52 SCIENCE "PROGRESS 
physiologists have recognised to be, next to the colour, its most 
distinctive feature outside the body. A gradual change in 
consistence takes place from the moment blood is shed, until, 
as a firm jelly, it forms a clot which is a cast of the vessel 
containing it. The plasma or fluid in which during life the 
red corpuscles and leucocytes floated has thus formed a single 
coherent mass of fibrin and corpuscles. This forthwith com- 
mences to shrink, with the consequent production of a spon- 
taneously non-coagulable liquid or blood-serum. 
In 1891 Lister! drew attention to the fact that, although 
blood may clot, no marked retraction of the clot due to a 
shrinking of the fibrin occurs when blood has been received 
in a vessel rendered sterile by heat or by washing it out with 
weak perchloride of mercury. A similar phenomenon is 
frequently seen when blood is collected for obtaining antitoxic 
sera. Hayem?and his pupils have also observed the absence 
of serum from the clotted blood of patients with pernicious 
anzemia or purpura hemorrhagica. They attach a special 
importance to this non-retraction of the clot for the positive 
diagnosis of pernicious anemia and most forms of purpura 
hzemorrhagica, and consider the phenomenon is of even greater 
value than a histological examination for the recognition of 
the former disease. This non-retraction of the clot is at present 
unexplained, but since it is a feature of clotted plasma in con- 
trast to normal clotted blood, it is probably dependent in some 
way upon influences exerted on the fibrin by the imprisoned 
corpuscles or, according to Hayem, the blood-platelets, since 
the plasma of birds, which may contain only a few or no 
corpuscles, will, on being caused to clot by the addition of 
tissue-extract, show no subsequent retraction of the clot—a fact 
which has been noticed by Delezenne,’ Fuld,t and Spangaro.° 
The fibrin, which forms but a very small amount of the bulk of 
the clot, subsequently alters in its physical characters, becoming 
more soluble. The extent of fibrinolysis and the rate at which 
this occurs differs in different animals; it is also a subject of much 
interest, since a study of the conditions under which it occurs 
' Lancet, p. 1082, 1891. 
? La Semaine médicale, November 1896 ; Comptes rend. January 1901. 
3 Arch. de Physiol. p. 333, 1897. 
4 Hofmeisters Bettrage, ii. p. 514, 1903. 
5 Arch. ital. de Biologie, xxxii. p. 210, 1900. 
