BACILLUS TYPHOSUS. 423 



the effect of substances on the cover-glass. The reac- 

 tion 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. 



USE OF SERUM. Mode of Obtaining Serum for Ex- 

 amination. Fluid blood-serum can be easily 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 wax, and as the blood clots a few drops of 

 serum separate. This method of obtaining blood- 

 serum has the advantage of rapidity; but it has also 

 disadvantages namely, that the serum thus separated 

 is apt to contain more or less blood-cells, which some- 

 what obscure the field when the liquid serum is imme- 

 diately mixed with the culture, and the needle stab is 

 often objected to. Second, the serum may be obtained 

 from blisters. This gives more satisfactory results, 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 abdomen. 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 be easily obtained from a blister so small that it is 

 practically painless and harmless. The serum obtained 

 is clear and admirablv suited for the test. 



