CHAP, i.] BLOOD. 17 



Putting these facts together, it is very clear that the pheno- 

 mena 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 coloured clot and 

 the colourless serum. 



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

 or by washing either a normal clot, or a clot obtained from colour- 

 less 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 attempting 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 ^th their weight of nitrogen ; 

 and yet as we shall see they are eminently the nitrogenous sub- 

 stances of the body. They usually contain a small quantity 

 (1 or 2 p.c.) 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 recognised ; of these the most characteristic are the following : 

 Boiled with nitric acid they give a yellow colour, which deepens 

 into orange upon the addition of ammonia. This is called the 

 xanthoproteic test ; the colour is due to a product of decomposi- 

 tion. Boiled with the mixture of mercuric and mercurous 

 nitrates known as Millon's reagent they give a pink colour. 

 Mixed with a strong solution of sodic hydrate they give on the 

 addition of a drop or two of a very weak solution of cupric sul- 

 phate a violet colour which deepens on heating. These are artificial 

 reactions, not throwing much if any light on the constitution of 

 proteids ; but they are useful as practical tests enabling us to 

 detect the presence of proteids. 



The several members of the proteid group are at present dis- 

 tinguished from each other chiefly by their respective solubilities, 

 especially in various saline solutions. Fibrin is one of the least 

 soluble ; it is insoluble in water, almost insoluble in dilute neutral 

 saline solutions, very sparingly soluble in more concentrated 



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