ICSE 9 Biology Skeletal Short

⚡ Quick Revision : Functions & Bone Structure

1. Functions of the Skeleton

The human skeleton is an endoskeleton consisting of 206 bones in an adult. It serves several vital roles:

• Support & Shape: Provides a framework that keeps the body upright.
• Protection: Shields delicate organs (e.g., Skull protects the brain, Rib cage protects heart/lungs).
• Movement: Acts as levers for muscles to pull against.
• Haemopoiesis: Red bone marrow produces Red Blood Cells (RBCs) and White Blood Cells (WBCs).
• Storage: Stores minerals like Calcium and Phosphorus.

2. Structure of a Long Bone

A typical long bone (like the Femur or Humerus) has a specific internal anatomy designed for strength and blood cell production.

• A. Periosteum: The tough, fibrous outer membrane that protects the bone and aids in repair.
• B. Compact Bone: The hard, dense outer layer that provides strength.
• C. Spongy Bone: Located at the ends (epiphyses), containing Red Bone Marrow.
• D. Bone Marrow Cavity: The central hollow space containing Yellow Bone Marrow (stores fat).

3. Constituents of Bone

Component	Percentage	Property Provided
Organic (Collagen/Ossein)	~33%	Flexibility and resilience.
Inorganic (Calcium Salts)	~67%	Hardness and rigidity.

Exam Reasoning

Question: Why do bones become brittle in old age?
Answer: With age, the proportion of organic matter (collagen) decreases while the inorganic mineral content remains high. This loss of flexibility makes the bones more prone to fractures.

❌ Don't Confuse:

Ligaments connect bone to bone. Tendons connect muscle to bone. Both are essential for movement, but their attachments are different!

⚡ Quick Revision : The Axial Skeleton

1. Overview of Axial Skeleton

The axial skeleton forms the central axis of the body and consists of 80 bones. It includes the Skull, Vertebral Column, Ribs, and Sternum.

2. The Vertebral Column (Backbone)

It consists of 33 ring-like bones called Vertebrae. It protects the spinal cord and supports the head.

The Five Regions:

• Cervical (7): Neck region. The first is Atlas, second is Axis.
• Thoracic (12): Chest region; ribs are attached here.
• Lumbar (5): Near the waist; largest and strongest vertebrae.
• Sacrum (5): Fused bones in the pelvic region.
• Coccyx (4): Fused tailbone (vestigial).

3. The Rib Cage

There are 12 pairs of ribs that protect the heart and lungs. They are classified based on their attachment to the Sternum (Breastbone):

Type of Ribs	Pairs	Attachment
True Ribs	1 to 7	Directly attached to the Sternum.
False Ribs	8, 9, 10	Attached to the 7th rib by cartilage.
Floating Ribs	11 & 12	Not attached to the Sternum at all.

Critical Reasoning

Question: What is the significance of intervertebral discs?
Answer: These are cushions of cartilage between vertebrae. They act as shock absorbers and allow the backbone to bend and twist without the bones grinding against each other.

❌ Don't Confuse:

The Cranium (brain box) consists of 8 bones, while the Face consists of 14 bones. Together they form the Skull.

⚡ Quick Revision : Appendicular Skeleton

1. The Appendicular Framework

The appendicular skeleton consists of 126 bones. It includes the limbs (arms and legs) and the girdles (pectoral and pelvic) that attach the limbs to the axial skeleton.

2. The Girdles (Support Systems)

• A. Pectoral Girdle (Shoulder): Consists of the Clavicle (Collar bone) and Scapula (Shoulder blade). It connects the arms to the sternum.
• B. Pelvic Girdle (Hip): Formed by two hip bones (Innominate bones). Each hip bone is made of three fused bones: Ilium, Ischium, and Pubis.

3. Bones of the Limbs

Region	Bones (Forelimbs - Arm)	Bones (Hindlimbs - Leg)
Upper Part	Humerus	Femur (Thigh bone)
Lower Part	Radius & Ulna	Tibia & Fibula
Joint Bone	---	Patella (Knee cap)
Ankle/Wrist	Carpals (8)	Tarsals (7)
Palm/Sole	Metacarpals (5)	Metatarsals (5)
Digits	Phalanges (14)	Phalanges (14)

Critical Reasoning

Question: Why is the Femur considered the most important long bone?
Answer: The Femur is the longest and strongest bone in the body. It supports the entire weight of the upper body during locomotion and provides a large surface area for the attachment of powerful leg muscles.

❌ Don't Confuse:

The Patella is only found in the leg (knee). There is no equivalent "cap" bone in the human elbow joint.

⚡ Quick Revision : Joints & Locomotion

1. What is a Joint?

A joint is a point where two or more bones meet. In ICSE Biology, joints are classified based on the degree of mobility they allow.

2. Classification of Joints

Type	Mobility	Example
Immovable (Fixed)	No movement allowed.	Sutures of the Skull.
Partially Movable	Very slight movement.	Joints between Vertebrae.
Freely Movable (Synovial)	Considerable movement.	Shoulder, Knee, Elbow.

3. Types of Movable (Synovial) Joints

• A. Ball and Socket Joint: Allows movement in all directions (360°).
  Examples: Shoulder joint, Hip joint.
• B. Hinge Joint: Allows movement in only one plane (like a door).
  Examples: Elbow, Knee, Finger joints.
• C. Pivot Joint: Allows rotation only.
  Example: Between Atlas and Axis vertebrae (neck).
• D. Gliding Joint: Bones slide over each other.
  Examples: Wrist (Carpals), Ankle (Tarsals).

Structure of a Synovial Joint

To prevent friction, these joints have:
1. Articular Cartilage: Smooth covering on bone ends.
2. Synovial Membrane: Secretes Synovial Fluid (lubricant).
3. Ligaments: Tough cords holding bones together.

Exam Reasoning

Question: Why do the joints of old people often creak or become painful?
Answer: With age, the synovial fluid may decrease or the articular cartilage may wear out, causing bones to rub against each other, leading to inflammation known as Arthritis.

❌ Don't Confuse:

The Knee joint is a hinge joint, but unlike the elbow, it also features the Patella (knee cap) for extra protection and leverage.