162 THE BLOOD 



was drawn into the test-tubes and set the test-tubes in a test-tube rack. 

 Examine at intervals of 30 seconds by gently inclining the test tubes. 

 Presently it will be noted that the blood becomes more viscous and does 

 not flow freely up the sides of the test-tubes. Later the whole mass will 

 become jelly-like and will retain the form of the test-tube. Note the time 

 of the first slight change, and also when the clot becomes more perfect. 

 The sample in the oiled test-tube will be found to clot more slowly. 



If the test-tubes of clotted blood are left standing for a day, the coagu- 

 lum will become smaller in size and a transparent yellowish blood will make 

 its appearance on the surface or between the sides of the clot and the test- 

 tube wall. This fluid is the serum and it is squeezed out by the shrinking 

 of the fibrin which holds the corpuscles in its meshes. 



b. The Time of Blood Clotting. The speed of clotting is measured more 

 accurately by Cannon's coagulometer, see figure 108. A sample of blood is 

 carefully drawn from an artery under conditions which insure fresh circulat- 

 ing blood (Cannon and Mendenhall, American Journal of Physiology, Vol. 34, 

 p. 225, for fuller details). This sample is inserted in the coagulometer and 

 successive tests for coagulation made every 30 seconds. Even a thread or two 

 of fibrin is indicated by the apparatus if the lever is accurately counterpoised. 



FIG. 1320. Successive tests of the coagulation of blood drawn from the femoral 

 artery of an animal in uniform condition. The mark below the time record signi- 

 fies when the sample was drawn. Time in 30 second intervals. (From Cannon and 

 Mendenhall.) 



c. Microscopic Examination of the Process of Clotting. Take a drop of 

 fresh blood from the tip of your finger under sterile conditions and mount 

 on a microscopic slide in a few drops of salt solution, and examine immediately 

 under the high power. Small threads of fibrin will presently be seen to form 

 across the field, usually being most clearly obvious where fragments of 

 white corpuscles are noted, see figures 107 and 132. The threads of fibrin 

 become more apparent when stained with rosanilin. 



d. Whipped Blood. Draw a sample of blood into a glass tumbler, 

 enough to fill it one-half or two-thirds full. Immediately begin vigorously 

 stirring the blood with a bunch of stiff wires or a pencil, and keep it up until 

 the time of clotting has passed, 5 or 10 minutes. In this instance the wires 

 will break up and collect the fibrin as fast as it forms, and no firm mass will 

 be produced. The remaining fluid is called whipped blood. The fibrin can 

 be removed from the wires and washed in tap water until all the adherent 

 red corpuscles are removed. This mass of fibrin is white, elastic, and com- 

 posed of a network of thread-like fibers. It is these fibers extending 



