In sleep the muscles of inhalation and exhalation are relaxed, inhalation becomes long and deep, exhalation short and exhaustive, and the rhythmic intervals of respiration much lengthened.

Now degrees of relative wakefulness are indicated with surprising delicacy by the slight respiration sounds given forth by the sleeper.

Professional nurses learn to interpret these indications with great skill.

This exaltation of hearing became very pronounced in my operations with the child.

After some experience the peculiar breathing of advancing or actual wakefulness in her was sufficient to wake me.