Again, an inquirer might ask, If it is so dilatable, why should we run the risk of cutting the prostate at all? Why should we not introduce instruments gradually increasing in size into the membranous portion of the urethra, and thus dilate prostate and neck of bladder? For this reason, that the urethral canal passing through the prostate is itself lined immediately outside of the mucous membrane by a firm membranous sheath (Fig. XXXIV. RR), which resists dilatation to the utmost. Experience tells us that any attempts to dilate or even forcibly to tear this ring of fibrous texture are both ineffectual and dangerous, while a clean cut into it and through it into the substance of the prostate is at once effectual and comparatively safe. In a word, we can describe the relation of the prostate to the operation of lithotomy somewhat in this manner:--Its fibrous sheath surrounding the urethra must be cut freely. The gland substance may be cut and freely dilated by the finger. Its fibrous envelope must, as far as possible, be preserved intact, but this interferes the less with the operation, as it is comparatively freely dilatable. The firm lining of the urethra, which must be cut, is specially strong at its base, forming a tough resisting band just at the aperture of the bladder, which, unfortunately, is often so high up in the pelvis in tall patients, or in cases in which the prostate is much enlarged, as to be almost out of reach of the finger, and so far up the staff as perhaps to escape division. You will be warned of such an occurrence by the urine in the bladder failing to make its appearance; and if any attempt be made to dilate the opening and introduce the forceps without further incision of the base of the prostate, the result will very likely be fatal, generally from pyæmic symptoms depending on a suppurative inflammation of the prostatic plexus of veins (Fig. XXXIV.). In fact, upon a recognition of this fact is founded the aphorism, "that cases in which the forceps have been introduced before the bladder fairly begins to empty its contents are generally fatal." [Illustration: FIG. XXXV.[151]] We have thus traced the necessary guiding principles as to our incisions from the bladder outwards through the prostatic portion of the urethra. We have next to discover what sort of an opening is necessary in the membranous portion of the urethra consistent with the fulfilment of the same conditions, namely, freedom of escape for the urine, and room enough to remove the stone. Both of these are gained at once by a free incision of the membranous portion, dividing especially those anterior fibres of the great sphincter muscle of the pelvis, the levator ani, which embrace the membranous portion, under the special names of compressor (Fig. XXV.) and levator urethræ (Guthrie's and Wilson's muscles).