum

  • Irregular bones: Complex shape, vary in amount of bone tissue, compact and spongy e.g. facial bones vertebral bones.

12.3 The Human Skeleton

The human skeleton consists of 206 bones, including tiny bones in each ear. It has two main divisions:

  1. Axial Skeleton: Includes the basic central framework of the body.
  2. Appendicular Skeleton: Includes the bones of the limbs and girdles.
12.3.1 Axial Skeleton

The axial skeleton consists of the skull, the vertebral column, the ribs, and the sternum.

  • A. Skull: The skeleton of the head, containing two parts:
    • Cranium (Brain-box): Made of eight bones joined together.
    • Face: Contains fourteen bones, including the upper and lower jaws.
    • The back of the cranium has a large hole, the foramen magnum, through which the spinal cord emerges.
  • B. Vertebral Column: Composed of 33 ring-like bones called vertebrae, divided into five groups:
    • Cervical (neck region): 7 vertebrae
    • Thoracic: 12 vertebrae
    • Lumbar: 5 vertebrae
    • Sacrum: 1 (fused 5) vertebrae
    • Coccyx: 1 (fused 4) vertebrae
    • The vertebral column is curved to maintain balance and absorb pressure during movement.
Structure of a Vertebra:

Each vertebra has a ring-like structure with a solid cylinder of bone called the centrum. The spinal cord runs through the neural canal.

  • Atlas: The first cervical vertebra.
  • Axis: The second vertebra.
  • Thoracic vertebrae: Have long neural spines directed backward.
  • Lumbar vertebrae: Have well-developed neural spines and transverse processes.
  • Sacrum: Formed by the fusion of five vertebrae, articulating with the hip bones.
  • Coccyx: Made up of four fused vertebrae, representing the rudimentary tail.
  • C. Ribs: Twelve pairs of ribs along with the thoracic vertebrae and breast-bone constitute the rib cage.
    • True ribs: (first 7 pairs) attached to the sternum with hyaline cartilage.
    • 8th, 9th, and 10th ribs: Join the seventh rib with hyaline cartilage.
    • Floating ribs: (11th and 12th pairs) not attached to the sternum.
  • D. Sternum: A long flat bone lying in the middle of the front part of the chest.
12.3.2 Appendicular Skeleton (Bones of Limbs and Girdles)

A. Bones of the Limbs: Forelimbs and hind-limbs largely correspond to each other.

  • Forelimbs: Humerus (upper arm), radius and ulna (lower arm), 8 carpals (wrist), 5 metacarpals (palm), and 14 phalanges (fingers - 2 in the thumb and 3 in each of the remaining four fingers).
  • Hind-limbs: Femur (thigh), tibia and fibula (shank), 7 tarsal bones (ankle), 5 metatarsal bones (middle of the foot), and 14 phalanges (toes - 2 in the big toe and 3 in each of the other four toes).
    • The hind-limbs include patella (knee-cap), which is joined to the lower end of the femur.

B. Girdles: Articulate the limb bones to the axial skeleton. There are two girdles:

  • Shoulder (Pectoral) Girdle: Consists of two scapulae (shoulder-blades).
    • The glenoid cavity of the scapula articulates with the head of the humerus.
    • The clavicle (collar-bone) is attached to the scapula and sternum.
  • Hip (Pelvic) Girdle: Formed by two hip bones joined to the sacrum.
    • Each hip bone consists of the ilium, ischium, and pubis.
    • The acetabulum is a cavity in which the head of the femur fits.
Difference between Male and Female Skeleton:

Male skeletons are generally larger and heavier. The female pelvis is wider to accommodate the foetus during pregnancy.

Regionwise classification of bones and their number in humans:

  • Axial skeleton
    • Skull(including ear bones): 29
    • Vertebrae: 33
    • Ribs: 24 (12 pairs)
  • Appendicular skeleton
    • Shoulder girdle: 4
    • Arms: 60
    • Pelvic girdle: 2
    • Legs: 60
    • Patella: 2
  • Total: 213

12.4 Joints

The point at which two separate bones meet is called a joint. There are three major categories of joints:

  1. Immovable Joints: No movement is possible between the bones (e.g., sutures between the bones of the skull).
  2. Partially Movable Joints: Very little movement occurs between the bones (e.g., the joint between a rib and the breast-bone or between vertebrae).
  3. Freely Movable Joints: Varying degrees of movement are possible between the bones.
    • (a) Gliding Joint: Occurs between the bones of the wrist and ankle, as well as between vertebrae.
    • (b) Pivot Joint: One bone is rotated over a pivot-like end of another bone (e.g., rotation of the skull on the axis vertebra).
    • (c) Hinge Joint: Moves like a hinge in one plane only (e.g., elbow joint, joints between the bones of fingers and toes, and the knee joint).
    • (d) Ball and Socket Joint: One end of a bone (rounded and ball-like) fits into a cup-like depression of another bone, providing movement in any direction (e.g., shoulder joint and hip joint).
Synovial Joints

Freely movable joints, like the shoulder and knee joints, are also called synovial joints. They allow considerable movement and have two special requirements:

  • They should be held firmly in position.

  • The surfaces coming into contact should be well lubricated to remove friction.

  • The lubricating fluid is called synovial fluid.

  • Ligaments prevent dislocation during normal movement.

12.5 Muscles

Muscles provide the means for all movements, cover the skeletal framework, and maintain body posture.

  • Most muscles are long bundles of contractile tissue.
  • Each muscle has two ends: a fixed end (origin) and a movable end (insertion).
  • The movable end forms a tendon that attaches to the bone.
  • When stimulated by a nerve, the muscle contracts, becoming shorter and thicker, and pulling the bone at the movable end.
  • Muscles can only contract and relax; they cannot lengthen.
Antagonistic Muscles

A structure moved by a muscle cannot return to its original position without the action of another muscle. Such muscles causing opposing movements are called antagonistic muscles.

  • Example: The biceps (flexor) muscle bends the lower arm, while the triceps (extensor) muscle straightens it.
Kinds of Muscles
  • Voluntary: Under control of our will.
  • Involuntary: Not controlled by our will.
  • Cardiac: Make up the walls of the heart, capable of rapid movements and do not tire easily.
Coordination of Muscles

Actions like standing, walking, running, and playing tennis require combined action of several muscles that must be coordinated.

14.6 Lever Mechanisms

Movements of different parts of the body are made easier due to lever actions in the skeleton.

  • Lever of the First Order: Fulcrum (F) between power (P) and weight (W). Represented by extension of the arm at the elbow by the triceps muscle.
  • Lever of the Second Order: Fulcrum at one end, power at the other, with weight in between. Represented by the gastrocnemius muscle raising weight of the body on the toes.
  • Lever of the Third Order: Fulcrum at one end, weight at the other, with power in between. Represented by the biceps muscle flexing the arm.
Additional Information
  • The longest bone in our body is the thigh bone (femur).
  • The smallest bones in our body are the three tiny bones (malleus, incus, and stapes) in the ear.
  • Nails and teeth, though hard, are not bones.
  • Most muscles are attached to bones, but some are not (e.g., muscles moving eyelids or pushing food along the food canal).
  • Some muscles in our body are vestigial (non-functional), such as the muscles of the projecting ears.

Points to Remember

  • The skeleton performs four primary functions: support, protection, movement, and leverage.
  • Bones are a storehouse of calcium and phosphorus, and the hollow long bones produce blood cells.
  • The human skeleton consists of the axial skeleton (backbone and ribs) and the appendicular skeleton (bones of limbs and girdles).
  • Joints between two bones may be immovable, partially movable, and freely movable.
  • Movable joints are of four kinds: gliding joint, hinge joint, pivot joint, and ball and socket joint.
  • Most movable joints have a lubricating synovial fluid between the two bones.
  • Antagonistic muscles bring about opposite movements of the part concerned.
  • The human skeleton provides examples of all three orders of lever.