_Infection from without_, so far as the male urethra is concerned, is a question of the venereal origin of the disease. _Infection from within_, in the specific form, is a matter of recrudescence of a formerly active lesion supposed to have disappeared. Infection of _non-specific type from without_ affects the introduction of germs either by venereal contact or by the medium of an unclean catheter, sound, or some other instrument, or _from within_ by the extension forward of an inflammation higher up in the genito-urinary tract, such as may be due to stone in the bladder, cystitis, enlarged prostate, or uric acid or oxalic acid crystals. Urethritis, usually of mild type, is not infrequent in old men from either of these causes. It may also be produced by the infection of a too strong or irritant injection, whether used either for prophylaxis or for ordinary treatment. Classifying them we may then have urethritis of the following four types: _A._ Gonorrheal of extrinsic origin. _B._ Gonorrheal of intrinsic origin (originally extrinsic). _C._ Non-specific urethritis of extrinsic origin. _D._ Non-specific urethritis of intrinsic origin. _A._ The period of incubation is short, usually two to six days, and the resulting inflammation is severe; the consequences are often remote and sometimes disastrous. Gonococci will be found in the pus and epithelial debris. _B._ This has been described as “_bastard clap_.” It is really an auto-infection, with an incubation period of a few hours, and is practically the reawakening of a quiescent stage of _A_. It is characterized by abundant purulent discharge; this latter is thin and mucilaginous, more like that of so-called gleet, with an abundance of pus threads, or “clap threads,” in the urine. In this form gonococci are also found, but less frequently. _C._ Much like _A_ in its clinical course, but less violent, and with less widespread reaction. Its period of incubation is rarely over thirty-six hours. This type is most common after alcoholic and sexual excesses; the latter especially with one already suffering from vaginal discharge, particularly so when near the time of menstruation. Here the microscope will show few if any true gonococci, but a profusion of pyogenic organisms. _D._ This form of disease is of non-venereal origin, and is susceptible of easy explanation and of satisfactory treatment if the cause be properly treated. No case of urethral discharge which does not seem to fall easily into one of the above categories should be treated without a search of the anterior urethra, especially the fossa navicularis, for possible _chancre_ or _chancroid_, as well as for stricture.