1.0 The Skeletal System: An Overview
The skeletal system is the hard, internal framework of the body. In an adult human, it consists of 206 bones, along with cartilages and ligaments that connect them.
Major Functions of the Skeleton
Provides a rigid frame for the body.
Shields delicate organs like the brain and heart.
Acts as levers for muscle attachment.
RBCs and WBCs are made in the bone marrow.
2.0 Composition of Bone
Bones are not "dead" structures; they are living tissues consisting of both organic and inorganic matter.
| Constituent | Percentage | Properties |
|---|---|---|
| Organic (Ossein protein) | ~33% | Gives the bone flexibility and toughness. |
| Inorganic (Calcium/Phosphorus) | ~67% | Gives the bone hardness and rigidity. |
1. If you put a bone in weak HCl, the minerals dissolve, leaving a decalcified bone (flexible/soft).
2. If you burn a bone, the organic matter is destroyed, leaving behind bone ash (brittle/crumbly).
2.0 The Axial Skeleton
The axial skeleton consists of 80 bones and forms the central pillar of the body. It includes the Skull, Vertebral Column, Ribs, and Sternum.
1. The Skull
The skull protects the brain and supports the structure of the face. It is divided into two parts:
- Cranium (Brain Box): Made of 8 flat bones fused together by immovable joints called Sutures.
- Facial Bones: Consists of 14 bones. The Lower Jaw (Mandible) is the only movable bone in the skull.
2. The Vertebral Column (Backbone)
The backbone is not a single bone but a series of 33 ring-like bones called vertebrae. It protects the spinal cord and supports the head.
| Region | No. of Vertebrae | Location |
|---|---|---|
| Cervical | 7 | Neck region |
| Thoracic | 12 | Chest region (Rib attachment) |
| Lumbar | 5 | Lower back |
| Sacrum | 5 (Fused) | Hip region |
| Coccyx | 4 (Fused) | Tail bone |
Between any two vertebrae, there is a pad of cartilage. These discs act as shock absorbers and allow the backbone to bend, preventing the bones from rubbing against each other.
In the cervical region, the 1st vertebra is called the Atlas (supports the head) and the 2nd is the Axis. ICSE papers often ask for the names of these specific vertebrae in "Name the following" sections.
3.0 Rib Cage and Appendicular Skeleton
1. The Rib Cage
The rib cage consists of 12 pairs of ribs that protect the heart and lungs. Not all ribs are attached to the breastbone (Sternum) in the same way.
| Type of Rib | Pairs | Connection Detail |
|---|---|---|
| True Ribs | 1 to 7 | Attached directly to the Sternum. |
| False Ribs | 8, 9, 10 | Joined to the 7th rib by cartilage. |
| Floating Ribs | 11 and 12 | Not attached to the Sternum at all. |
2. The Appendicular Skeleton
This includes the bones of the limbs (arms and legs) and the girdles (pectoral and pelvic) that connect them to the axial skeleton.
- Humerus: Upper arm bone.
- Radius & Ulna: Forearm bones.
- Carpals: 8 wrist bones.
- Metacarpals: 5 palm bones.
- Phalanges: 14 finger bones.
- Femur: Thigh bone (Longest & strongest).
- Tibia & Fibula: Shank bones.
- Patella: Knee cap.
- Tarsals: 7 ankle bones.
- Metatarsals: 5 sole bones.
In "Identify the bone" or "Label the diagram" questions, look for the bone with the large rounded head that fits into the hip socket. That is the Femur. It is the longest bone in the human body!
4.0 Girdles and Joints
1. The Girdles
Girdles provide support and serve as attachment points for the limb bones.
Consists of the Clavicle (Collar bone) and Scapula (Shoulder blade). It connects the arms to the axial skeleton.
Consists of two hip bones (Innominate bones). It supports the weight of the upper body and protects reproductive organs.
2. Types of Joints
A joint is a point where two or more bones meet. Based on mobility, they are classified into three types:
| Type | Movement | Example |
|---|---|---|
| Immovable (Fibrous) | No movement possible. | Sutures of the Skull. |
| Partially Movable | Slight movement. | Joints between Vertebrae. |
| Freely Movable (Synovial) | Considerable movement. | Shoulder, Knee, Wrist. |
Major Freely Movable Joints:
- Ball and Socket Joint: Allows movement in all directions (e.g., Shoulder and Hip joints).
- Hinge Joint: Allows movement in only one plane, like a door (e.g., Elbow and Knee joints).
- Pivot Joint: Allows rotation (e.g., joint between Atlas and Axis vertebrae).
- Gliding Joint: Bones slide over each other (e.g., Wrist and Ankle bones).
In synovial joints, the space between bones is filled with Synovial Fluid. This acts as a lubricant to reduce friction between the bones during movement. This is a very common "Give Reason" or "Label" question.
5.0 Muscles and Movement
Bones alone cannot move. Movement is caused by the contraction and relaxation of Skeletal Muscles attached to the bones.
1. Antagonistic Muscles
Muscles can only pull; they cannot push. Therefore, they work in pairs called Antagonistic Pairs—when one muscle contracts, the other relaxes.
Example: The Biceps and Triceps
- Flexion (Bending the arm): The Biceps (Flexor) contract, and the Triceps relax.
- Extension (Straightening the arm): The Triceps (Extensor) contract, and the Biceps relax.
2. Tendons vs. Ligaments
These are two types of connective tissues often confused in exams. Understanding their difference is vital.
| Feature | Tendon | Ligament |
|---|---|---|
| Connects... | Muscle to Bone | Bone to Bone |
| Nature | Tough and Inelastic | Strong and Elastic |
3. Common Skeletal Disorders
- Arthritis: Inflammation of the joints, causing pain and stiffness.
- Osteoporosis: A condition where bones become weak and brittle due to loss of bone density (common in old age).
- Sprain: Excessive stretching or tearing of a ligament.
- Dislocation: When the end of a bone is forced out of its normal position in a joint.
If you see a diagram of a joint with a capsule, synovial membrane, and cartilage on the bone ends, it is a Synovial Joint. Label the fluid-filled space as the Synovial Cavity—its function is to act as a lubricant to prevent friction.
- Can you distinguish between Axial and Appendicular skeletons?
- Do you know the dental formula (from the previous chapter) vs. the vertebral formula?
- Can you define Antagonistic Muscles with the example of Biceps/Triceps?
- Do you understand the difference between a Sprain and a Fracture?