Thrombus formation has been studied in normal rabbits with an experimental method of establishing extracorporeal circulation. In the normal animal circulation in the extracorporeal loop usually ceases in from 6 to 10 minutes, or at most, in very large and vigorous animals, in 25 minutes.1 Cessation of the circulation is due most frequently to obstruction of the venous cannula (sometimes of the arterial cannula) by a mass of white thrombi and secondary fibrin formation around it. The site of the clot is determined somewhat by the swiftness of the blood stream.

In the collodion tube red mural thrombi are obtainable as a rule. They are flat and present a wide base resting on tiny white thrombi. After the complete obstruction of the circulation, the blood in the apparatus clots very rapidly. When the obstruction occurs very quickly either in the arterial or venous cannula, or when the blood stream is very slow, the clotting in the loop may occur before platelets are laid down in large numbers or before the formation of the white thrombi is evident.

Mixed thrombi are found in the jugular vein, and have their inception in the white thrombi, in the cannula or from the injured intima. They extend in the direction of the blood flow.

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