248 BIOLOGY. [BOOK IT. 



The nutrimentary matters, prepared by the labour of digestion 

 and diffused into the circulation, are exchanged through the wall 

 of the capillaries with the products of denutrition. These last 

 are the result of an oxydation, the principal seat of which is 

 probably in the very substance of the anatomical elements, and 

 not in the capillaries themselves, as is still affirmed in a number 

 of special treatises. The capillaries are organs marvellously 

 adapted to the exchanges between the blood and the anatomical 

 elements ; but these last are the laboratory itself, where the 

 principal transformations are accomplished. The comparison of 

 the two kinds of blood permits us to fix, in some degree, the 

 balance of gain and loss. After having served as nutrition, the 

 ternary matters not azotized are brought back to the state of 

 carbonic acid and water; their combustion is, then, complete. 

 They have attained their maximum of oxydation. On the 

 contrary, the quaternary or albuminoidal substances only 

 undergo an incomplete combustion, of which the product is a 

 certain quantity of carbonic acid and water, besides a residue 

 of crystallizable quaternary elements, of which we have already 

 spoken, and which are eliminated by the various emunctories of 

 the economy. 



From the comparison of the two kinds of blood springs another 

 interesting fact, namely, that the arterial blood has a composition 

 sensibly identical in the various regions of the organism, whilst 

 the composition of the venous blood varies much according to 

 different organic localities. In effect, the arterial blood is the 

 general nutritive reservoir; from it each anatomical element 

 satisfies its needs ; but these needs vary according to the chemical 

 constitution and function of the elements. Each anatomical 

 element, then, takes, according to its affinities, one substance in 

 preference to another. Besides, and above all, it makes the 

 matters which it absorbs undergo a special elaboration, and 

 restores to the circulation also a special nutritive residue, 

 whence results necessarily the local diversity of the venous 

 blood. 



