During the course of the last few years the study of the cerebrospinal fluid in both mental and nervous disorders has made rapid progress, and with the introduction of methods of precision much useful knowledge in regard to its physical, chemical, and serological properties has been acquired. Nevertheless, much remains to be learnt, and we propose to submit observations on one aspect of the subject which is still in its infancy, namely, the pressure of the cerebrospinal fluid, and the modifications which it may undergo in the insane. At the outset, however, it will be convenient to refer in as few words as possible to the distribution and circulation of this fluid.