230 ANTITOXINS 



in the laboratory. A series of guinea-pigs weighing from 250 to 300 

 grams are injected subcutaneously with increasing doses of a diphtheria 

 toxin to determine the smallest dose that will just kill a pig at the end 

 of four days after injection. This is known as the minimal lethal dose 

 (M.L.D.), and one-fortieth to one-fiftieth this amount is the proper dose 

 to inject into the skin. To facilitate injecting so small an amount of fluid 

 this dose is so diluted with sterile normal salt solution as to be contained 

 in 0.1 c.c.; a preservative, as 0.25 per cent, tricresol or phenol, is added. 



FIG. 69. SHOWING THE SMALL ANEMIC AREA IMMEDIATELY AFTER AN INTRACU- 



TANEOTJS INJECTION. 



The toxin does not contain the bacilli and this dose is so small as to 

 be without danger. 



The method of injection is very important. A sterile syringe equipped 

 with a fine needle is required. I use and can recommend the 1 c.c. Record 

 syringe fitted with No. 26 platinum iridium needle, but any accurate 

 syringe so made that 0.1 c.c. can be measured and injected will suffice. 

 The skin of the upper arm is wiped off with a pledget of cotton and al- 

 cohol, the skin pinched up between the forefinger and thumb of the 

 left hand, and the needle entered into the epidermis and not under it, so 



