OF VITAL MOTION. 97 



dilatation of the respiratory passages is occasioned by 

 the descent of the atmosphere into the space which 

 has been formed by the contraction of the diaphragm 

 and thoracic muscles ; but, if we reflect a moment, we 

 may perceive that this explanation will not meet the 

 whole requirements of the case. The consideration, 

 indeed, that the chest cannot be filled with irre- 

 spirable air is sufficient to show that the efficient cause 

 of inspiration is something more than mere atmo- 

 spheric pressure. From the difficulty which is expe- 

 rienced under these circumstances, it seems almost as 

 necessary that the air should possess certain qualities 

 by which it could overcome the contracted state of the 

 respiratory passages, as that the blood should possess 

 certain properties in order to secure a free entrance 

 into the bloodvessels. Indeed, the consideration that 

 the blood exerts this action upon the vessel is a strong 

 argument that the air acts upon the respiratory 

 passages in the way we suppose, for in each case the 

 activity is mainly to be ascribed to the oxygen, and in 

 each case the action is exerted upon an irritable tissue, 

 which presents many grounds of affinity and analogy. 

 Be this as it may, however, the necessity of oxygenated 

 air to inspiration is a sufficient proof that there is an 

 active dilatation of the respiratory passages, in addi- 

 tion to the passive distention which will result from 

 the mere weight of the incumbent atmosphere. 



When, therefore, we find that an expanded state of 



H 



