THE BLOOD 69 



means of a small plug of cotton fastened to the roughened end of a short 

 wire. 



Annotation. In operations upon animals the work should be divided in such a 

 way that every student has his own particular task to perform. Thus, one should 

 be assigned to give ether, another to perform the tracheotomy, and still another to 

 cannulaize the artery. During the next laboratory period the students should be 

 made to rotate, so that each has a different task to perform. 



3. Coagulation of the Blood. Expose the opposite external jugular 

 vein widely. Place two ligatures about 3 mm. apart around its central 

 portion. Again ligate twice its distal portion. Remove the interven- 

 ing segment of vein in its entirety by cutting between the two central 

 and two peripheral ligatures. Suspend this preparation for a period of 

 about forty minutes. Meanwhile perform the following experiments. 



Annotation. Unless the walls of the vein have been injured the blood in this 

 segment will remain fluid for an indefinite period of time, because no agent is present 

 therein to destroy the thrombocytes and to liberate thrombokinase. 



Withdraw a small quantity of blood from the artery into a watch- 

 glass and observe the formation of the coagulum. 



Allow a drop of blood to fall upon a glass slide, and observe under 

 the microscope the clumping of the corpuscles produced by the deposi- 

 tion of fibrin shreds. 



Draw blood into a beaker and vigorously whip it for a few minutes 

 with a roughened piece of wood. Wash the fibrin attached to the stick, 

 and note its appearance, texture, and elastic properties. State why 

 blood from which the fibrin has been removed remains fluid. Thor- 

 oughly cleanse the cannula. 



Collect a few cubic centimeters of blood in a test-tube and upon a 

 plate. Which specimen clots more rapidly? 



Annotation. Since the walls of the test-tube present a larger destructive sur- 

 face to the blood, this portion will clot more speedily. 



Draw a small quantity of blood into a test-tube about 1 cm. in 

 diameter. Note the time of its withdrawal. Hold the test-tube steady 

 in your hand, slightly tilting it after one minute and again at intervals 

 of one-half minute until it can be inverted without the blood flowing 

 out. The time intervening between the withdrawal of the blood and 

 its coagulation is known as the coagulation time. 



Withdraw a sample of blood into a test-tube and place the latter in 

 crushed ice. Since cold retards all chemical processes, it prolongs the 

 coagulation time. 



Withdraw equal quantities of blood into two test-tubes, one of 

 which has been thoroughly anointed with vaselin. Give an explana- 

 tion for the fact that the blood in the latter clots less speedily. 



Draw blood into one-quarter of its volume of a 1 per cent, solution 

 of potassium oxalate. Explain the fact that this blood remains fluid. 



