126 BACTERIOLOGY. 



protected against evaporation by sterilized rubber caps 

 or sterilized corks in the way already described, and set 

 aside until needed. 



Unless the coagulation in the dry sterilizer be com- 

 plete, the surface of the serum will be found to be blis- 

 tered and pitted by bubbles and cavities after it has 

 been subjected to the steam sterilization. A similar 

 formation of cavities over the surface of the serum will 

 occur if the temperature of the hot-air sterilizer, in 

 which it is solidified, is allowed to get above 90 C., 

 or if it be elevated to this point too quickly. 



It is of no special advantage to have the serum clear, 

 as the admixture of blood-coloring-matter does not affect 

 its nutritive properties. 



METHODS OF OBTAINING BLOOD-SERUM. It is often 

 desirable to obtain small quantities of blood-serum 

 under strictly aseptic precautions, and for this purpose 

 Nuttall 1 suggests a very convenient method. By the 

 use of a sterilized vessel, of the shape shown in Fig. 

 20, from 10 to 100 c.c. of blood can be collected, and if 

 proper precautions are observed no contamination by 

 bacteria need occur. The collecting bulb is used in the 

 following way : an artery, either femoral or carotid, is 

 exposed, and around it two ligatures are placed ; that 

 distant from the heart is tightened, while the one near- 

 est the heart is left loose ; between the latter and the 

 heart the artery is clamped. A small slit is then made 

 in its wall, into which the point a of the bulb is intro- 

 duced and the artery bound tightly around it with the 

 hitherto loose ligature ; the clamp is removed and the 

 bulb quickly fills with blood. The clamp is now again 

 put in position, the point of the bulb removed and 



1 Nuttall : Centralbl. f. Bakt. u. Parasitenkunde, 1892, vol. xi. p. 539. 



