They teach us to laugh aright. They dis cover for us relationships where they are least expected, and the reaction is laughter. Or they show us how relationships appearing to exist are preposterous, which being done sud denly we laugh. The expression of laughter is not always indicative of a physical apprecia tion of enjoyment, for it is well known that idiots are prone to laughter which is often with out any such significance. Many idiots con stantly show a laughing countenance, the smile being more or less stereotyped; or they may grin, giggle or chuckle at the slightest stimulus, whether of food, color, music or personal con tact. It is probable that in such cases laughter is purely an expression of physical content ment, rarely associated with higher or more complex ideas.
In children, laughter is more sensible, but the expressions of joy usually contain an ele ment of uncontrolled exuberance. Thus they clap their hands, stamp their feet and jump around in pure excess of vital spirits. In adults the subject of laughter is extremely com plex. As a rule, during laughter, the mouth is more or less open, the corners being drawn backward and usually somewhat upward. The upper lip is commonly raised. The drawing back movement is seen best in the broad smile or in moderate laughter; in out-and-out mirth the teeth are usually exposed by the raising of the upper lip. The cheeks are ordinarily drawn upward at the same time and wrinkles are formed under the eyes. This movement in old persons makes a very characteristic feature, and the wrinkles so frequently found in their faces largely assist to interpret the sense of content ment indicated. Associated with laughter, there is often a change in the character of the eye. The bright and sparkling eye described by Darwin and others who have devoted much time to this subject is the eye of laughter. Often tears suffuse the eyes and destroy this appearance of brightness, but this is usually due to excessive laughter.
Numerous signs are during these expressions of joy. During laughter, the move ments of the chest and larynx are almost ex actly opposite to those that accompany the screams and cries of distress. In these latter the expirations are prolonged andcontihuous, i and the inspirations short and whereas in joy the expirations are usually short and the inspirations long.
"In all races of men,)) says Darwin, "the ex pression of good spirits seems to be the same, and it is easily recognized)); and he adds that °from the natives of New Zealand to the highly civilized Caucasian, much the same forms of emotional expression are to be observed.* Laughter is often an indication of general char acter; it is seldom two persons laugh exactly alike; and the study of laughter becomes a study of muscular movements. The °spontane ous, hearty laughter of sincere feeling is very different from the affected and constrained laughter of insanity.)) Moreover, there are laughs which betoken peculiar constitutions of mind and character ; laughs that are mechanical, nervous spasms, expressing nothing and ex pressed when there is nothing to laugh at, or perhaps something not to laugh at; laughs which, when they have not been acquired and are unconsciously formed, are signs of neu rotic instability, but sure signs of guile when they are affected and consciously used — untrustworthy, anyhow, as laughter. Maudsley describes what he terms a "quasi-pathological which is the abortive and incoherent laugh of the person of insane temperament, which is laughter pulled up abruptly, followed by a sudden facial seriousness or a change which affects only a part of the features, while the rest are unmoved. Consult Bergson, Henri, 'Laughter' (New York 1912); Darwin, 'Ex pression of the Emotions in Man and Ani mals' (latest ed., New York 1910); Hughes, 'Die Mimik des Menschen> (1900) ; and Sidis, B., the 'Psychology of Laughter) (New York 1913); Sully, James, 'An Essay on (ib. 1902).