124 PHYSIOLOGY CHAP. 



If the plasma thus obtained by simple cooling of horse's blood 

 is warmed to the temperature of the atmosphere, it coagulates, 

 like the blood in toto, and an incoagulable fluid separates out, 

 which is pure serum. If the clot is squeezed and washed out, 

 the purest fibrin is obtained. Serum is, therefore, nothing but 

 plasma in which the protein which gives rise to the formation 

 of fibrin, and was therefore termed fibrinogen, is wanting. Besides 

 this cardinal difference, however, we shall see that other secondary 

 differences that can be demonstrated between plasma and serum 

 are the result of the coagulation process. 



Since the plasma of horse's blood can coagulate, it is not 

 suitable for the examination of the true proteins which it normally 

 contains. In order to obtain a purer plasma, as free as possible 

 from corpuscles and haemoglobin, and at the same time incoagul- 

 able, the blood of a dog, into whose veins a certain quantity of 

 albumoses (pro-peptones) has been injected intravenously a few 

 minutes before the bleeding (Schmidt-Miihlheirn, Albertoni, Fano), 

 is employed. The peptonised blood obtained in this way has lost 

 its faculty of spontaneous coagulation, so that it is easy by 

 prolonged centrifuging to separate the plasma completely from 

 the mass of corpuscles. The same effect is arrived at by intra- 

 venous injection of leech-extract (Haycraft). Apparently all 

 blood-sucking animals, independent of their zoological position, 

 and merely in relation to the nature of their food and their mode 

 of obtaining it, possess substances in their buccal secretions which 

 impede coagulation : such are the leech (Haycraft), the tick 

 (Sabbatani), and the mosquito (Grassi). 



The plasma obtained from peptonised blood is a transparent, 

 light yellow fluid, absolutely free from haemoglobin ; under the 

 microscope it is found to contain no erythrocytes, and only a few 

 leucocytes and blood-platelets. It does not coagulate spontane- 

 ously ; but when diluted with an equal volume of water, or when 

 a stream of carbonic acid gas is passed through it for a couple of 

 minutes, it is soon converted into a quivering, gelatinous mass, 

 from which the serum, in which floats the snow-white cake of 

 pure fibrin, separates out (Fano). 



Incoagulable plasma can also be obtained by receiving the 

 blood that issues from the veins in a vessel which contains a 

 certain quantity of salt solution, since, with hardly any exceptions, 

 all salts render the blood incoagulable in greater or less degree 

 owing to various physical and chemical reasons (Buglia and 

 Gardella). fwThe solutions most frequently employed in the 

 chemical physiology of the blood are sodium sulphate, sodium 

 chloride, and magnesium sulphate. Twenty -four hours after 

 extraction, or sooner if the centrifuge is used, the mass of the 

 corpuscles separates from the plasma. One inconvenience of this 

 method is that the corpuscles are deformed, and a considerable 



