302 PATHOGENIC BACTERIA. 



The Preparation of the Serum for Therapeutic Pztr- 

 floses.When, because of the tolerance to large quanti- 

 ties of toxin, the horse seems to possess antitoxic blood, 

 a "twitch" is applied to the upper lip, the eyes are 

 blindfolded, a small incision is made through the skin, 

 a trocar thrust into the jugular vein, and the blood al- 

 lowed to flow through a cannulated tube into sterile 

 bottles. It is allowed to coagulate, and kept upon ice 

 for two days or so, that the clear serum may be pi- 

 petted off. This serum is the antitoxic serum. It does 

 not always materialize according to the desires of the 

 experimenter, sometimes proving surprisingly strong in 

 a short time, sometimes very weak after months of 

 patient preparation. 



The serums are preserved by Roux with camphor, by 

 Behring with carbolic acid (0.5 per cent.), and by Aron- 

 son with trikresol (0.4 per cent.). I prefer to use tri- 

 kresol, as it is not poisonous, is a reliable antiseptic, and 

 has a very pronounced local anesthetic action. Formalin 

 has been tried, but it gelatinizes the serum and causes 

 much local pain when injected beneath the skin. 



Dried antitoxic serum has also been placed upon the 

 market under the impression that it will keep longer and 

 bear shipment better than any other. This is not, how- 

 ever, shown to be the case, and as the dried serum dis- 

 solves with difficulty it is much less convenient than the 

 usual preparations. It is also less likely to be sterile 

 than the liquid forms. 



The strength of the serum is expressed in what are 

 known as immunizing units. This denomination origin- 

 ated with Behring and Ehrlich, whose normal serum 

 was of such strength that o.i c.cm. of it would protect 

 against ten times the least certainly fatal dose of toxin 

 when simultaneously injected into guinea-pigs. Each 

 cubic centimeter of this normal serum they called an 

 immunizing unit. Later it was shown that the strength 

 of the serum could easily be increased tenfold, so that 

 o.oi c.cm. of the serum would protect the guinea-pig 



