152 ELEMENTARY PHYSIOLOGY 



into play, and also the muscles of the abdomen, which latter 

 act on the organs of the abdomen, thus exerting indirectly an 

 upward pressure on the diaphragm. 



Although the respiratory movements are generally purely 

 involuntary, yet they are brought about by muscles which are 

 under the control of the will ; and are sometimes partially or 

 entirely voluntary, as in speaking, singing, and blowing. 



We may now notice some of the more important respiratory 

 acts which differ from ordinary breathing. Generally speaking, 

 while respiration is going on, the glottis (the aperture at the upper 

 end of the trachea) is open so as to allow a free passage to the 

 air. In speaking or singing the vocal cords are stretched across 

 the larynx, leaving only a narrow space for the passage of air (see 

 fig. 217). Under these circumstances the cords are made to vibrate 

 by the air current forced between them, thus producing the voice. 



A sigh is simply a prolonged inspiration, followed by a rather 

 sudden expiratory movement. The former is produced chiefly by 

 the action of the abdominal muscles, and the latter is due to the 

 elastic recoil of the lungs and the walls of the chest. When the 

 mind is intensely concentrated on some particular subject for 

 a long time, the breathing becomes so quiet that the blood is 

 imperfectly aerated ; and this feeble respiration is compensated 

 for by deep involuntary respirations (sighs) at intervals. 



Hiccough is a sudden inspiration, produced by the spasmodic 

 action of the diaphragm. Being so sudden, the aperture of the 

 glottis is unprepared for the rush of air, and hence the peculiar 

 sound produced by the vibration of the vocal cords. 



Coughing is generally preceded by a deep inspiration. The 

 glottis is then closed firmly by the muscles of the larynx, and the 

 expiratory muscles suddenly contract. At first the force is not 

 sufficient to open the glottis, but at last the vocal cords are sud- 

 denly burst open, and the air is noisily expelled. Coughing is 

 a means by which any mucus or other matter which irritates the 

 walls of the air passages may be got rid of. 



Sneezing is produced much in the same way as coughing, but 

 the tongue is raised against the soft palate so as to divert the 

 air current through the nose. 



Sniffing consists of a series of short inspirations ; but, the 

 mouth being closed, the whole of the air inspired is made to pass 

 through the nostrils. 



Sobbing is a series of convulsive inspirations, each taking place 

 while the glottis is more or less closed. 



Laughing consists of short and rapid expirations. 



Yawning is a long inspiratory act, accompanied by a stretch- 

 ing of the muscles of the mouth and face. 



