Blood-serum 197 



But as the medium cannot be subsequently sterilized, the greatest dexterity 

 in filling the tubes or dishes is required to keep them from contamination, and it 

 is best to incubate them for 24 hours before using to make sure that they are 

 sterile. When the "blood-agar" is intended to be employed for the purpose of 

 determining the hemolyzing power of the micro-organismal products, the corpus- 

 cles are not laked, but the melted agar-agar receives an addition of one per cent, 

 of the defibrinated blood. 



BLOOD-SERUM 



The advantage possessed by this medium is that it is primarily a 

 constituent of the animal body, and hence offers conditions favor- 

 able for the development of the parasitic forms of bacteria. If the 

 blood-serum is to be employed fresh, it must either be heated or kept 

 sufficiently long to lose its natural germicidal properties. The 

 statement that serum represents the normal body-juice is erroneous, 

 as it is minus the fibrin factors and some of the salts, and contains 

 new bodies liberated from the destroyed leukocytes. Solidified 

 blood-serum, exposed to the heat of the sterilizing apparatus, in no 

 sense resembles the body-juices. 



It is one of the most difficult media to prepare. The blood must 

 be obtained either by bleeding some good-sized animal, or from a 

 slaughter-house, in appropriate receptacles, the best things for the 

 purpose being i-quart fruit jars with tightly fitting lids. The jars 

 are steriUzed by heat, closed, and carried to the slaughter-house, 

 where the blood is permitted to flow into them from the severed 

 vessels of the animal. It seems advisable to allow the first blood to 

 escape, as it is likely to become contaminated from the hair. By 

 waiting until a coagulum forms upon the hair the danger of con- 

 tamination is diminished. The jars, when full, are allowed to stand 

 undisturbed until firm coagula form within them, after which they 

 are carried to the laboratory and stood upon ice for forty-eight 

 hours, by which time the clots will have retracted considerably, and 

 a moderate amount of clear serum can be removed by sterile pipets 

 and placed in sterile tubes. If the serum obtained be red and 

 clouded from the presence of corpuscles, it may be pipetted into 

 sterile cylinders and allowed to sediment for twelve hours, then 

 repipetted into tubes. 



As the demand for sermn has been considerable during the last 

 few years, commercial houses deahng in biologic products now 

 market fresh horse serum, preserved with chloroform, in hter bottles. 

 This can be employed with great satisfaction, the chloroform being 

 driven off during coagulation and sterilization. 



If it be desirable to use the serum as a liquid medium, it is exposed 

 to a temperature of 6o°C. for one hour upon each of five consecutive 

 days. To coagulate the serum and make a solid culture medium, 

 it may be exposed twice, for an hour each time — or three times if 

 there be reason to think it badly contaminated — to a temperature 

 just short of the boiling-point. During the process coagulation 

 occurs, and the tubes should be incHned, so as to offer an obhque 



