Gentlemen,—It is customary for the President of a Section to open the proceedings by an address, but I feel that it would serve a more useful purpose if I opened the discussion on the diagnosis and treatment of parenchymatous syphilis by giving an account of the observations which I have been making regarding the microscopic and bio-chemical pathology of tabes and general paralysis; for it is by an understanding of the pathological processes underlying these diseases that we are able to make a correct diagnosis, and prevent their occurrence or arrest their progress. My personal experience of the effect of treatment is somewhat limited as compared with that of many of those who are to take part in the discussion, and therefore I shall especially devote my attention to the observations which have been made in the Pathological Laboratory of the London County Asylums.