226 BLOOD. 



We certainly cannot apply all the controlling checks which we' 

 have here described in every analysis of the blood ; but as the 

 little regarded rule holds good (see p. 3 of this volume), that the 

 smallest possible quantities give the most accurate results for each 

 individual determination, by no means so large a quantity of 

 material is requisite for a good analysis of the blood, as we are 

 commonly in the habit of supposing necessary. It is only for 

 analyses of the ash that larger quantities are required, and even 

 here the accuracy attainable in inorganic analysis is now so great, 

 that a comparatively small quantity suffices. 



Sugar and urea may sometimes be determined quantitatively in 

 the blood ; on these points it is sufficient to refer to what has 

 been already stated in p. 91 of the present volume, and pp. 159 

 and 1 64 of vol. i. 



In relation to the determination of the specific gravity, we shall 

 more fully notice this subject in the chapter on " the urine," when 

 we shall examine the different methods which have been proposed 

 for this purpose. With regard to the blood, we need only remark 

 that it is very often almost impossible to determine the density of 

 the cruor free from fibrin, and of the defibriiiated blood, in con- 

 sequence of the viscidity of this fluid, and of the air-bubbles 

 suspended in the blood, and, in particular, adhering to the vessel. 



A full consideration of all the circumstances and accidents 

 appertaining to chemical analysis, must shake our confidence in 

 the relative accuracy of those analyses of the blood of which we 

 are at present in possession, and we might even hesitate in ascrib- 

 ing any degree of value to the deductions and hypotheses which 

 have incautiously been drawn from them. Then, moreover, it 

 must be remembered that in many diseases in which the admix- 

 ture of the blood is most altered, good analyses of blood cannot, 

 from considerations of humanity, be adequately prosecuted, and 

 that in reporting such analyses, we have generally been contented 

 with a vague and abstract diagnosis, although the course of the 

 individual morbid process is of the highest importance in a 

 scientific point of view : hence no great weight can be attached to 

 a humoral pathology which is based on such slender supports. If, 

 finally, we consider that in all kinds of analysed blood, the result 

 refers only to the greater or lesser fluctuations in the relations of 

 the main constituents of the blood, and not to a new alteration, 

 admixture, or decomposition of that fluid, and since these relations 

 have not yet been adequately elucidated in a chemical point of 

 view, we can only wonder that it should ever have been supposed 



