274 BACTERIA PATHOGENIC TO MAN 



formed. According to the completeness of the reaction, either all of 

 the bacilli may finally become clumped and immobilized or only a small 

 portion of them, the rest remaining freely motile, and those clumped 

 may appear to be struggling for freedom. With blood containing a 

 large amount of agglutinating substances all the gradations in the 

 intensity of the reaction may be observed, from those shown in a 

 marked and immediate reaction to those appearing in a late and in- 

 definite one, by simply varying the proportion of blood added to the 

 culture fluid. 



PSEUDOREACTIONS. If too concentrated a solution of dried blood 

 from a healthy person is employed a picture is often obtained which 

 may be mistaken for a reaction. Dissolved blood always shows a vary- 

 ing amount of detritus, partly in the form of fibrinous clumps; and 

 prolonged microscopic examination of the mixture of dissolved blood 

 with a culture fluid shows that the bacilli, inhibited by substances in the 

 blood, often become more or less entangled in these clumps, and in 

 the course of one-half to one hour very few isolated motile bacteria are 

 seen. The fibrinous clumps alone, especially if examined with a poor 

 light by a beginner, may be easily mistaken for clumps of bacilli. Again, 

 the bacilli may become fixed after remaining for one-half to two hours, 

 by slight drying of the drop or the effect of substances on the cover- 

 glass. The reaction in typhoid is chiefly due to specific substances, 

 but clumping and inhibition of movement similar in character may be 

 caused by other substances such as exist in normal horse and other 

 serums. This is a very important fact to keep in mind. (For details 

 of technique see pages 81-83.) 



In pseudoreactions Wilson has noticed that many free bacilli are 

 apt to be gathered at the margin of the hanging drop. 



Use of Serum. MODE OF OBTAINING SERUM FOR EXAMINATION. 

 Fluid blood serum can easily be obtained in two ways : First, the serum 

 may be obtained directly from the blood, thus : The tip of the finger 

 or ear is pricked with a lancet-shaped needle, and the blood as it issues 

 is allowed to fill by gravity a capillary tube having a central bulb. The 

 ends of the tube are then sealed by heat or melted wax, or candle grease, 

 and as the blood clots a few drops of serum separate. To obtain larger 

 amounts of serum for a microscopic examination the blood is milked 

 out from the puncture into a small homoeopathic vial or test-tube. One 

 cubic centimetre of blood can easily be collected in this way. The 

 vial is then corked and placed on the ice to allow the serum to sepa- 

 rate. As a rule one or two drops of serum are obtainable at the end 

 of three or four hours. Second, the serum may be obtained from 

 blisters. This gives more serum, but causes twelve hours' delay. The 

 method is as follows : A section of cantharides plaster, the size of a 

 5-cent piece, is applied to the skin at some spot on the chest or abdo- 

 men. A blister forms in from six to eighteen hours. This should be 

 protected from injury by a vaccine shield or bunion plaster. The serum 

 from the blister is collected in a capillary tube, the ends of which are 

 then sealed. Several drops of the serum can easily be obtained from a 



