Hair-pins have, above all, been found in the female bladder with special frequency; this point is worth some consideration as an illustration of the enormous frequency of this form of auto-erotism. The female urethra is undoubtedly a normal centre of sexual feeling, as Pouillet pointed out many years ago; a woman medical correspondent, also, writes that in some women the maximum of voluptuous sensation is at the vesical sphincter or orifice, though not always so limited. E.H. Smith, indeed, considers that "the urethra is the part in which the orgasm occurs," and remarks that in sexual excitement mucus always flows largely from the urethra.[198] It should be added that when once introduced the physiological mechanism of the bladder apparently causes the organ to tend to "swallow" the foreign object. Yet for every case in which the hair-pin disappears and is lost in the bladder, from carelessness or the oblivion of the sexual spasm, there must be a vast number of cases in which the instrument is used without any such unfortunate result. There is thus great significance in the frequency with which cases of hair-pin in the bladder are strewn through the medical literature of all countries. In 1862, a German surgeon found the accident so common that he invented a special instrument for extracting hair-pins from the female bladder, as, indeed, Italian and French surgeons have also done. In France, Denucé, of Bordeaux, came to the conclusion that hair-pin in the bladder is the commonest result of masturbation as known to the surgeon. In England cases are constantly being recorded. Lawson Tait, stating that most cases of stone in the bladder in women are due to the introduction of a foreign body, very often a hair-pin, adds: "I have removed hair-pins encrusted with phosphates from ten different female bladders, and not one of the owners of these bladders would give any account of the incident."[199] Stokes, again, records that during four years he had four cases of hair-pin in the female urethra.[200] In New York one physician met with four cases in a short experience.[201] In Switzerland Professor Reverdin had a precisely similar experience.[202]