When the child is crying and the mother does not know what ails it, she holds it in her arms and pats it on the back. This sets the circulation of the blood, the pulsations and the whole mechanism of the body in rhythm; in other words sets the body in order, and soothes the child. The nursery rhyme "Pat-a-cake", which is known all the world over in some form or other, cures a child of fretfulness by setting its whole being in rhythm.