140 RESPIRATION 



reason above given, namely the collapse of the bronchioles be- 

 fore all the air can escape, is inoperative under normal condi- 

 tions and need not be considered. 



Causes of Respiratory Movements. Seeing that the lung 

 structure has always a tendency to empty itself of air, it follows 

 that inspiration cannot be dependent upon the lung itself. 

 Granting, from the physical conditions present, that the lungs 

 and thorax must expand and contract together, the expansion of 

 the lungs in inspiration is a consequence and not a cause of the 

 thoracic expansion, and the contraction of the lungs in expiration 

 is a cause and not a consequence of thoracic contraction. This 

 statement as to expiration applies only to ordinary tranquil 

 respiration, as will be seen later. Speaking broadly then, inspira- 

 tion is an active and expiration a passive process. That is, 

 inspiration occurs as a result of the activity of certain muscles 

 which operate to expand the thorax, and expiration as a conse- 

 quence simply of the cessation of activity on the part of those 

 muscles and the passive contraction of the lung tissue. 



The relation of the thorax and lungs and the action of each in 

 respiration may be illustrated. Suppose a bellows, which, say 

 for some mechanical reason, cannot completely collapse and 

 which is itself air-tight, to contain a thin rubber bag commu- 

 nicating by a tube with the external air; suppose the bag con- 

 forms in general outline to the shape of the bellows, and under 

 a moderate degree of distention completely fills the cavity of the 

 bellows when the latter is collapsed as far as possible. Now, it 

 being understood that the bag was somewhat distended to cause 

 it to fill the bellows, and that all air has been allowed to escape 

 by a temporary opening from between the walls of the two and 

 the bellows itself made air-tight afterwards, it follows that unless 

 the bellows can contract the bag will remain distended and will 

 not leave the bellows wall, although it will have a constant tend- 

 ency to do so. It is also apparent that, since the bag exerts a 

 continual compressing effect on its contents, the pressure inside 



