The human skeletal system consists of:

$A$: On repeated application of stimuli,involuntary striped muscles undergo fatigue.
$R$: This is due to non-availability of $ATP$ molecules.

Locomotion requires a perfect coordinated activity of muscular system with $.......$ systems.

We swing our arms while walking for $......$.

Fill in the blank spaces:
$(a)$ All mammals (except a few) have ............. cervical vertebra.
$(b)$ The number of phalanges in each limb of human is ............. .
$(c)$ Thin filament of myofibril contains $2 \,'F'$ actins and two other proteins namely .............. and ........... .
$(d)$ In a muscle fibre $Ca^{++}$ is stored in ............. .
$(e)$ .............. and .............. pairs of ribs are called floating ribs.
$(f)$ The human cranium is made of .............. bones.

What is the function of the iris in the eye of a frog?