RESPIRATION: THE MECHANISM OF BREATHING 369 



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. 



