The sudden shadow or sound constitutes a disagreeable fact, and the disturbed nervous system responds by a reflex movement, just as many children cry when they hear the first clap of thunder, although they do not know what it is, and start when they suddenly hear a door bang or some object fall. Preyer noticed that in the seventh week his child started and lifted its hands at any sudden noise without waking. An expression of the greatest wonder can be produced in a child of seven months old by opening and shutting a fan before it; but the wide-open eye and mouth and the fixed look are not merely signs of astonishment, for when one draws the infant away from the breast, it expresses its lively desire to be fed again by the same attitude. In these cases the eyes shine with a more abundant secretion of tears. Wide-open eyes accompany the first smile. One notices that children have a tendency to open their eyes in joy and close them in displeasure. Children, like the insane and like animals, when they have had some disagreeable experience, are frightened at everything which they do not know. Sometimes fear appears suddenly; from one day to another a child may become timid and frightened when it sees an unknown person, or if the father or mother makes some unusual gesture, or calls loudly. The fear which children have of dogs and cats, before they have learnt why they are to be feared, is a consequence of heredity; even later, when they have gained some experience, they are overcome with fear at the sight of sucking pups or kittens, which would be ridiculous if it were not an innate aversion. The same may be said of the fear of falling when they make the first steps, although they have never yet fallen, and of the fear which children have at the first sight of the sea. III _Pavor nocturnus_ is a malady peculiar to children from the third to the seventh year, and must not be confounded with nightmare. The symptoms are the following: Sudden awaking of the child after a few hours of profound sleep.--A vivid expression of great terror, the eyes fixed on some point, as though on some apparition standing before them.--Failure of consciousness: the child recognises no one, and does not reply to questions.--Skin bathed in perspiration.--Stronger cardiac pulsations.--Rapid pulse.--Laboured breath.--Trembling of the limbs.--Temperature normal. The intensity, duration, and frequency of these attacks vary greatly; they last generally from five to twenty minutes, after which the children recover consciousness and fall asleep. In the morning they remember nothing. Rarely do the attacks occur several times in the same night; they appear as a rule at intervals of a few days, often disappearing altogether after occurring two or three times.