Functions of skeletal system :

• Gives shape to the body.
• Provides support to the body.
• Protects delicate internal organs.
• Minerals and salts are stored in the bones.
• Movement of the body is possible due to skeleton.

The structure of the bones :

• Bones are hard and strong. Cartilages are flexible.
• There are two constituents in the bones, Viz. biotic and abiotic.
• The bone cells are biotic.
• The calcium carbonate, calcium phosphate and minerals and salts together form abiotic constituents.
• Shape of the bones is different.
• As the body growth takes place, the bones become strong and powerful. The bone growth occurs only till a certain age. A tall person has longer bones.

Types of bones : According to the shape, bones are of four types, i.e. (1) Flat bone (2) Small bones (3) Irregular bones (4) Long bones.

The skeleton is made up of the following bones :

Axial skeleton:

1)Skull : Skull is the first part of the axial skeleton that is made up of 22 bones.

• These are 14 facial bones (bones of the face) and 8 cranial bones (bones of the head).
• The bones of the skull are very hard and flat.
• None of the bones except the lower jaw or mandible bone is able to move.
• All the skull bones are joined by immovable joints.
• Skull is like a bony case in which the brain is well protected.

2) Rib cage : The rib cage is the cage like structure in the chest region.

• It is made up of 12 pairs of ribs and a single central bone in front called sternum.
• Thus in all 25 bones form a rib cage.
• Ribs are joined to the vertebral column or spine on the back side.
• Inside this boney cage two lungs and a heart are very well protected.

3) Vertebral columns / Spine : The spine is formed by padlock-shaped bones placed straight one above the other.

• There are altogether 33 bones in the spine, each called a vertebra.
• These bones are arranged one above the other flexibly.
• The spine protects the spinal cord that originates from the brain.
• Peculiarity : Irregular bones

4) Appendages : The appendicular skeleton is made up of bones of arms and legs along with upper and lower girdle which joins the arms and legs to the torso. There are all types of joints in the appendicular skeleton due to which we are able to make all possible movements of our limbs.

Types of movable joints :

• Hinge joint :  Allows the movements of bones only in one direction. It moves in a 180⁰ angle. Examples : the elbow and knee joints.
• Ball and socket joint : Bones can move in two or more directions - in a 360⁰ angle. Examples : shoulder and hip joints.
• Gliding joint : Bones can only slide over each other. Examples : wrist and ankle joints.

The structure of the skin :

Two main layers of skin : Outermost epidermis and inner is dermis.

• Epidermis, the outermost layer of skin has many layers in it. Pigment melanin is present in the cells of the epidermis.
• Dermis is the inner layer of the skin. It contains sweat glands.
• Below dermis there is subcutaneous layer. It has network of capillaries. It maintains normal body temperature.

Functions of the skin :

• Protection of the internal parts of the body, i.e. muscles, bones and organ systems
• Preserving moisture of the body.
• Synthesising vitamin ‘D’.
• Regulation of body temperature by releasing sweat.
• Protection against heat and cold.
• Sensory organ of touch.

Regulation of body temperature : When we walk or play in the hot sun, the temperature of our body may rise. But due to automatic regulation, the sweat glands get activated and we start sweating. We also feel thirsty.

The sweat glands are present in the dermis layer of the skin. The sweat helps to reduce the increased temperature. Our body temperature is normally regulated approximately at 37 °C or 98.4° F.

Melanin : Melanin is the pigment which decides the colour of our skin and

• Melanin protects our internal parts of the body from ultraviolet radiations present in the sunlight.
• Jet black hair are due to pure melanin.
• Darker persons have more melanin.
• Melanin is secreted by specialized glands present in the epidermis layer of the skin.
• Skin colour can change according to weather and exposure to sunlight.
• One should not discriminate between dark and fair skin colour.