And certain cautions may be given:-- (1.) In every exploration of an unknown urethra the surgeon should commence with an instrument of medium size, certainly not less than No. 7 or 8. (2.) In cases of difficulty occurring in the urethra behind the scrotum, invariably use the forefinger of the left hand in the rectum as a guide. (3.) Expression of pain on the part of the patient is no indication that a false passage is being made, nor its absence that the instrument is in the passage, for it is a remark of Mr. Syme, that passing an instrument through a stricture is generally more painful than making a false passage through the walls of the urethra. An instrument may be passed, while the patient is erect, with the following precautions:--The patient should stand with his back against a wall, his arms supported on the back of a chair on each side, heels eight or ten inches apart, and four or five inches from the wall; his clothes thoroughly down, not merely opened. The bougie should then be held nearly horizontal, with its concavity over the left groin of the patient, the penis being raised in the surgeon's left hand. Introduced thus for four or five inches, the handle is gradually raised into the middle line of the abdomen, and to the perpendicular; it is then to be lightly depressed, and, as the point enters the bladder, brought down towards the operator until it sinks beneath the horizontal line. _b._ _Mechanical dilatation_ is of two kinds, both very rarely used:--(1.) When an instrument cannot be passed, it consists of passing down day after day the point of an instrument (sometimes armed with caustic, sometimes not), and pressing it against the stricture till it is overcome.[155] (2.) When an instrument is introduced through an intractable stricture, and is left there either for some hours, or for some days, to excite what is called "suppuration" of the stricture.[156] _c._ _Forced dilatation._--Under this head we might describe at great length mechanical contrivances to force or rupture a stricture. A word or two on a few of the most important:-- (1.) Conical bougies of steel or silver. (2.) Mr. Wakley's method, on which many others have been founded. He passed a small bougie or wire into the bladder, over which were slipped straight tubes of varying size, with perfect certainty that they could not leave the urethra. (3.) Mr. Holt's method.[157]--The principle of it is to rupture the stricture at once, so that a No. 12 catheter can immediately be passed into the bladder.