112 READINGS IN BIOLOGICAL SCIENCE 



sarily injured in the experiment at no time comes into contact with the 

 blood, clotting fails to occur, or is greatly delayed. This fluid blood, even 

 after hours, can then be made to clot within a few minutes by adding juices 

 compressed from almost any tissue. Something present in cells generally, 

 hberated when they are injured, is able to initiate the clotting process. 



Contact with foreign surfaces: platelet disintegration — If blood is drawn 

 carefully, so as to avoid contact with injured tissues, it will clot in the 

 normal time if the container is glass. But, if the glass beaker is lined with 

 parafHne, clotting will be delayed. The essential difference here seems to 

 be the physical nature of the glass or paraffine surface to which the blood 

 is exposed. Just what this difference is, is not clear, except that watery 

 solutions "wet" a clean glass surface but drain from a paraffine or waxy 

 surface, leaving it quite dry. In general, surfaces which are "wet" by 

 water, on which a thin film of water tends to remain after most of the 

 water is drained off, behave like glass as regards clotting. Surfaces which 

 are not "wet" by water, in general, retard clotting. 



Clotting can even be induced in the blood vessels when the blood is 

 exposed to the proper surfaces. If a pin is stuck through a vein, a thin 

 clot forms on the pin as the blood flows by. Or, if particulate matter of a 

 suitable kind is injected, each particle soon comes to be covered with a 

 thin clot. 



Just why this happens is not clear. But, if the process is carefully ob- 

 served under the microscope, it will be seen that certain of the formed 

 elements of the blood — the blood platelets — collect upon surfaces like 

 glass and quickly disintegrate. At a paraffined surface this occurs only 

 slowly. The platelets seem to be involved in these surface relationships, 

 and their disintegration apparently initiates clotting. This can be verified 

 in other ways. If blood is drawn into an ice-cold vessel, and is immediately 

 cooled sufficiently, clotting does not take place, and examination reveals 

 that the platelets have remained intact. As soon as the blood is again 

 warmed, the platelets quickly disintegrate, and clotting occurs. Why 

 platelets respond in this way to temperature changes is not known, but 

 the important consideration here is that the clotting seems to be dependent 

 upon platelet disintegration. Various chemicals are also known which de- 

 lay the breakdown of platelets. They all retard clotting. 



THE RED BLOOD CORPUSCLES 



The red blood cells, or erythrocytes, are the most numerous of the 

 formed elements, each cubic millimeter of human blood containing four 

 and a half to five millions. Human erythrocytes are biconcave disks, a little 

 less than 0.008 mm. or about H,2oo inch in diameter. They are normally 

 of such uniformity in size that histologists frequently use them as handy 

 units of measurement, including a red cell in a drawing to indicate the 

 size of the cells in any tissue, relative to this nearly uniform red-cell size. 



