LUMBAR PUNCTURE. I29 
If no fluid flows through the needle it is presumptive 
evidence against the presence of acute meningitis. A 
“dry tap” may, however, occur from plugging of the 
needle with fibrin, or from its point coming in contact 
with a nerve root (Osler), and in some cases of menin- 
gitis the purulent exudation is too thick to flow through 
the needle. 
4. Examination of the fluid.—a. Naked eye.— When 
meningitis is present the fluid is always more or less 
turbid, and some observers hold that the turbidity is 
greater in proportion to the severity of the case. Osler 
has pointed out that the fluid may be alternately turbid 
and clear, being clear during the remissions, and turbid 
during the exacerbations of the disease. Blood-stained 
fluid may occur in meningitis or from hemorrhage into 
the cerebral or spinal meninges apart from inflamma- 
tion. The presence of clear fluid affords strong evi- 
dence of the absence of meningitis, but in tubercular 
meningitis the amount of turbidity may be very slight. 
b. Micvoscopical.—Prepare films of the exudate in the 
manner recommended on p. 131 if the fluid is thin and 
watery; if it is thick and purulent treat it like ordinary 
pus. Stain by any of the methods recommended for 
the examination of the blood (Jenner’s stain being most 
convenient) and examine. 
The presence of leucocytes (except in very small 
numbers) indicates meningitis. If the bulk of the 
leucocytes are lymphocytes (indicated by their small 
size, large, circular, deeply staining nuclei, and ab- 
sence of granules) the presumption is that the case 
is one of tubercular meningitis. In meningitis due to 
other bacteria the chief cell is the polynuclear leuco- 
cyte; this may be recognised by its larger size, 
its twisted (apparently multiple) nucleus, and, if 
K 
