The flexible catheters are of many kinds; the English gum-elastic (fig. 85), the French black flexible (fig. 86), and the vulcanised india-rubber (fig. 87), catheters, being the three varieties most generally employed. English flexible catheters should be kept on stylets well curved at the last 3 inches, that, when the stylet is withdrawn, for the catheter to be passed, the latter may retain sufficient curve to pass over the neck of the bladder easily. [Illustration: Fig. 86.—French bulbous-ended catheter.] [Illustration: Fig. 87.—Vulcanised india-rubber catheter.] =Sounds= are solid, being of steel, plated or gilt. Their curve varies, and is generally 20 or 30 degrees more obtuse than that of the catheters. =Bougies= are made of the same materials as the flexible catheters; they are kept straight, and the more supple they are the better, the black bulbous-ended bougies being the most useful variety for dilating the urethra. [Illustration: Fig. 88.—The Olive-headed bougie.] =Olive-headed Bougies= (_Bougies olivaires_) are used for exploring the urethra in cases of gleet, where the discharge is often kept up by a stricture or a tender patch of chronic inflammation of the mucous membrane. They are made of metal, or of black gum mounted on a very flexible leaden wire; the latter kind are far preferable. The stem of the instrument is slender, no bigger than a No. 3 or No. 4 bougie; the end terminates in a conical point about ¼ or ⅜ of an inch long, expanding at its base to any required size. These bougies are most useful from No. 4 to No. 16 of the English scale, or from No. 10 to No. 24 of the millimetrical scale. The stem should be marked with white rings an inch apart, so that when the instrument is passing over a tender part, or is arrested by a stricture, the distance of the impediment down the urethra can be at once estimated. In withdrawing the instrument, the wide base of the olive shows the exact position and length of those strictures which are not too narrow for the olive-head to slip by, for it is nipped by the stricture and released as soon as the narrowing is passed. By using instruments large enough to fill the normal urethra, an induration beneath the mucous membrane can be detected in its earliest stage before it has produced symptoms diagnostic of stricture.