I4O RESPIRATION 



cause the thorax can contract only so far, and when its con- 

 traction has reached its limit, for the lung to contract far- 

 ther would mean a separation of the pulmonary and tho- 

 racic walls and the formation of a vacuum between them. 

 The additional reason above given, namely the collapse of 

 the bronchioles before all the air can escape, is inoperative 

 under normal conditions 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 contraction of the 

 lungs in expiration is a cause and not a consequence of tho- 

 racic contraction. This statement as to expiration applies 

 only to ordinary tranquil respiration, as will be seen later. 

 Speaking broadly then, inspiration is an active and expira- 

 tion a passive process. That is, inspiration occurs as a re- 

 sult of the activity of certain muscles which operate to ex- 

 pand the thorax, and expiration, as a consequence, 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. communicating by a tube with the external air ; suppose 

 the bag conforms in general outline to the shape of the bel- 

 lows, 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 



