242 



sufficient evidence*, and conceives, with Mr Davy, 

 that the pulmonary exhalation proceeds from a se- 

 cretion carried on by the vessels of the lungs. More- 

 over, we have endeavoured to shew, that no oxygen 

 gas can enter into the blood-vessels, but that the 

 whole of it that disappears is decomposed in the 

 lungs, and is actually expended in forming the car- 

 bonic acid (123.) which is there produced : so that 

 none remains for the formation of water by uniting 

 with the supposed hydrogen of the blood. Even if 

 oxygen gas did enter into the blood, and hydrogen 

 also existed in that fluid, we have no proof that their 

 affinity for each other is sufficient to form water ; 

 for, out of the body, actual ignition, or a great de- 

 gree of compression, is required to reduce them to 

 a fluid form. 



197. If we consider the vast extent of the cellular 

 surface (1 14.) of the lungs, and the similarity which 

 in its structure and functions (157.) it bears to that 

 of the external surface of the body, we are naturally 

 led to the belief, that the pulmonary excretion, like 

 every other, and especially like that from the skin, 

 is carried on by an appropriate structure, and accord- 

 ing to the ordinary laws of the exhalent system. 

 That the great bulk of fluid exhaled from the lungs 

 is derived from this source, none will venture to deny. 

 Why then should we imagine another mode of produc- 

 tion for this fluid, which is so totally at variance with 

 it? At least, the insufficiency of the ordinary function to 



* On Respiration, p. 129. 



