$36 MEDICAL BACTERIOLOGY 



a. Make a hanging drop preparation from a 24 to 72-hour old 

 agar, or bouillon, culture of Bacillus typhosus. 



5. If the bacilli be actively motile, remove the cover-glass, add 

 to the culture a small drop of a solution of typhoid blood (diluted 

 from 10-50 times), return the cover glass to the slide and seal well 

 with vaselin. 



c. Examine with a high dry power (1-6 in obj.) rather than with 

 the oil immersion. 



The dilution is made in the following way : Nine drops of sterile 

 water are placed around the drop of dried blood. (The drops of 

 water should be of about the same size as that of the original drop of 

 blood.) The drops are all mixed together and allowed to soak up 

 the blood for about two minutes. In this way an approximate dilu- 

 tion of one to ten is obtained. One drop of this is added to the 

 hanging-drop culture. This gives a dilution of one to twenty which 

 is the one usually employed. 



More exact dilutions of dried blood may be made by weighing out 

 the blood and adding it to a measured amount of water. 



Where possible the blood should be collected so that the clear 

 serum may be separated and used for the test. This can be done in 

 hospital work and wherever it is possible to get the blood to the 

 laboratory a few hours after it is collected. For this purpose a glass 

 pipette is prepared by drawing out a glass tube, as indicated 

 in Fig. 39, which represents the pipette natural size. The 

 skin is cleaned and the blood drawn as indicated above and 

 when a large drop has collected on the skin one of the points 

 of the pipette is introduced when the blood is drawn up by 

 capillary attraction. The bulb ought to be about one-half 

 filled. The pipette is then placed in a horizontal position 

 until the blood has clotted, when it may be taken to the 

 laboratory. It should then be placed in the ice chest, still 

 in a horizontal position, for two or three hours. The end 

 which was used to draw up the blood is then scratched with 

 a file and broken off. By holding the tube in a vertical 

 F position the clear serum may now be dropped from the 



B . ! oo a opposite end into a glass or porcelain capsule. The clear 



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serum is then taken up with a clean capillary pipette and a 

 drop placed in another capsule and then after rinsing out the same 

 pipette is used to add the requisite number of drops of bouillon or 

 salt solution to make the required dilution. The test is then made 

 in exactly the same way as described for the dried blood, 



