Muscular Action.—­ Muscular action in the new-born infant is entirely involuntary, there being no voluntary acts until about the end of the third month.  Sucking and licking are largely instinctive.  The movements of the arms and legs are impulsive acts, and occur during sleep, just as they did in the intra-uterine life.  The act of raising the head, which is attempted about the fourth month in healthy children, is volitional, requiring not so much added strength of muscle as power of coordination.  As volition develops the power of coordination gradually increases, and the child learns to perform voluntary or purposeful acts.  Voluntary grasping is done after the fourth month.  As the child learns to balance its head, it attempts to sit up.  This act is not successfully accomplished until about the fortieth week; the child sits firmly alone when ten or eleven months old.  Before this time it is necessary to support the head and spine of the child with the hand.  By the third or fourth month the infant should be able to grasp things.  The child begins to creep about the ninth month.  The clothing should be so arranged as to allow entire freedom of motion.

It should be able to stand up by a chair by the tenth month, and be able to walk alone at the end of the first year.  It is important that parents should know this, since not knowing what a normal baby ought to be able to do, cases of birth palsy, or even an attack of paralysis due to teething, are not infrequently overlooked, not only by the mother, but even by the doctor, who attributes the inability of the child to do what other children can do at this age simply to weakness, which the child will outgrow; and thus the time passes in which the most could be done to cure the child and to prevent the subsequent deformity.

A baby should not be forced to stand or walk; a very stout baby, on account of its weight, will stand up and walk much later than a slight one, the two being equally healthy.  Or if a baby has been sick, it will feel no inclination to stand up.  Naturally, a child creeps before it walks, and this develops the muscles of the lower limbs, so that they will support the weight of the child in standing.  By prematurely forcing a child to stand up and walk, there is danger of causing bow-legs, as the bones of the legs are still weak; the child should be discouraged from standing up too much rather than encouraged to stand up more.

Sleep.—­ A large proportion of the time of early infancy is spent in sleep; for the first few weeks the infant only wakens up to be fed.  During sleep the eyelids should be tightly closed; a partial opening of the lids, showing the whites of the eyes, is an indication of ill health.  Up to the age of six, children require twelve hours of sleep at night, besides an hour or more in the middle of the day; the child should be permitted to sleep as long in the morning as it will.