THE PRINCIPLE OF VITAL AFFINITY. 327 
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 less 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 the dark colour, 
until it has passed along the arteries, and through the capillaries of the system, 
and entered the veins? This fact is noticed both by Prout and Linpic. “ The 
oxygen absorbed at the lungs,” says Dr Prout, ‘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 the oxygen changes its mode of action ; it combines with a portion of 
carbon, and is converted into carbonic acid.’”—(Bridgewater Treatise, p. 536.) 
Li=zsBiG 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- 
