132 CLINICAL BACTERIOLOGY AND HEMATOLOGY 



If the patient is able to do so without harm, it is much easier 

 to perform the operation with him sitting in a chair leaning over 

 the back or on the edge of the bed with his head nearly between 

 his knees, so that the spine is bent into a convex curve ; by this 

 means the spinous processes are rendered more prominent and 

 the spaces between the laminae are made wider (see Fig. 31). The 

 operation is thus made much more easy, and the method is usually 

 preferable in cases of tabes or general paralysis, where the fluid 

 is collected for a cytological examination only, and the patient is 

 in good general health. But the method with the patient on his 

 side should also be learnt. 



3. Collection of Fluid and Inoculation of Media. — The first few 

 drops of fluid which escape may be stained with blood ; in this 

 case they should be rejected. Allow a few drops of the fluid to 

 flow directly on to the surface of the medium without touching the 

 glass. Collect also some of the fluid (i to 2 drachms) in the 

 sterilized empty tube. 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 

 (Osier), and in some cases of meningitis the purulent exudation is 

 too thick to flow through the needle. In one case in which no 

 fluid could be obtained by repeated punctures the venous sinuses 

 of the brain were found to be thrombosed : a frequent cause in 

 meningitis is the closure of the foramina in the roof of the fourth 

 ventricle. 



The force of the flow should be noted. In health it flows 

 out slowly, whilst in meningitis it runs faster, and may even spurt 

 out a foot or more; the same thing may happen if there is a 

 cerebral tumour, uraemia, or other cause of increased pressure. 



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. Osier 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 haemorrhage into the 

 cerebral or spinal meninges apart from inflammation.* The 

 presence of clear fluid affords strong evidence of the absence of 

 meningitis, but in tuberculous meningitis the amount of turbidity 



* See also p. 242. 



