BLOOD SEEUM 39 



special purposes the reaction may be standardised. In such a 

 solution the cholera vibrio grows with remarkable rapidity. It 

 is also much used for testing the formation of indol by a 

 particular bacterium ; and by the addition of one of the sugars 

 to it the fermentative powers of an organism may be tested 

 (p. 75). Litmus may be added to show any change in reaction. 



Blood Serum. 



Koch introduced this medium, and it is prepared as follows : 

 Plug the mouth of a tall cylindrical glass vessel (say of 1000 c.c. 

 capacity) with cotton wool, and sterilise by steaming it in a 

 Koch's steriliser for one and a half hours. Take it to the place 

 where a horse, ox, or sheep is to be killed. When the artery 

 or vein of the animal is opened, allow the first blood which 

 flows, and which may be contaminated from the hair, etc., to 

 escape ; fill the vessel with the blood subsequently shed. Carry 

 carefully back to the laboratory without shaking, and place for 

 twenty-four hours in a cool place, preferably an ice chest. The 

 clear serum will separate from the clotted blood. If a centrifuge 

 is available, a large yield of serum may be obtained by centri- 

 fugalising the freshly drawn blood. If coagulation has occurred, 

 the clot must first be thoroughly broken up. With a sterile 10 

 c.c. pipette, transfer this quantity of serum to each of a series 

 of test-tubes which must previously have been sterilised by dry 

 heat. The serum may, with all precautions, have been con- 

 taminated during the manipulations, and must be sterilised. 

 As it will coagulate if heated above 68 C., advantage must be 

 taken of the intermittent process of sterilisation at 57 C. 

 [method B (4)]. It is therefore kept for one hour at this 

 temperature on each of eight successive days. It is always 

 well to incubate it for a day at 37 C. before use, to see that 

 the result is successful. After sterilisation it is "inspissated," 

 by which process a clear solid medium is obtained. "Inspissa- 

 tion " is probably an initial stage of coagulation, and is effected 

 by keeping the serum at 65 C. till it stiffens. This temperature 

 is just below the coagulation point of the serum. The more 

 slowly the operation is performed the clearer will be the serum. 

 The apparatus used for the purpose is one of the various forms 

 of serum steriliser (e.g. Fig. 8), generally a chamber with water- 

 jacket heated with a Bunsen below. The temperature is con- 

 trolled by a gas regulator, and such an apparatus can, by altering 

 the temperature, be used either for sterilisation or inspissation. 

 As is evident, the preparation of this medium is tedious, but its 



