36 PROXIMATE PRINCIPLES IN GENERAL. 



proximate principle, since the fibrous tissue which produces it is not at 

 first soluble, even in boiling water, and its ingredients become altered 

 and converted into a gelatinous matter only by prolonged ebullition. 

 So, again, an animal substance containing the alkaline acetates or lac- 

 tates will, upon incineration in the air, yield carbonates of the same 

 bases, the original acid having been destroyed, and replaced by car- 

 bonic acid. In either case, the analysis of the tissue, so conducted, 

 would be a deceptive one, and useless for anatomical and physiological 

 purposes, because its real ingredients have been decomposed, and re- 

 placed by others, in the process of manipulation. 



It should, therefore, be kept constantly in view, in the examination 

 of an animal tissue or fluid, that the object of the operation is simply 

 the separation of its ingredients from each other, and not their decom- 

 position or ultimate analysis. Only the simplest forms of chemical 

 manipulation, if possible, should be employed. The substance to be 

 examined should first be subjected to evaporation, in order to extract 

 and estimate its water. This evaporation must be conducted at a heat 

 not above 100 (212 p.), since a higher temperature would destroy or 

 alter some of the animal ingredients. Then, from the dried residue, 

 sodium chloride, alkaline sulphates, carbonates, and phosphates may be 

 extracted with water. Coloring matters may be separated by alcohol, 

 and oils may be dissolved out by ether. When a chemical decomposi- 

 tion is unavoidable, it must be kept in sight and afterward corrected. 

 Thus the sodium glyko-cholate of the bile is separated from certain 

 other ingredients by precipitating it with plumbic acetate, forming lead 

 glyko-cholate; but this is afterwards decomposed, in its turn, by sodium 

 carbonate, reproducing the original sodium glyko-cholate. Sometimes 

 it is impossible to extract a proximate principle in an entirely unaltered 

 form. Thus the fibrine of the blood can be separated only by allowing 

 it to coagulate ; and once coagulated, it is permanently altered, and no 

 longer presents its original characters as an ingredient of the blood. 

 In such instances as this, we can only make allowance for an unavoid- 

 able difficulty, and endeavor by other means to ascertain under what 

 form the substance originally existed in the animal fluids, being careful 

 that the substance suffers no further alteration. By bearing in mind 

 the above considerations, we may form a tolerably correct estimate of 

 the nature and quantity of all the proximate principles in the tissue or 

 fluid under examination. 



The manner in which the proximate principles are associated to- 

 gether is also deserving of notice. In every animal solid and fluid, 

 there is a considerable number of proximate principles which are 

 present in certain proportions, and which are so united with each other 

 that the mixture presents a homogeneous appearance. But this union 

 is of a complicated character ; and the presence of each ingredient de- 

 pends, to a certain extent, upon that of the others. Some- of them, 

 such as the alkaline carbonates and phosphates, are in solution directly 

 in the water. Some, which are insoluble in water, are retained in solu- 



