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 or 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 alsu 
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 admirably suited for the test. 
