138 HOUSEHOLD BACTERIOLOGY 



immunized. But these did not furnish enough. So at 

 last the horse was tried, and was found admirably 

 adapted to the purpose. He lends himself readily to 

 the increasing doses of the potent diphtheria poison ; 

 he is easily rendered immune, and he furnishes without 

 especial inconvenience a large quantity of blood. In 

 fact, he makes no more fuss about losing blood than 

 did the old people along in the early part of the last 

 century, who were quite accustomed in the springtime, 

 when they felt a bit heavy and had a little headache, 

 to drop into the nearest barber shop to be bled. 



The preparation of diphtheria antitoxin has been 

 brought to a high state of perfection. The horses 

 are first very carefully tested so as to be certain that 

 they have no disease. They are well fed and groomed, 

 and suitably exercised. At first a small amount of the 

 diphtheria toxin is injected beneath the skin. After 

 a few days a larger dose is given, and then at intervals 

 larger and larger quantities, until at last the horse is 

 receiving such an amount in a single dose as if given 

 at first would have sufficed to kill not only one but 

 many horses. He has not had diphtheria at all, but 

 he is now poison-proof immune. 



The animal is then bled from the large vein in the 

 neck, the greatest care being taken, by cleansing of the 

 skin, the use of sterilized instruments, etc., that no 

 outside germ shall get into the blood as it flows. This 

 blood is set aside in a cool place, and presently, as the 

 clot forms, the serum separates in considerable quan- 



