110 BACTERIOLOGY. 



the mass a mean of the composition of the several pota- 

 toes, or bits of potatoes, used in making it, an advan- 

 tage where uniformity is desired. 



Care must be given to the sterilization of potatoes, 

 because they always have adhering to them the organ- 

 isms commonly found in the ground, the spores of which 

 are among the most resistant known. The so-called 

 potato bacillus is one of this group ; it is an organism 

 which is not infrequently more or less of an obstacle 

 to the work of the beginner. 



Blood-.seeum. — By the original method of prej)aring 

 blood-serum a great many precautions were taken that 

 have been found unnecessary to the success of the more 

 modern plans. 



It is possible to collect serum from small animals and 

 in small quantities under such precautions that it is per- 

 haps not contaminated ; but, ordinarily, for laboratory 

 purposes a larger quantity is needed, so that slaughter- 

 houses are the source from which it is usually obtained, 

 and here a certain amount of contamination is unavoid- 

 able, though its extent may be limited by proper pre- 

 cautions. 



The precautions to be taken at the slaughter-house in 

 the collection of blood and the preparation of serum for 

 culture purposes are about as follows : 



The animal from which the blood is to be collected 

 should be drawn up to the ceiling by the hind legs ; the 

 head should be held well back, and with one pass of a 

 very sharp knife the throat should be cut. The blood 

 which spurts from the severed vessels should be col- 

 lected in large glass jars which have been previously 

 cleaned and disinfected, and all traces of the disin- 

 fectant removed with alcohol and, finally, with ether. 



