4 IMMUNE SERA 



than 150 units per cc. are discarded. If moderate 

 grades only are desired, all that yield 100 units may be 

 retained. The retained horses receive steadily in- 

 creasing doses, the rapidity of the increase and the 

 interval of time between the doses (three days to one 

 week) depending somewhat on the reaction following 

 the injection, an elevation of temperature of more than 

 3 F. being undesirable. At the end of three months 

 the antitoxic serum of all the horses should contain 

 over 300 units and in about 10 per cent as much as 800 

 units per cc. Very few horses ever give over 1000 

 units, and none so far has given as much as 2000 units 

 per cc. The very best horses, if pushed to their 

 limit continue 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. 



Collecting the Serum. 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 the blood is withdrawn from the jugular vein by means 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 attached to the 

 canula, 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 4 days by means of sterile glass and 

 rubber tubing, and is stored in large flasks in a refrige- 

 rator. From this as needed small vials are filled. 

 The vials and their stoppers, as indeed all the utensils 

 used for holding the serum, must be absolutely sterile 

 and evqry possible precaution must be taken to avoid 

 contamination of the serum. An antiseptic may be 

 added as a preservative, but is not necessary. Diph- 



