910 PHYSIOLOGY 



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 solubility 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. Fib- 

 rinogen in salt solution coagulates at 56 C. A small amount however 

 remains in solution and is not coagulated until 65 C. is reached. Fibrinogen 

 can be therefore described as a globulin occurring in the plasma and con- 

 verted on coagulation into fibrin. The other precursors of fibrin, namely, 

 those involved in the production of thrombin and called thrombokinase 

 and thrombogen, seem to be phosphorus-containing proteins, perhaps 

 belonging to the class of nucleo-proteins. Their chief characteristics have 

 already been dealt with. 



FIBRIN. Fibrin is easily obtained by whipping blood as it flows from 

 the vessels with a bundle of wires or twigs, and then washing the stringy 

 threads so obtained under a stream of water. As prepared in this way 

 it always contains fragments of leucocytes, blood platelets, and stromata, 

 which have become entangled in its meshes. In order to prepare fibrin 

 in a pure state, it is necessary to get it by the action of fibrin ferment on 

 a pure solution of fibrinogen. Fibrin is a white stringy substance insoluble 

 in water and in dilute salt solutions. It slowly dissolves in 5 per cent, 

 solutions of sodium chloride, sodium sulphate, potassium nitrate, etc., 

 but is converted in this process into soluble globulins. It is probable 

 that its solution is effected by the agency of minute traces of proteolytic 

 ferment present in the blood and adherent to the ;fibrin as it is precipitated. 

 This probability is strengthened by the fact that a certain amount of 

 albumoses is always found in the fluid along with the soluble globulins. 

 In dilute acid, such as 0-2 per cent, hydrochloric acid, fibrin swells into a 

 clear jelly which very slowly undergoes solution with the formation of acid 

 albumen and proteoses. 



THE PROTEINS OF THE SERUM. The serum proteins are generaUy 

 grouped in two classes, namely, the serum albumens and the serum globulins. 

 All the proteins are completely precipitated by saturation with ammonium 

 sulphate. By half -saturation with this salt only the globulins are pre- 

 cipitated and can be separated from the serum albumens by filtration. 

 The proportion of globulin to albumen as determined in this way is known 

 as the ' protein quotient.' It varies in different animals, but in the same 

 individual it is almost constant in the blood, seruni, lymph, and serous 

 transudations, though the total amounts of protein in these may be very 

 different. 



SERUM ALBUMEN. Serum albumen remains in the serum after half- 

 saturation with ammonium sulphate. It can be precipitated from this by 



