90 



while in a healthy state ; and if we consider the great- 

 extent of surface, and extreme minuteness of the 

 vessels in the gills of fishes (65.), we cannot but 

 consider them as well adapted to produce an exten- 

 sive contact of surfaces, and but little fitted to ab- 

 sorb, decompose, and again emit aeriform fluids. 



75. If to account for this supposed entrance of 

 air into the vascular system, the agency of chemical 

 affinity be had recourse to, by what means, we would 

 ask, can its operation be in this case explained ? No 

 sensible .or obvious principle, equal to such an ef- 

 fect, can be held to reside in the blood, since the 

 changes go on equally in all these animals, though 

 the blood be of various colours, and, in many in- 

 stances, where it is totally devoid of colour. During 

 a torpid state also, Spallanzani has shewn, that no 

 change is effected by the animal on the air * ; and 

 consequently, no oxygen gas is then attracted by the 

 blood, although, if the supposed carbon of that 

 fluid be considered to attract this gas, the union 

 ought still to proceed, because, according to the re- 

 ceived opinions, the animal system is at this period 

 surcharged with carbon. Neither can the condi- 

 tions, indispensable to the operation of chemical affi- 

 nity, be in these cases fulfilled; for the interposi- 

 tion of organized substance between the air and the 

 blood, altogether precludes that degree of absolute 

 contact, which is held to be essential to chemical ac- 

 tion. Even if the oxygen gas were attracted into 

 thi blood by the operation of chemical affinity, by 



* Memoirs on Respiration, p. 334-. 



