DIPHTHERIA BACILLUS. 361 
ever give above 1000 units, and none so far has given 
as much as 2000 units perc.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In order to obtain the serum the blood is withdrawn 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 asterile 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 
