COMPOUND ANIMAL SUBSTANCES. 89 



and salts, and determine their individual proportions. This is 

 the plan that I have usually adopted, and in some cases I have 

 added the determination of sugar, urea, and hsemaphsein. The 

 execution of such a comparatively simple scheme as this is a 

 matter requiring considerable time and labour ; and if it Avere 

 required that we should carry out the analysis still further, and 

 separate the various fats, the different combinations of the fatty 

 acids, the varieties of extractive matter, and finally the different 

 salts, our task, in the present state of our knowledge, would be 

 one of great difficulty ; and in consequence of the minute pro- 

 portions in which some of these substances exist in the blood, 

 it would be necessary for us to operate upon a much larger 

 quantity of the fluid than we are usually able to obtain. This 

 method of investigation will probably in a short time be deemed 

 insufficient, for as soon as we have an accurate knowledge of 

 the mode of formation of the extractive matter, its separation and 

 determination will be of the highest importance in explaining 

 many of the phenomena of the metamorphoses of the blood. 



The same is the case with respect to the urine. The forma- 

 tion of a perfect quantitative analysis of this complicated fluid 

 is an extremely difficult (if not an impossible) task, in conse- 

 quence of the facility with which new products are developed 

 during the progress of the investigation. The course usually 

 pursued has been, therefore, the separation of those constituents 

 Avhich are apparently most important, the urea, uric acid, salts, 

 and extractive matter; in some cases the estimation of sugar 

 and albumen has been added. The instances in Avhich the se- 

 paration of the extractive matter into its three principal groups, 

 and the indiAidual analysis of the salts, have been undertaken, 

 are still more rare. 



It has been already observed that a single isolated analysis 

 is of very little intrinsic value, in substances of so varying a na- 

 ture as the blood or urine. The only method by which we can 

 hope to throAV any light upon the leading alterations that occur 

 in these fluids is by the comparison of the results obtained 

 from a series of analyses ; and if we were desnous of merely 

 ascertaining so simple a fact as the determination of the pa- 

 thological states in which either an excess or a deficiency of 

 fibrin and blood-corpuscles occurs in the blood, and the relation 

 that exists between such pathological states and such modifica- 



