The operation should be done slowly and with great gentleness, giving the urethra time “to swallow the instrument,” as the French surgeons express it. Hasty or forcible movements tend to thrust the point against the wall of the urethra, where it hitches, if it does not penetrate and make a false passage. However easy the introduction may have been, the withdrawal of the catheter should be always done slowly to avoid giving pain to the patient. When the canal suddenly contracts, as from a stricture, the point of the sound often stops at the obstruction; by withdrawing the instrument a little, and diverting its point to another side or along the upper part of the urethra, a part where the obstruction is less abrupt will often be found to let the catheter glide into the stricture. The floor of the urethra should always be avoided, as false passages nearly always branch off from the floor close to the stricture. Difficult narrow strictures are most easily overcome by injecting a drachm of warm olive oil into the urethra, and then passing fine black gum or whalebone bougies (_bougies filiformes_) along the urethra. These, from their fineness (their diameter is only ⅓ or ⅔ of a millimetre, about 1/100 inch), are very apt to catch in false passages; if so, the bougie should be left engaged in the false passage, and held in the left hand while another bougie is passed along the urethra; if, in its turn, this one gets into a false passage, it also should be left, and a third passed; and so on till all the false routes are occupied, or a bougie enters the stricture and reaches the bladder, which is known by the readiness with which it will pass backwards and forwards. The other bougies should then be withdrawn, and the bougie which has passed the stricture be tied in for twenty-four hours, until the passage is sufficiently dilated to allow a small catheter to replace it. If the patient is not suffering from retention of urine, there need be no anxiety about evacuating his bladder, as urine will find its way alongside the bougie when he attempts to make water. In passing to relieve retention, No. ½ English flexible catheter should be used instead of bougies; but when the stricture is too narrow for these, a bougie may still be tried, as the urine will generally dribble alongside the bougie with sufficient rapidity to relieve the patient. _English flexible Catheters_ should be kept on stylets curved as represented in fig. 85, that the first 3 inches of the instrument, when the stylet is withdrawn, may retain sufficient curve to ride over the impediment at the neck of the bladder. In warm weather, after being oiled, they should be dipped in cold water just before using, to render them a little stiffer, and less likely to lose their curve while traversing the urethra. They may also be passed while the patient lies or stands, and the movements are the same as for the silver catheter.