112 MICROSCOPIC METHODS. 



To observe sedimentation mix equal parts of diluted serum and of bacterial 

 emulsion and place in a thin glass tube — a simple tube with closed end or 

 a U-tube. Keep in upright position for twenty-four hours. One of Wright's 

 sedimentation tubes is shown in Fig. 53, d. Diluted serum is drawn up to 

 fill the space nm, a small quantity of air is sucked up after it to separate it 

 from the bacterial emulsion, which is then drawn up in the same quantity ; 

 the diluted serum will then occupy the position kl. The fluids are then 

 drawn several times up into the bulb and returned to the capillary tube so 

 as to mix, and finally blown carefully down close to the lower end, which is 

 then sealed off. The sediment collects at the lower extremity. 



General Bacteriological Diagnosis. 



Under this heading we have to consider the general routine 

 which is to be observed by the bacteriologist when any material 

 is submitted to him for examination. The object of such 

 examination may be to determine whether any organisms are 

 present, and if so, what organisms ; or the bacteriologist may 

 simply be asked whether a particular organism is or is not pres- 

 ent. In any case his inquiry must consist (i) of a microscopic 

 examination of the material submitted ; (2) of an attempt to 

 isolate the organisms present ; and (3) of the identification of the 

 organisms isolated. We must, however, before considering these 

 points look at a matter often neglected by those who seek a 

 bacteriological opinion, viz. : the proper methods of obtaining and 

 transferriitg to the bacteriologist the material which he is to be 

 asked to examine. The general principles here are (i) that every 

 precaution must be adopted to prevent the material from being 

 contaminated with extraneous organisms ; (2) that nothing be 

 done which may kill any organisms which may be proper to the 

 inquiry; and (3) that the bacteriologist obtain the material as 

 soon as possible after it has been removed from its natural 

 surroundings. 



The sources of materials to be examined, even in patho- 

 logical bacteriology alone, are of course so varied that we can 

 but mention a few examples. It is, for instance, often necessary 

 to examine the contents of an abscess. Here the skin must be 

 carefully purified by the usual surgical methods ; the knife used 

 for the incision is preferably to be sterilised by boiling, the first 

 part of the pus which escapes allowed to flow away (as it might 

 be spoiled by containing some of the antiseptics used in the purifi- 

 cation) and a little of what subsequently escapes allowed to flow 



