976 PHYSIOLOGY 



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 thrcftnbin 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, &c., 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 

 generally 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 precipitated 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, serum, 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 complete saturation with ammonium sulphate or sodio- 

 magnesium sulphate, or in the crystalline form by slight acidification, 

 as in Hopkin's method described on p. 80. Serum albumen is 

 soluble in distilled water. Its solutions therefore can be dialysed 

 indefinitely without any precipitation taking place. 



