QUANTITY AND COMPOSITION OF BLOOD 975 



The important points to be drawn from these analyses may be 

 summarised as follows. Blood contains from rather over one-third 

 to one-half of its weight of corpuscles. It contains from 20 per cent, 

 to 25 per cent, solids. Blood- plasma is resolved by clotting into serum 

 and fibrin. The fibrin forms only 0-2 to 0-4 per cent, of the total 

 weight of blood. The serum contains in 100 parts 8 to 9 parts of 

 solids, of which 7 to 8 parts consist of proteins, while the salts make 

 up about 1 part. The chief salt present in the serum is sodium 

 chloride, which constitutes 60 per cent, of the ash. Next to this comes 

 sodium carbonate, about 30 per cent., and besides these two we find 

 traces of potassium, sodium, and calcium chlorides and phosphates. 

 Traces of fats, cholesterin, lecithin, dextrose, urea, and other nitro- 

 genous extractives are constantly found in the serum. The fats are 

 much increased after a meal rich in them and may give the serum a 

 milky appearance. The red corpuscles contain from 30 to 40 per cent, 

 total solids. Of the solid constituents haemoglobin forms nine-tenths ; 

 the other tenth corresponds to the stroma consisting of stroma protein 

 (nucleo-protein), lecithin, cholesterin, and salts. There is a striking con- 

 trast between the salts of the corpuscles and those in the serum, the 

 former consisting chiefly of potassium phosphate, the latter of sodium 

 chloride, which in some animals is entirely wanting in the corpuscles. 



THE PROTEINS OF THE PLASMA 



The plasma is generally described as containing a number of 

 different proteins belonging to the class of coagulable proteins. No 

 albumoses or peptones are present. Since the plasma in clotting 

 gives rise to fibrin and serum we may divide its protein constituents 

 into those which are the precursors of fibrin and those which are still 

 contained in the serum. 



THE PRECURSORS OF FIBRIN. Most of these have been dealt 

 with in discussing the causation of coagulation. It only remains for 

 us here to mention some of the chemical features of fibrinogen and its 

 product fibrin. Fibrinogen is best separated by Hammarsten's 

 method, namely, half -saturation with sodium chloride, or by the use of 

 ammonium sulphate. Fibrinogen is precipitated between 13 and 

 28 per cent, saturation with ammonium sulphate, whereas no other 

 globulins are precipitated until the saturation amounts to 29 per cent, 

 of ammonium sulphate. Fibrinogen obtained in either of these ways 

 can be purified by re-solution and re-precipitation, but loses its solu- 

 bility in the process, so that every time it is precipitated some of 

 the substance becomes insoluble. The insoluble fibrinogen resembles 

 fibrin in many characters, but does not swell in the presence of dilute 

 acids as fibrin does. Fibrinogen is soluble in dilute alkali, from which 

 it may be precipitated by careful neutralisation. Fibrinogen in salt 



