Chap. 32.] UJes of Refyiraticn. 331 



uneafinefs, but that which is felt when the lungs arc 

 kept too long inflated after a full infpiration, is of a 

 different kind from that which is perceived when they 

 are preferved too long empty after expiration. In the 

 former cafe the uneafinefs is referred to the head, in 

 the. latter to the cheft. To what thefe fenfanons arc 

 owing we canno: altogether determine ; they are pro- 

 bably, however, to be attributed to the anterior cavi- 

 ties of the heart and the veffels of the head being 

 overloaded with bloodf which cannot fo readily pafs 

 through the lungs while their motion is fufpended. 

 The truth of this opinion is much confirmed by the 

 fluffing of the face, and the burfting of blood veflels, 

 which fometimes happens from impeded refpiration. 



The air, after paffing through the windpipe, is con- 

 veyed by its ramifications to. the air veffels of the lungs. 

 After infpiration the air veffels, which are to be con- 

 fidered as very minute bladders with thin coats, arc 

 fully diftended. The minute and very numerous ra- 

 mifications of the pulmonary artery are diftributed on 

 the membranes of thefe air vcffds -, and through the ' 

 membranes, without coming into direct contact wich 

 the blood, the air produces thofe changes on it, which 

 are found to be abfolutely neceffary for the continu- 

 ance of life. 



The chief ufes of refpiration, as far as our know- 

 ledge extends, are, ift. To effect certain changes in 

 the mafs of blood; and id. To produce animal hear. 

 Thefe effects, though no doubt intimately connected, 

 I fhall take the liberty of confidering feparately, for 

 the fake of greater perfpicuity. 



The compofition of the atmofphere has already been 

 defcribed, and it is at prefent only neceffary to remind 

 the reader, that rather lefs than three -fourths of the 

 gtmofphere is azotic gas, rather more than one fourth 



oxygen 



