356 PROTEIN-COMPOUNDS. 



Tests. It is only seldom that a case occurs in which any ques- 

 tion can arise, as to whether the substance separated from an 

 animal fluid is, or is not fibrin ; thus, by way of illustration, the 

 plasma surrounding the organs of insects coagulates on exposure to 

 the air, and we may term this substance fibrin ; yet it is by no 

 means identical with the fibrin of vertebrate animals ; for it does 

 not separate under the microscope into threads, and it is insoluble 

 in saline solutions., and even in solutions of the alkaline carbonates. 

 Pathological fluids, on exposure to the air, occasionally deposit a 

 sediment. But here the form of the coagulum as well as the 

 microscopic texture of the sediment must decide whether or not the 

 substance which is separated is fibrin ; and the action of salts upon it 

 may be observed in the further investigation. In many cases of 

 this kind the separated substance is not fibrin, but consists of 

 albuminous products, which appear under the microscope as minute 

 masses or molecular granules, and whose chemical characters may 

 be recognised by their behaviour with hydrochloric and nitric acids 

 and other reagents ; or finally it often consists of fatty or earthy 

 matters that can easily be distinguished by the ordinary tests from 

 true fibrin. 



It is often perfectly impossible to distinguish coagulated fibrin 

 from other protein-compounds; and we are therefore not justified 

 in regarding every insoluble mass contained in an exudation as 

 fibrin : the fibrin has, in these cases, already assumed an or- 

 ganised condition, and exhibits the elements of tissues under the 

 microscope ; or we find an unorganised, amorphous mass, which is 

 usually not fibrin, although it may be a derivative of that body, and 

 exhibits no property of fibrin that is not common to all the protein- 

 compounds, as we see, for instance, in tubercular deposits. Many 

 obvious reasons conspire to render a quantitative analysis imprac- 

 ticable in determinations of this kind. It is therefore unchemical, 

 to say the least, for pathological anatomists to designate every 

 unorganised exudation as fibrin ; nor shall we learn to distinguish 

 the chemical substrata of these exudations until we shall have 

 thoroughly investigated, in a chemical point of view, the actual 

 constitution of the protein-compounds. 



The quantitative determination of fibrin in animal fluids has 

 probably been more frequently attempted than that of any other 

 substance ; but we nevertheless are still without any method that 

 fulfils the requirements of a good analysis. The usual method of 

 determining fibrin quantitatively is by pressing the clot and 

 washing out the blood, or more frequently by shaking or whipping 



