2. _Expiration_.--When we forcibly expel the air from our lungs, the operation is reversed. We bend forward, draw in the walls of the abdomen, and press the diaphragm upward, while the ribs are pulled downward,--all together diminishing the size of the chest, and forcing the air outward. Ordinary, quiet breathing is performed mainly by the diaphragm,--one breath to every four beats of the heart, or eighteen per minute. (See p. 299.) MODIFICATIONS OF THE BREATH.--_Sighing_ is merely a prolonged inspiration followed by an audible expiration. _Coughing_ is a violent expiration in which the air is driven through the mouth. _Sneezing_ differs from coughing, the air being forced through the nose. _Snoring_ is produced by the passage of the breath through the pharynx when the tongue and soft palate are in certain positions. [Footnote: The soft palate must have fallen back in such a manner as nearly or quite to close the entrance to the nasal cavity from the throat, and the tongue must also be thrown back so far as to leave only a narrow opening between it and the soft palate. The noise is produced by the air being forced either inward or outward through this opening. A snore results also when, with a closed mouth, the air is forced between the soft palate and the back wall of the pharynx into the nasal cavity. With deep breathing, perhaps accompanied by a variation in the position of the soft palate, a rattling noise may be heard in addition to the snoring, which is due to a vibration of the soft palate.--F. A. FERNALD, in "How we Sneeze, Laugh, Stammer, and Sigh."--_Popular Science Monthly_, Feb., 1884.] _Laughing_ and _crying_ are very much alike. The expression of the face is necessary to distinguish between them. The sounds are produced by short, rapid contractions of the diaphragm. _Hiccough_ is confined to inspiration. It is caused by a contraction of the diaphragm and a constriction of the glottis; the current of air just entering, as it strikes the closed glottis, gives rise to the well-known sound. _Yawning_, or _gaping_, is like sighing. [Footnote: The usefulness of a yawn lies in bringing up the arrears, as it were, of respiration, when it has fallen behindhand, either through fatigue or close attention to other occupation. The stretching of the jaws and limbs may also serve to equalize the nervous influence, certain muscles having become uneasy on account of being stretched or contracted for a long time.] It is distinguished by a wide opening of the mouth and a deep, profound inspiration. Both processes furnish additional air, and therefore probably meet a demand of the system for more oxygen. Frequently, however, they are like laughing, sobbing, etc., merely a sort of contagion, which runs through an audience, and seems almost irresistible.