Question

It moves by the extension and contraction of its muscles.
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Answer 1

Answer:

I think it's brain

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Answer 2

Our arm consists of two parts ,upper arm and lower arm, which are joined at the elbow. The upper arm has

one bone. The top end of upper arm bone is joined to shoulder blade by a ball and socket joint. The lower

arm has two bones. The upper arm bone and lower arm bone and joined at the elbow by a hinge joint.

The upper arm bone has a pair of muscles called biceps and triceps on its two sides.

The upper end of both the muscles is attached to the fixed bone called shoulder blade by tendons. The

lower end of both the muscles are attached to the bones of lower arm by tendons.

(1) When we want to bent the arm, the biceps muscle contracts. The contraction of biceps muscle pulls the

lower arm bones due to which the lower arm moves up. Biceps muscle is called flexor muscles because it

flexes the arm. When biceps muscle contracts and bends the arm, the tricep muscle stretches or relaxes

(becomes long and thin).

(2) When we want to straighten the arm, the triceps muscle contract (it becomes shorter and fatter).The

contraction of tricep muscles pulls the lower arm bones at the end due to which the lower arm moves out

and straighten. Triceps muscle is called extensor muscle because it extends the arm. Triceps muscles

contract and straightens the arm ,then the biceps muscles stretches or relaxes.

By alternate contraction and relaxation of biceps and triceps muscles, we can bend or straighten the arm

i.e. we can move the lower arm up and down. Biceps and triceps muscle just bent and straighten the arm.

Other muscle move the arm in other direction.