The term "meningitis" as used in this paper is restricted to conditions in which living organisms are present in the cerebrospinal fluid. It includes the common forms of acute purulent meningitis, such as are caused by the meningococcus, the streptococcus, the pneumococcus and the staphylococcus, as well as tuberculous meningitis, acute syphilitic meningitis, influenza bacillus meningitis, colon bacillus meningitis and meningitis caused by mycoses. In all these types of meningitis certain characteristic changes take place in the cerebrospinal fluid, namely, an increase in pressure, cells and protein, and a decrease in sugar and chlorides. It is the purpose in this paper to explain so far as is possible the mechanisms by which these alterations take place. Such an explanation, however, depends on a clear understanding of the nature of the cerebrospinal fluid.1In the light of present knowledge this may be summarized as follows: The cerebrospinal fluid is a