212 PHYSIOLOGY. 



Fifth. THE PRECIPITIN TEST. Strong rabbits are injected 

 subcutaneously with 5 cubic centimeters of sterile, human blood, the 

 injections being repeated every two or five days, depending upon the 

 condition of the test animal. The occurrence of a rise of tempera- 

 ture above 101 F. or a decided loss in weight are considered 

 counter-indications to further injections until after this reaction has 

 subsided. It is better to give injections of only 5 cubic centimeters 

 each and always with great care as to asepsis, since abscesses often 

 develop at or near the site of puncture. Usually 20 to 30 cubic 

 centimeters make a sufficient quantity for the average-sized rabbit, 

 and with due care a specific anti-serum can always be produced in 

 from three to four weeks. After a sufficient quantity of blood has 

 been injected to insure obtaining an anti-serum, the rabbit is chloro- 

 formed, the chest-cavity opened, and the blood drawn from the heart 

 into a sterile receptacle by means of a sterile trocar and cannula. 

 The drawn blood is placed in an icebox for one hour until well coag- 

 ulated. Carbolic acid is now added to the serum, which has separ- 

 ated sufficiently to make the mixture approximately 0.5 per cent, 

 acid. The serum is then drawn up into sterile pipettes and sealed. 

 It will remain potent indefinitely if kept at a low temperature. 



The test is made as follows : A given amount of the test-serum 

 is diluted to the desired extent with sterile water or normal saline 

 solution. To a few cubic centimeters of this diluted solution in a 

 sterile test-tube is added an equal quantity of a similarly diluted 

 solution of the blood to be tested and the tube left at room tempera- 

 ture or placed in an incubator for two or three hours at 37 C. The 

 reaction, if it occurs, will be more rapid and marked if the tube is 

 exposed to the higher temperature. If the dilution be sufficient the 

 reaction will not occur at room temperature. If the test-serum is 

 used undiluted and pure human blood is added to it, the reaction is 

 immediate. 



If only the sample of blood to be tested is diluted and pure test- 

 serum is used, the reaction is almost immediate. The reaction is 

 marked by a turbidity of the solution, becoming constantly more 

 intense. If an old stain is to be examined by the serum test the 

 material containing it is washed in sterile water or in sterile normal 

 saline, the mixture is repeatedly filtered and finally added to some 

 of the test-serum, as in the examination of fresh blood already 

 described. 



Contamination with monkey blood can be excluded first by a 

 great dilution of the blood tested, and a dilution of the test-serum 



