The rectum, W C, at its middle and upper parts, occupies the hollow of the sacrum, A Q, and is behind the bladder. The lower third of the rectum, W, not being covered by the peritonaeum, is that part on which the various surgical operations are performed. At its upper three-fifths, the rectum describes a curve corresponding to that of the sacrum; and if the bladder be full, its convex back part presses the bowel against the bone, causing its curve to be greater than if the bladder were empty and collapsed. This fact requires to be borne in mind, for, in order to introduce a bougie, or to allow a large injection to pass with freedom into the bowel, the bladder should be first evacuated. The coccygeal bones, Q, continuing in the curve of the sacrum, bear the rectum, W, forwards against the base of the bladder, and give to this part a degree of obliquity upwards and backwards, in respect to the perinaeum and anus. From the point where the prostate, V, lies in contact with the rectum, W, this latter curves downwards, and slightly backwards, to the anus, P. The prostate is situated at a distance of about an inch and a half or two inches from the anus--the distance varying according to whether the bladder and bowel be distended or not. [Footnote] [Footnote: The distance between any two given parts is found to vary in different cases. "In subjects of an advanced age," Mr. Stanley remarks, "a deep perinaeum, as it is termed, is frequently met with. This may be occasioned either by an unusual quantity of fat in the perinaeum, or by an enlarged prostate, or by the dilatation of that part of the rectum which is contiguous to the prostate and bladder. Under either of these circumstances, the prostate and bladder become situated higher in the pelvis than naturally, and consequently at a greater distance from the perinaeum."--On the Lateral Operation of Lithotomy.] The arteries of the bladder are derived from the branches of the internal iliac, S. The rectum receives its arteries from the inferior mesenteric and pudic. The veins which course upwards from the rectum are large and numerous, and devoid of valves. When these veins become varicose, owing to a stagnation of their circulation, produced from whatever cause, the bowel is liable to be affected with haemorrhoids or to assume a haemorrhagic tendency.