THE RESPIRATION 285 



wall drives the abdominal viscera against the relaxed dia- 

 phragm and again arches it towards the thorax, squeezing its 

 marginal portion against the ribs and occluding the comple- 

 mental pleura. Experimental evidence shows that the inter- 

 nal intercostals contract with each expiration, and help to 

 draw the ribs downwards. 



Ordinary expiration is thus normally mainly a passive 

 act, being simply a return of the thorax to the position of 

 rest. But voluntarily, and in certain conditions involuntarily, 

 expiration may be forced. Forced expiration is then partly 

 due to the above factors, and partly due to the action of 

 muscles. Every muscle which can in any way diminish the 

 size of the thorax comes into play. 



Chief of these are the abdominal muscles, which by 

 compressing the viscera push them upwards and press 

 the diaphragm further up into the thorax. At the same 

 time by acting from the pelvis to pull down the ribs 

 they decrease the thorax from side to side, and from before 

 backwards. 



The serratus posticus inferior and part of the sacro-lumbalis 

 pull downwards the lower ribs, and the triangularis sterni also 

 assists in this. 



By this constriction of the thorax, brought about by 

 ordinary or by forced expiration, the air inside is compressed 

 and the pressure raised. During ordinary expiration the 

 highest pressure reached is about 2 to 3 mm. Hg, in forced 

 expiration about 80 mm. 



The pressure of the air outside is less than this, and the air 

 inside the chest is driven out. 



Special Respiratory Movements. -- There are several 

 peculiar and special reflex actions of the respiratory muscles, 

 each caused by the stimulation of a special district, and 

 each having a special purpose. 



Coughing. This consists of an inspiration followed by a 

 strong expiratory effort during which the glottis is constricted 

 but is forced open repeatedly by the current of expired air. 

 It is generally due to irritation of the respiratory tract, and 

 its object is to expel foreign matters. 



Sneezing. This is generally produced by irritation of the 

 nasal mucous membrane. It consists in an inspiratory act 



