28 BLOOD. 



If a few cubic centimetres of this colorless plasma, or of a similar plasma 

 which may be obtained from almost any blood by means which we will 

 presently describe, be diluted with many times its bulk of a 0.6 per cent, 

 solution of sodium chloride 1 clotting is much retarded, and the various stages 

 may be more easily watched. As the fluid is becoming viscid, fine fibrils 

 of fibrin will be seen to be developed in it, especially at the sides of the con- 

 taining vessel. As these fibrils multiply in number, the fluid becomes more 

 and more of the consistence of a jelly and at the same time somewhat opaque. 

 Stirred or pulled about with a needle, the fibrils shrink up into a small 

 opaque stringy mass ; and a very considerable bulk of the jelly may by 

 agitation be resolved into a minute fragment of shrunken fibrin floating in a 

 quantity of what is really diluted serum. If a specimen of such diluted 

 plasma be stirred from time to time, as soon as clotting begins, with a needle 

 or glass rod, the fibrin may be removed piecemeal as it forms, and the jelly 

 stage may be altogether done away with. When fresh blood which has not 

 yet had time to clot is stirred or whipped with a bundle of rods (or anything 

 presenting a large amount of rough surface), no jelly-like clotting takes 

 place, but the rods become covered with a mass of shrunken fibrin. Blood 

 thus whipped until fibrin ceases to be deposited, is found to have entirely 

 lost its power of clotting. 



Putting these facts together, it is very clear that the phenomena of the 

 clotting of blood are caused by the appearance in the plasma of fine fibrils 

 of fibrin. So long as these are scanty, the blood is simply viscid. When 

 they become sufficiently numerous, they give the blood the firmness of a 

 jelly. Soon after their formation they begin to shrink, and while shrinking 

 enclose in their meshes the corpuscles, but squeeze out the fluid parts of 

 the blood. Hence the appearance of the shrunken colored clot and the 

 colorless serum. 



15. Fibrin, whether obtained by whipping freshly shed blood, or by 

 washing either a normal clot, or a clot obtained from colorless plasma, 

 exhibits the same general characters. It belongs to that class of complex 

 unstable nitrogenous bodies called proteids, which form a large portion of 

 all living bodies and an essential part of all living structures. 



Our knowledge of proteids is at present too imperfect, and probably none 

 of them have yet been prepared in adequate purity to justify us in attempt- 

 ing to assign to them any definite formula ; but it is important to remember 

 their general composition. 100 parts of a proteid contain rather more than 

 50 parts of carbon, rather more than 15 of nitrogen, about 7 of hydrogen, 

 and rather more than 20 of oxygen ; that is to say, they contain about half 

 their weight of carbon, and only about i their weight of nitrogen ; and yet, 

 as we shall see, they are eminently the nitrogenous substances of the body. 

 They usually contain a small quantity (1 or 2 per cent.) of sulphur, and 

 many also have some phosphorus attached to them in some way or other. 

 When burnt they leave a variable quantity of ash, consisting of inorganic 

 salts of which the bases are chiefly sodium and potassium, and the acids 

 chiefly hydrochloric, sulphuric, phosphoric, and carbonic. 



They all give certain reactions, by which their presence may be recog- 

 nized; of these the most characteristic are the following: Boiled with nitric 

 acid they give a yellow color, which deepens into orange upon the addition of 

 ammonia. This is called the xanthoproteie test ; the color is due to a product 

 of decomposition. Boiled with the mixture of mercuric and mercurous 

 nitrates known as Millon's reagent, they give a pink color. Mixed with a 

 strong solution of sodic hydrate they give, on the addition of a drop or two of 



1 A solution of sodium chloride of this strength will hereafter be spoken of as " normal 

 saline solution." 



