818 SPECIAL PHYSIOLOGY. 



trachea, and larynx. Sneezing consists of a quick, noisy expiration, 

 following a decided, sudden, and deep inspiration ; but the glottis is 

 not closed, as in coughing, and the column of air is not driven through 

 the mouth, but is directed, by the closure of the fauces, into and through 

 the nasal fossae. The irritation which causes sneezing, has its seat in 

 those fossae, whilst that which induces coughing, as is well known, re- 

 sides in the fauces, the larynx, especially at the glottis, or in the air- 

 tubes. The stimuli which excite coughing are cold air, irritating gases, 

 fluids or solids, and diseased secretions. The noise of sneezing is pro- 

 duced in the nose ; whilst that of coughing originates at the glottis. 

 Snoring is produced by the resonance of the air passing, in or out, 

 through the nasal cavities and the throat, owing to some irregular vi- 

 brations of the soft palate and uvula; it is sometimes dependent on 

 narrowing of the fauces by enlargement of the tonsils, or on other pe- 

 culiarities of conformation. In snoring, there is no special modification 

 of the respiratory movements themselves, either as to force, frequency, 

 or quickness. Yawning consists of a deep prolonged inspiration, the 

 air being drawn in through the mouth, which is widely opened, by a 

 consentaneous spasmodic action of the muscles of the lower jaw; this 

 is then followed by a slow expiration, accompanied usually by a lifting 

 of the soft palate, and sometimes by a prolonged characteristic sound. 

 It may be accomplished by the will, and is often the result of involun- 

 tary imitation. Sighing also consists of a slow, deep inspiration, 

 mostly accomplished through the mouth, and followed by a prolonged 

 expiration, likewise associated with a peculiar sound; it often occurs 

 after the attention has been strongly fixed; it is usually emotional. 

 Sobbing is produced by rapid convulsive contractions of the diaphragm, 

 associated with closure of the glottis. In crying, the movements re- 

 semble those of laughter, to be next described, although they are excited 

 by very different emotions. Laughing consists of a series of sudden, 

 short expirations, quickly succeeding each other, and divided, as it 

 were, by intermediate closures of the glottis, giving rise to peculiar 

 interrupted sounds. Laughter furnishes an excellent example of a re- 

 flex respiratory movement, excited either by sensori-motor impressions 

 acting through certain cutaneous nerves, as when it is caused by tick- 

 ling, or by emotional stimuli, as when it is the result of joy, or by a 

 volitional stimulus, as when it is imitated by the actor. Lastly, hiccup 

 is a short, sudden inspiration, produced by a sharp, convulsive con- 

 traction of the diaphragm, at the end of which the glottis is suddenly 

 and spasmodically closed, so that the air strikes it from below. Of 

 these varied movements, some, such as sighing and yawning, may be 

 induced by certain conditions of the respiratory organs themselves, 

 whilst others, such as laughing and crying, are never so excited. 



Movement of the Air in Respiration, and Capacity of the Lungs. 

 After the lungs have been once inflated, as in the newborn infant, they 

 are never, except from disease, entirely emptied of air. The most for- 

 cible expiration fails to accomplish this, and the quantity of air then 

 retained in their tissue is termed the residual air. The quantity above 

 this, held in the lungs after an ordinary expiration, but which may be 

 expelled by a voluntary forced expiration, is called the reserve air, or 





