Muscle is a band or bundle of fibrous tissue in a body that has the ability to contract, producing movement or maintaining the position of parts of the body.

(i) A band of strong fibrous tissue is called ligament.
(ii) It connects bone to bone. 

Our backbone which comprises of vertebrae is a cartilaginous gliding joint whịch allows limited movements. So our backbone is slightly movable.

(i) It provides structure and shape to the body.
(ii) It supports and surrounds the internal organs of the body.
(iii) Calcium and phosphorous, the two minerals that the body needs for important regulatory functions, are stored inside the bones.
(iv) Red blood cells are produced in the bone marrow.
(v) The bones of the skeletal system act as levers for muscular action. Muscular movement would not be possible without tendons and ligaments.

► A bird has streamlined body.
► Its bones are light and strong.
► They are hollow and have air spaces between them.
► The hind limbs of birds are modified as claws, which help them to walk and to perch.
► The breast bones are modified to hold massive flight muscles which help in moving wings up and down
►  Birds have special flight muscles and the forelimbs are modified as wings.
► The wings and tail have long feathers which help in flying.

► Muscles often work in pairs which work against each other. These are called antagonistic pairs.
► The two muscles, the biceps and triceps are working against each other.
►When the biceps contracts the lower arm is raised and the arm bends. In this position the triceps muscle is relaxed.
► To straighten the arm the reverse happens. The triceps contracts straightening the arm, while the biceps relaxes.

(a)

(b)

(c)

(d)

(e)

Muscles are of three types
(i) Striated Muscles
1. These are also called as skeletal muscles or voluntary muscles
2. These are multi nucleated, and unbranched.
3. These are attached to bones.
4. Striated muscles are found in arms, legs, neck.
ii) Non-striated Muscles
1. These are also called as smooth muscles or involuntary muscles.
2. They possess single central nucleus.
3. Non-striated muscles are attached to soft parts of the body like blood vessels, iris, bronchi and the skin.
iii) Cardiac muscles
1. These are branched muscles with 1-3 central nuclei
2. These are involuntary muscles.
3. These muscles are found in Heart.

Streamlined body
(i) A streamlined body is the shape of the body that reduces resistance when moving
(ii) Streamlined bodies often have a teardrop shape.
(iii) Streamline shape reduces the friction of movement to a minimum thus decreasing overall drag
(iv) Streamlined body of birds help the particles of air to move upload them quickly and come together smoothly on the other side. The air does not provide much resistance to the body so that they can fly easily in air spending less energy.
(v) Streanilined body of fishes help them to move smoothly with the flow of water. Its body shape reduces friction while moving in water. It helps in the orientation of the fish movements in a particular direction.