OF THE DOCTRINE OF VITAL AFFINITY. 393 
still farther diminished from their alternate contraction from the stimulus of the 
blood. There can be no doubt that in compounds the force of attraction subsist- 
ing among their constituent particles, is modified by the distance at which these 
are placed ; and in compounds especially, which consist of four or more prin- 
ciples, the slightest alteration in their relative situation is sufficient to change 
entirely their existing attraction, and induce new combinations. The blood is a 
compound of this kind ; its ultimate principles, too, are capable of entering into 
an innumerable variety of combinations with each other; we may conceive, 
therefore, that when subjected to the contraction of the extremely minute vessels 
through which it is forced to circulate, the relative position of its elements will be 
changed, and new combinations formed. And if we suppose a fluid thus passing 
through tubes of different diameters, and undergoing successive decompositions, 
we may easily conceive that very different products may be formed from the 
same original compound. This affords a very simple view of the nature of Secre- 
tion. No complicated apparatus is requisite ; all that is necessary being the pro- 
pulsion of the blood through extremely minute vessels capable of contraction. 
And it is easy to account for the variations to which secretion is liable, as the 
contraction of the vessels must vary from variations in the state of their irritabi- 
lity and of the stimuli acting on them.” [Murray's System of Chemistry, vol. iv., 
p- 518.] In regard to the Nutrition of solids, Dr Murray says merely that they 
appear to attract immediately from the blood the materials which it contains ready 
formed, as there is probably “no solid in the animal body, of which the imme- 
diate principles do not exist in the blood.” [Jéid., p. 516.] But I need hardly 
say that subsequent researches have not only completely demonstrated the insuf- 
- ficiency of this explanation, but have shewn that the cause of the difference of 
os 
products formed apparently from the same blood must be essentially different 
from that here assigned; and I would say farther, have shewn that the pecu- 
liarity of the compounds formed in living bodies cannot be reasonably ascribed to 
any modification of those movements of fluids, which Dr Daupeny regards as the 
only results of the vital principle. To shew this, I need not go into the question 
_ of the mode of action of arteries on the blood, or the portion of the changes essen- 
tial to secretion, which takes place in cells, exterior to vessels, and, of course, can- 
_ not be ascribed merely to the pressure to which the blood passing along the vessels 
may have been subjected; which had certainly been misapprehended by Murray, 
as by most other physiologists of that day. It is sufficient to quote a brief state- 
ment from Cuvier, which seems to me quife conclusive as to the question, 
whether difference of secreted fluids in the animal economy can be ascribed to 
difference in the structure of, and therefore of the movement of the blood through, 
q the organs in which they appear. “The same organ,” he says, 7.¢., the organ 
- secreting the same fluid from the blood, “ presents in different classes of animals, 
sometimes in the same class, perfectly distinct structures. This is true of the 
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