426 THE HUMAN BODY. 



the laryngeal opening is kept closed ; being afterwards sud- 

 denly opened, the air issues forth with a rush, tending to 

 carry out with it anything lodged in the windpipe or larynx. 

 Sneezing is much like coughing, except that, while in a 

 cough the isthmus of the fauces is held open and the air 

 mainly passes out through the mouth, in sneezing the fauces 

 are closed and the blast is driven through the nostrils. It is 

 t commonly excited by irritation of the nasal mucous mem- 

 brane, but in many persons a sudden bright light falling into 

 the eye will produce a sneeze. Laugh ing consists of a series 

 of short expirations following a single inspiration; the 

 larynx is open all the time, and the vocal cords (Chap. 

 XXXVII.) are set in vibration. Crying is, physiologically, 

 much like laughing and, as we all know, one often passes 

 into the other. The accompanying contractions of ,the face 

 muscles giving expression to the countenance are, however, 

 different in the two. 



All these modified respiratory acts are essentially reflex 

 and they serve to show to what a great extent the discharges 

 of the respiratory centre can be modified by afferent nerve 

 impulses; but, with the exception of hiccough, they are to a 

 certain extent, like natural breathing, under the control of 

 the will. Most of them, too, can be imitated more or less 

 perfectly by voluntary muscular movements; though a good 

 stage sneeze or cough is rare. 



