218 



CHAP. III. Arterial Blood. 



1. IT has not been presumed to assign the seat of the 

 conversion of chyle into blood: but the conversion of venous into 

 arterial blood takes place in the lungs. It has been suggested, in 

 speaking of these organs, that the obvious changes, as that of 

 colour, &c. may not be the most important ones. Although the 

 exposure of venous blood to the air, by which, as they say, it i* 

 oxygenated, is a thing necessary to the support of life; yet the 

 mode, or process by which it becomes necessary to life has not been 

 minutely investigated. The change which takes place in the 

 blood in its passage through the lungs is one interesting chiefly 

 the vital properties of this fluid, it is a preparation of the blood 

 for future purposes. We know in the gross that the admission of 

 air into the lungs is requisite to this preparation; but that the 

 preparation consists in the mere oxygenation of blood is more than 

 we have a right to conclude. Such an inference can be sanctioned 

 only by facts which shew that blood exposed to air and oxygenated 

 in any other seat is capable of maintaining life. We know no such 

 facts, and until they are attained we have a right to suspect that 

 the vital properties of the structure have a relation with those of 

 the blood and contribute towards its preparation in this stage ; and 

 that the change which blood suffers in the lungs is one produced 

 by the complex relations of blood, air, and the properties of the 

 structure: for it must be remembered that although earth and air 

 have been said to contain informal life, and to furnish the elements 

 of constituted life, yet these sources are modified by the living 

 form in every variety of animal existence, and, so far as we can 

 trace the matter perceptibly, in every stage of their relation with 

 each. If we could not satisfy ourselves by observation to the con- 

 trary, we may as well suppose that food containing its share of the 

 elements of life is diffused among the structures which are the 

 seats of it, without the previous changes elaborated by the vital 

 properties of those which have been termed the preparatory organs, 

 as that air, mixing with blood, is at once fitted for all the future 

 purposes of this fluid, without any concurrence in its adaptation of 

 the vital properties of the organs into which it is first received. 

 At least we are justified by analogy in this suspicion in the absence 

 of proofs on either side, the want of which we are still likely to 

 lament, as our analyses do not reach the order or constitution of 

 spiritual change. 



