DIPHTHERIA BACILLUS. 361 



ever give above 1000 units, and none so far has given 

 as much as 2000 units per c.c. The very best horses 

 continue to furnish blood containing the maximum 

 amount of antitoxin for several months, and then, in 

 spite of increasing injections of toxin, begin to furnish 

 blood of gradually decreasing strength. If every nine 

 months an interval of three months' freedom from 

 inoculations is given, the best horses furnish high- 

 grade serum during their periods of treatment for from 

 two to four years. 



In order to obtain the serum the blood is withdrawn 

 from the jugular vein by means of a sharp- pointed 

 canula, which is plunged through the vein wall, a slit 

 having been made in the skin. The blood is carried 

 by a sterile rubber tube into large Erlenmeyer flasks 

 and allowed to clot, the flasks, however, being placed 

 in a slanting position before clotting has commenced. 

 The serum is drawn off after four days by means of sterile 

 glass and rubber tubing, and is stored in large flasks. 

 From this, as needed, small phials are filled. The 

 phials and their stoppers, as indeed all the utensils used 

 for holding the serum, must be absolutely sterile, and 

 every possible precaution must be taken to avoid 

 contamination of the serum. An antiseptic may be 

 added to the serum as a preservative, but it is not 

 necessary and probably inadvisable, except when the 

 serum is to be sent to great distances, where it cannot 

 be kept under supervision. 



Kept from access of air and light and in a cold place 

 it is fairly stable, deteriorating not more than 40 per 

 cent., and often much less, within a year. Diphtheria 

 antitoxin, when stored in phials and kept under the 

 above conditions, contains within 10 per cent, of its 



