96 OF VITAL MOTION. 



depend chiefly upon the presence of oxygen. It is 

 difficult, however, to suppose that the contractions are^ 

 due to this presence. On the contrary, we find that 

 the air effects an entrance, remains for some time, 

 and is not expelled until a considerable quantity of 

 the vital element has been absorbed and replaced by 

 carbonic acid; and hence it would appear that the 

 contractions (if due to the air at all) are rather to be 

 ascribed to the last-named gas than to the oxygen. 

 In opposition to this notion, however, we find that the 

 respiratory passages of the foetus remain in a con- 

 tracted state in absence of any aeriform stimulus, and 

 also that there is a similar condition in the adult, in- 

 dependently of any direct stimulant, when an attempt 

 is made to inhale an irrespirable gas. When, there- 

 fore, we consider that the air passages, both before 

 and after birth, may remain in a contracted state, in 

 absence of any gaseous stimulant, and when we con- 

 sider that they immediately take in vital air, and 

 remain expanded until this is partly devitalized, the 

 natural conclusion appears to be that the atmosphere 

 produces, by virtue of its oxygen, a state the reverse 

 of contraction, and that this expansion disappears 

 when the efficient cause is removed by the require- 

 ments of respiration. 



In objection to this view it may be said, that the 

 lungs are filled or emptied in consequence of the move- 

 ments of the walls of the chest, and that the seeming 



