152 CLINICAL BACTERIOLOGY AND HEMATOLOGY 



" 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, 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 exacerba- 

 tions 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 slight. 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.! 



The presence of leucocytes (except in very small numbers) 

 indicates meningitis. If the bulk of the leucocytes are lympho- 

 cytes (indicated by their small size, large, circular, deeply staining 

 nuclei, and absence of granules) the presumption is that the case 

 is one of tuberculous meningitis. In acute meningitis due to other 

 bacteria the chief cell is the polynuclear leucocyte ; this may be 

 recognized by its larger size, its twisted (apparently multiple) 

 nucleus, and, if the staining method has been appropriate, by the 



* See also p. 158. 



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

 heading of Cytodiagnosis. 



