286 MANUAL OF BACTERIOLOGY. 



injections last over a long period usually about two or three months. 

 The general condition of the animal remaining good, the toleration of 

 these large doses of toxin is presumed to indicate the existence of a con- 

 centrated antitoxic substance in the blood. Small quantities of blood 

 may be withdrawn from time to time, and the serum tested for its 

 antitoxic strength. When a satisfactory serum has been attained, the 

 animal may be bled and the serum saved for therapeutic purposes. 

 Through an incision in the skin a trochar is inserted into the jugular 

 vein. The blood is conducted into sterilized flasks with every precau- 

 tion to insure sterility. The blood is allowed to coagulate and is placed 

 for a time in the ice-chest. The serum is then withdrawn with steril- 

 ized pipettes. Small amounts of chemical germicides, as carbolic acid 

 or chloroform, are sometimes added to assist in preserving it. This 

 serum is the so-called antitoxin used in medical practice. 



A standard to express the potency of the serum, called an immunity 

 unit, has been devised by Behring and modified by Ehrlich. Such an 

 immunity or antitoxic unit is the amount of antitoxic principle con- 

 tained in that quantity of serum, which, when mixed and injected with 

 one hundred times the fatal dose of toxin will preserve the life of a 

 guinea-pig weighing 250 grains for four days. 



It has been proposed to modify this test so as to lessen certain 

 sources of error. According to the new method, one hundred fatal 

 doses of toxin would not be used, but the amount of toxin, which, 

 when mixed with one unit of some special and previously standardized 1 

 antitoxin and injected into a guinea-pig weighing 250 grams, will kill 

 the animal in four days. This procedure would partly do away with 

 inaccuracies, unavoidable where one hundred fatal doses of toxin are 

 used as a standard, resulting from changes that occur in the toxin 

 (development of toxoids, presence of IOXOIK-S, etc, Ehrlich). 



It has been found possible to prepare antitoxin of a high 

 degree of concentration, so that 500 to 1,500 units may be 

 contained in a quantity of serum which it is practicable to 

 give at a single hypodermic injection. The large volume of 

 statistics that have been collected from hospitals, and from 

 physicians in private practice indicates that the use of this 

 serum has effected a very great reduction in the mortality 

 from diphtheria. 



'The standard largely used in this country is an antitoxin prepared 

 for this purpose by the lustitut fiir cxpcrimcntcllc Tlicrapic, Frankfort 

 a Main, (iermany. Prof. Ehrlich, Director. 



