60 The Precipitins: Methods 



the nock on the stretch. The animal being held in this position by an 

 assistant, the operator's left hand is placed upon the back of the rabbit's 

 neck, which is uppermost, the fingers and thumb encircling it. These, 

 being drawn upward, put the skin on the stretch transversely to the 

 long axis of the animal. By pressing downward, the neck arches ven- 

 trally. A shai'p sterilized knife is now alloAved to make a clean sweep 

 through the tense skin to and through the vessels. The skin retracts, 

 and the blood spurts out into a large sterile dish which is innnediately 

 covered when the main floAv has ceased. Drippings are caught in a 

 similar manner in Petri dishes, the thorax etc. being compressed to 

 expel as much blood as possible from the body. 



The dishes are placed horizontally until a clot has formed ; they are 

 then slightly tilted, and as soon as serum enough has been expressed, 

 this is pipetted off into sterile test-tubes, and thence transferred to 

 sealed bulbs of small size. The serum which conies off later is similarly 

 collected, but it may be necessary to add chloroform thereto or to pass 

 it through a Chamberland filter to exclude bacterial developments 

 Even when a hair or two falls in, the first serum will generally be 

 sterile, the clot retaining the microorganisms, the bacteria being more- 

 over in part destroyed through the bactericidal properties of the fresh 

 serum. After serum has been drawn off once or twice, a further amount 

 may be gained by slashing the clot with a sterilized knife. This last 

 serum is bloody, and has to be centrifugalized or placed in the refrige- 

 rator to sediment. 



Storing Antisera. 



The most suitable form of bulb in which to store antisera is repre- 

 sented in figure 3 B (p. 65), being drawn out at both ends. The bulb 

 is sterilized in the process of drawing out the ends ; the latter are cut 

 otf with a diamond. After cooling, a number of bulbs are grasped in 

 the hand, the ends being sterilized in the flame, after which they are 

 allowed to cool, resting upon sterilized wire-gauze or otherwise. The 

 antiserum is now drawn up into the bulbs until these are filled, as little 

 air as possible being left in the portion where it narrows. By tilting 

 the tube a little serum flows into the opposite drawn-out portion. The 

 capillaries arc now sealed near to the serum by means of the small 

 flame, such as serves U) relight the large flame of certain Bunsen 

 burners. A large flame is unsuitable. The serum must not be heated 

 in the process of sealing. The tubes are now placed vertically in tin 



' The effects of filtration are noted on p. 118. 



