l8o CLINICAL BACTERIOLOGY AND H.EMATOLOGY 



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 by adhesive inflammation, 



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 men- 

 ingitis 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, but this is certainly not true. 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 may be very 

 slig'ht. It should be estimated by comparing the fluid with 

 some distilled water in a clean test-tube of the same size in 

 the two cases. 



(b) Microscopical. Centrifugalize some of the fluid and 

 prepare films of the sediment 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. f 



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 absence of g*ranules) the presump- 



* See also p. no. 



t The cytology of the cerebro- spinal fluid is dealt with more fully under 

 the heading of Cyto-diagnosis. 



