Besides the ordinary respiratory motions and the voluntary modifications made use of in speaking, singing, etc., the muscles of respiration perform a series of movements of an involuntary reflex nature indicative of certain emotions and mental states.

They will be seen to resemble each other in the mechanism of their production, though differing essentially in expression. The following are the more important: -

Coughing is caused by a stimulus applied to certain parts of the air passages, but more particularly to the larynx; the stimulus passing along the superior laryngeal branch of the pneumogastric. It consists of a deep inspiration, closure of the glottis, and then a more or less violent expiratory effort, accompanied by two, three, or more sudden openings and closures of the glottis, so that rapidly repeated blasts of air pass through the upper air passages and mouth, which~is generally held open.

Sneezing is caused by a stimulus applied to the nose or eyes, the impulses being carried to the respiratory centre by the nasal and other branches of the 5th nerve. It consists of a deep inspiration and closure of the glottis, followed by a single explosive expiration and sudden opening of the glottis and posterior nares and facial distortion.

Sneezing is a purely reflex act, since it is impossible to produce it voluntarily, except indirectly by the stimulation of the nasal mucous membrane with some irritating substance.

Laughing consists of a full inspiration, followed by a long series of very short, rapid, expiratory efforts. The facial muscles are at the same time thrown into a characteristic set of movements.

Crying is made up of a series of short, sudden expirations, accompanied by peculiar facial contortions, lachrymal secretion, and usually associated with the following: -

Sobbing consists of a rapid series of convulsive inspiratory efforts, causing but little air to enter the chest, followed by one long expiration.

Sighing is a long, slow inspiration, quickly followecl by a corresponding expiration.

Yawning is a very long, deep inspiration, completely filling the chest. It is accompanied by a peculiar depression of the lower jaw, wide open mouth, facial movements, and commonly stretching of the limbs.

Hiccough is an unexpected inspiratory spasm, chiefly of the diaphragm, the entrance of the air being checked by the sudden closure of the glottis.