Then, in view of the foregoing facts, and in consideration of the anatomical formation of the parts, being a collapsable tube, highly sensitive and extremely difficult of accessibility, quite unlike the vaginal canal, which is closed at one end, more capacious and dilatable, and designed by nature to be approached from the exterior, a speculum should be so constructed as to not only be easy of introduction and withdrawal, but to exclude all the surface except a limited portion, and permit the greatest amount of available light possible to fall on the exposed part shown _in situ_. The greatest barrier to the successful use of a speculum is the unruly external sphincter and the excessive mobility of the mucous and muco-cutaneous surfaces. The upper margin of the external sphincter terminates beneath the junction or the skin with the mucous membrane, which place also marks the beginning of the internal sphincter and its junction with the external muscle by a more dense connective tissue, sometimes appearing as a white line at the muco-cutaneous junction called the white line of Hilton. According to Dr. Andrews, Hilton has demonstrated that the locality where the two muscles join by the intervention of this fibrous ring forming the anal verge, the junction of the skin and mucous membrane, and the exit of the branches of the pudic nerve, is identical. [Illustration: FIG. 12.—Author’s Rectal Speculum.] The internal sphincter is a collection of the circular fibres of the muscular coat of the bowel, about five-eighths of an inch in width, and constitutes in reality the terminus of the gut. For the external sphincter is a thin band of distinct and separate muscular fibres, elliptical in shape, between three and four inches from its anterior to its posterior extremity, and expands out around the margin of the anus like the flaring end of a trumpet; with its superficial layer in close relation to the skin which it draws down in radiating folds. With this understanding of the anatomical relations, it will be seen that the external muscle contributes so slightly to the length of the canal, that it might be considered wholly on the outside, where it guards closely the entrance, and is nowise concerned in an examination with a speculum except as a feature of incumbrance. To correct an erroneous idea that there is any considerable depression or space intervening between the muscles, we mean, when we say between the sphincters, the distance bounded by the fibrous ring uniting the two muscles below, and the upper portion of the internal muscle above. More simplified, we mean all the surface included between the upper margin of the internal sphincter and its junction with the external muscle at the anal verge.