26 ORGANIC SUBSTRATA OF THE ANIMAL ORGANISM, 



&c., must be associated with very different physiological functions 

 from albuminous bodies, containing a large quantity of nitrogen : 

 but we should hardly have expected that the difference between 

 nitrogenous and non-nitrogenous bodies should be so clearly shown 

 in the two great kingdoms of living organisms ; the vital pheno- 

 mena of animals and plants, in a great measure owe their differences 

 to the diversity of these two classes of chemical substances. We 

 shall find in the course of our observations, that pure chemistry 

 cannot sever or group together organic substances, otherwise than 

 as physiological conditions shall require. 



When we speak of applying a purely chemical principle to the 

 classification of the objects embraced in zoo-chemistry, under- 

 standing by the term, the theory of the chemical substrata of 

 animal organisms, we do not refer to the old and bye-gone classi- 

 fication of organic substances into acids, bases, and indifferent or 

 amphoteric bodies ; for we are of opinion that a classification of 

 animal substances, according to their combined chemical relations and 

 their chemical import, (but not according to a single property, as for 

 instance their basicity or acidity), must be physiologically correct, 

 since it is a natural method of arrangement. On the other hand we 

 regard a purely physiological principle of classification in zoo-che- 

 mistry (such as we followed in the first edition of the present 

 work) as 110 less irrational and unnatural than that which has 

 originated in views based merely on a theory of affinities. Although 

 we might at first sight be disposed to regard as appropriate a 

 classification of organic substrata into nutrient matters and excreta, 

 the practical application of such a mode of treatment will exhibit 

 numerous deficiencies, which completely nullify the advantages 

 it might have been supposed to possess. For it soon becomes 

 apparent, that a body which appears in one part of the animal 

 organism, or in one process, strictly as a product of decomposition, 

 is applied in another to the formation of a tissue, or the accom- 

 plishment of a purely physiological function. A separation of 

 zoo-chemical substances into secreted and excreted matters, leads 

 to the greatest uncertainty and the most intricate confusion. We 

 must, however, admit that every systematic mode of arrangement 

 seems impracticable in a purely empirical science, which ought 

 only to follow a genetic or eetiological, and not a teleological method ; 

 since the latter can, at most, only indicate the direction in which 

 investigation should be pursued in an immature science. A new 

 phrase has, however, been recently employed by which it was con- 

 jectured that zoo -chemical processes might, according to their nature, 



