RESPIRATION. TIJK MECHANISM OF BREATHING 409 



the air issues forth with a rush, tending to carry out with it any- 

 thing 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 commonly excited by irritation of the nasal 

 mucous membrane, but in many persons a sudden bright light 

 falling into the eye will produce a sneeze. Laughing consists of 

 a series of short expirations following a single inspiration; the 

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

 are set in vibration. Crying is, physiologically, much like laughing 

 and, as we all know, one often passes into the other. The accom- 

 panying 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 center 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. 



