130 THE BLOOD. 



connection. In addition to the nutritive principles, we have 

 entering into the composition of the blood, urea, cholesterine, 

 urate of soda, creatine, creatinine, and other substances, the 

 characters of which are not yet fully determined, belonging 

 to the class of Excrementitious Principles. Their considera- 

 tion comes more appropriately under the head of Excretion, 

 and they will be fully taken up in the chapter devoted to 

 that subject. Though a knowledge of the exact proportions 

 of the various elements of the blood is not necessary in order 

 to appreciate the relations of this fluid to the tissues, the 

 great interest which is attached to this line of investigation, 

 and the important advantages which we may look for in the 

 future from extended inquiry in this direction, lead us to 

 discuss at some length the methods which have been employ- 

 ed by physiological chemists in quantitative analyses, with 

 some of the results which have already been obtained. 



Quantitative Analysis of the Blood. 



The methods which have been, and are now, commonly 

 employed for quantitative analysis of the blood vary very 

 little from the process recommended by Provost and Dumas 

 in 1823. They are based upon the supposition that the 

 organic constituents, fibrin and albumen, are solid substances 

 in solution in the watery elements, and that all the water of 

 the blood is to be attributed to the serum. As we have shown 

 in treating of organic substances that this view of their con- 

 dition in the fluids is erroneous, and that the desiccated ma- 

 terials obtained from the blood do not represent the real 

 quantities of its organic elements, a new method of analysis, 

 based on the view that these principles are naturally fluid, 

 seems necessary. The same process has been employed for 

 the estimation of the proportion of corpuscles. Here the 

 error is too manifest to require discussion. It is evident that 

 the blood-corpuscles are semi-solid bodies which become 

 altered by desiccation ; and an estimate which does not give 



