25S BACTERIOLOGICAL EXAMINATION OF MATERIAL 
It may be desirable to estimate the number of bacteria in the blood : 
1 cc. of blood is mixed at once with 10 cc. of agar previously melted 
and cooled to 42° C, and plated in a Petri dish. If desired, dilution 
may be made 1 to 10, 1 to 100 in succeeding tubes of agar. 
Typhoid and paratyphoid bacilli grow readily in the broth cultures. 
They may be identified by their cultural and agglutination reactions 
with highly potent specific sera. Streptococcus and pneumococcus 
cultures are obtained in a similar manner from the blood stream in 
blood bouillon. The organisms are isolated in pure culture by smear- 
ing the broth, after incubation, upon the surface of freshly prepared 
blood-agar plates. The streptococcus colonies usually exhibit a wide 
clear zone of hemolysis. Pneumococcus colonies are characterized by 
a narrower green zone of altered blood pigment around them. Plague 
bacilli and Micrococcus (Bacillus) melitensis are frequently detected in 
the blood stream.; occasionally the organisms are present in sufficient 
numbers to dcA'elop in blood-agar plates inoculated with 1 to 2 cc. 
of blood. The former produces characteristic lesions in guinea-pigs; 
the latter develops very slowly in artificial media. Frequently the 
colonies become \'isible only after five to seven days' incubation. 
Bacteriological Examination of Cerebrospinal Fluid. ^Spinal fluid 
for bacteriological examination is obtained by lumbar puncture with 
a sterile hypodermic needle, or fine trocar about 8 cm. long and 1 
mm. in bore. The needle is introduced preferably in the fourth intra- 
vertebral space; the fasciculi of the cauda equinum are not tense at 
this level and are readily pushed aside by the needle without injury. 
An imaginary line touching the crests of the ilia intersects the spinous 
process of the fourth lumbar vertebra; the sterile needle is inserted 
through the previously sterilized skin at a point 1 cm. to the right 
(or left) of the lower rim of the spinous process, and directed obliquely 
upward and inward to such a degree that the point of the needle will 
reach the median line at a depth of 5 to 6 cm. The subarachnoid space 
is reached at this level and resistance to the passage of the needle 
ceases, and spinal fluid should flow at once. The fluid should be col- 
lected in a sterile test-tube. Usually from 20 to 30 cc. of fluid flow 
spontaneously; the flow may be much greater, amounting to 75 cc. 
or even more. Rarely but a few drops, or even none at all may be 
obtained. Normal spinal fluid is clear and practically colorless. 
Only a few cells, chiefly lymphocytes, may be found in the sediment 
obtained by centrifugalization. Pathologically the fluid may con- 
tain numerous cellular elements. A blood-stained spinal fluid may 
be due to injury to bloodvessels during the passage of the needle, or 
to l)loo(l from hemorrhage in the brain or upper levels of the cord. 
In the former case the bloofi will clot if the spinal fluid is allowed 
to stand; in the latter case the blood settles to the bottom, but fails 
to clot. A turbid spinal fluid is indicative of an inflammatory process 
in the cerebrospinal axis. If the turbidity is uniform, pus cells are 
almost invariably present. Occasionally the fluid appears clear, but 
