125 



much more low. From this review of the effects 

 which take place between the blood and air, we con- 

 clude, that the chemical phenomena which arise when 

 these substances are placed in contact, do not prove 

 an attraction and diffusion of air through the blood ; 

 but shew only that a reciprocal action takes place, 

 by which a new product is formed : no inference, 

 therefore, in favour of an attraction of air by the 

 blood in the lungs, can be drawn from the recipro- 

 cal action which they exert on each other out of the 

 body. 



101. To the operation of chemical affinity also, a 

 degree of absolute contact is required, which may, 

 and does exist between air and venal blood out or, 

 the body ; but the intervention of the coats of the 

 cells and blood-vessels altogether forbids this neces- 

 sary condition in the lungs. The supposition, that 

 the coats of these vessels and cells are so thin, that, 

 when moist, they allow the air, or its oxygen gas, to 

 pervade them, is wholly gratuitous, and in opposition 

 to the results (94.) of direct experiment : and the be- 

 lief, that certain pores exist, through which elastic 

 fluids may permeate, is equally unsupported by ana- 

 tomical fact ; for the terminations of the absorbent 

 and exhalent vessels are the only orifices which are 

 known to open on the surface of the bronchial cells. 

 If, indeed, air did permeate the bronchial cells through 

 any supposed pores, it would more readily pass into 

 the cellular substance which connects them together 

 than into the pulmonary vessels, and thus would 

 create, at all times, an emphysematous state of those 

 organs ; but this is never known to be the case, for 

 these cells are impermeable by air. It is, lastly, ex- 



