THE PRINCIPLE OF VITAL AFFINITY. 827 



should hardly appear in their blood, even in that diseased state (diabetes) in 

 which it passes off so copiously, in the form of sugar, by the kidneys. Neither is 

 it easy to understand why the gelatin, formed probably in the course of the cir- 

 culation, and deposited in so large quantities from the bloodvessels, should not 

 appear in the blood. We are very imperfectly informed as to the origin, the use. 

 or even the composition, of that animal matter, or rather congeries of animal 

 matters, to which the name Extractive is applied. We are still in doubt as to 

 the purposes served by the globules of the blood, both red and white, and the 

 place and mode of their composition and decomposition. 



But, admitting all these difficulties as to the details of the chemical changes, 

 still these leading facts are ascertained : — that, in the cells of living vegetables, 

 amylaceous, fatty, and albuminous compounds are formed, — and that, in the cir- 

 culation through different parts of animal bodies, these compounds are selected 

 and appropriated, and, in some instances, farther transformed, so that a farther 

 formation of oily matter, and a new formation of gelatin, and probably of al- 

 buminous matter, takes place, applicable to the immediate nourishment of tex- 

 tures ; that all these materials are formed ultimately from carbonic acid, water, 

 and ammonia, existing in the atmosphere ; that the carbon, originally fixed from 

 the carbonic acid, is the most essential of all the ingredients, and the proportion of 

 oxygen in all these organic matters, much loss than in the inorganic compounds 

 from which they are derived : that the affinities whereby the carbon is enabled 

 to enter into these combinations with the other elements, existing in these organic 

 compounds, to the exclusion of much oxygen, are peculiar to the state of life, 

 and liable to variations by causes which do not affect dead matter ; and that, in 

 so far as the oxygen of the air is concerned in the formation of any of these com- 

 pounds, it acts only by carrying off such portions of carbon and hydrogen, as en- 

 able the remainder of those elements to fall into certain new combinations with 

 the others which are there present. 



We may state another difficulty here, as leading directly to the next important 

 question in vital chemistry, the rationale of the Excretions ; viz., Why does the 

 oxygen, which certainly attaches itself to the red globules in the lungs, not give 

 evidence of its combining with the carbon in them, by giving them tlie dark colour, 

 imtil it has passed along the arteries, and through the capOlaries of the system, 

 and entered the veins ? This fact is noticed both by Peout and Liebig. " The 

 oxygen absorbed at the lungs," says Dr Peout, " remains in some peculiar state 

 of union with the blood {query ^ As oxygenated water, or some analogous com- 

 pound ?) till the blood reaches the ultimate terminations of the arteries. In these 

 minute tubes tlie oxygen changes its mode of action ; it combines with a portion of 

 carbon, and is converted into carbonic acid." — {Bndgeivater Treatise, p. 5.36.) 



Liebig goes a step farther in explanation of the change of mode of action of 

 the oxygen, when he says, " The globules of the blood serve to transport the oxy- 



