250 INFECTION AND IMMUNITY 



bosis.) The artery is then raised out of the wound on a knife or forceps 

 haadle and, with sharp-pointed scissors, a small incision is made into 

 but not through the vessel. A small glass cannula is now introduced into 

 the proximal end of the artery through the incision and tied into place 

 by a thread. To this cannula a small rubber tube fitted with a pinch- 

 cock should have been attached, the whole being sterilized. The clamp 

 on the artery is then released and the blood allowed to flow into sterile 

 test tubes which are slanted and placed in the cold for separation of the 

 Serum. A larger yield of serum will be obtained if, after coagulation, 

 the clot is separated from the glass with a sterile platinum wire. 



In obtaining blood from larger animals, horses, sheep, etc., a cannula 

 may be introduced into the jugular or internal saphenous veins. The 

 skin is shaved and sterilized and a rubber tourniquet placed about the 

 neck or thigh, as the case may be, in order to cause the vein to stand 

 out. A small incision may be made through the skin over the vein, but 

 is not necessary. A cannula, with rubber tubing attached, is then plunged 

 into the vein and the blood caught in sterile high cylindrical jars, 

 allowed to clot, and placed in the refrigerator. The serimi is taken off 

 after twenty-four to forty-eight hours with sterile pipettes. 



Agglutination Tests. — For the determination of the agglutinating 

 power of serum it is necessary to make suitable dilutions of the serum, 

 and to prepare an even emulsion of the microorganisms to be tested. 

 The test may be made microscopically or macroscopically. The micro- 

 scopic test is the one in general use in the diagnosis of typhoid fever, 

 and is occasionally applied to some other diseases. In its application 

 to typhoid fever it is usually spoken of as the Gruber-Widal reaction. 



Twelve- to eighteen-hour broth cultures of the typhoid bacillus, 

 grown at incubator temperature, may be used. It is preferable, bow- 

 ever, to use an emulsion of a twelve to twenty-four hour old agar culture 

 in physiological salt solution (0.85 per cent) . The salt-solution emulsion 

 is made by adding about 10 c.c. of normal salt solution to the fresh agar 

 slant culture, carefully detaching the culture from the surface of the 

 agar with a flexible platinum wire, and pipetting off the emulsion thus 

 made. With some microorganisms it is sufficient simply to allow the 

 larger clumps to settle and to pipette off the supernatant turbid emulsion. 

 With other microorganisms, the tendency to form clumps makes it 

 necessary to resort to further methods of securing an even distribution 

 of the bacteria. This may be done either by sucking the emulsion in and 

 out through a narrow pipette held perpendicularly against the bottom of 

 a watch glass, as in Wright's technique for the opsonic test (see section 



