The Rectum commences opposite the body of the third sacral vertebra, and, descending in front of the sacrum and coccyx, runs forwards for 1½ inches upon the pelvic floor, formed by the two levatores ani, and then, bending downwards and backwards, it pierces the pelvic floor to terminate at the anus. About 6 inches in length, it has no mesentery and only a partial peritoneal covering, while instead of presenting three muscular bands and general sacculation as the colon does, it presents two wide longitudinal muscular bands, the one anterior, the other posterior, slight sacculation occurring laterally between these bands. In addition to the course described above, the rectum presents lateral curves, which are maintained by the muscular bands. These curves are generally three in number, an upper and lower with the concavity to the left, while the intermediate one, which is likewise the most prominent, has the concavity to the right. The concavities are marked by furrows externally, and by marked crescentic projections-the rectal valves, or valves of Houston-internally. As those valves occasionally obstruct the passage of long instruments, it is well to remember that the main one projecting from the right side is situated about 3 inches above the anus, while the other two, less marked and projecting from the left side, are situated respectively 1½ inches above and below the middle one. The curving increases the length of the rectum, and also delays the passage of and supports the bowel contents, and the rectal valves also play an important part in so doing. These valves become most prominent when the bowel is distended, and are excellently seen when, the patient lying inverted, a speculum is introduced through the anus, and the rectum distends with air. When distended the rectum occupies the greater portion of the posterior division of the pelvis, obliterating the lateral or pararectal fossae. About 1 inch in diameter at its upper extremity, it dilates in its lower two-thirds to form the rectal ampulla. At the point of the last sharp curve backwards, the anterior rectal wall sometimes projects forwards and even downwards, forming a small anterior pouch. This is most apt to occur in multiparas.
The relationship of the peritoneum to the rectum is of importance in excision of the rectum and similar operations. At its commencement the rectum is covered both in front and laterally by peritoneum, then the lateral portions become uncovered, and, finally, at an average distance of 3 inches from the anus, the peritoneum is reflected from the front of the bowel on to the posterior surface of the bladder and the vesiculi seminales, forming the recto-vesical peritoneal pouch. Thus this pouch is generally opened into, when, in excising the rectum, more than the last 3 inches are exposed, but the exact position of the pouch varies between 1 inch and 4½ inches above the anus, and is partly influenced probably by the amount of distension of bladder and rectum. In the female the peritoneum is reflected at the same level on to the upper part of the vagina, cervix, and broad ligaments, forming the fundus of the pouch of Douglas (recto-vaginal pouch). The lateral reflections are higher up, about 5 inches from the anus, and form pararectal fossee, when the rectum is empty. These fossae are generally occupied by small intestine, but, when the rectum is distended, it generally occupies and obliterates these spaces. Below the level of the recto-vesical pouch, the rectum is surrounded by the rectal fascia; which is derived from the visceral layer of the pelvic fascia. Here, also, but for the interposition of this fascia, bladder and rectum are in contact with one another over a triangular area about 1 inch long, whose base corresponds to the line of reflection of the peritoneum, and whose apex is situated at the base of the prostate, while the lateral limits are formed by the vesiculi seminales.
Through this triangular area the bladder has been tapped per rectum, and the examining finger, introduced per rectum, can sometimes detect the presence of a posterior prostatic pouch of the bladder in cases of enlarged prostate, and even perhaps a stone lying in it. The vesiculi seminales can also be examined per rectum, and various changes, as, for example, their hardness in certain stages of tubercular disease, noted. Sometimes they may be pressed on by straining at stool, and thus emptied, causing a so-called ' spermatorrhoea.' In front of the vesiculi seminales, close to the bladder wall, are the ureters, which are not easily felt. The vasa deferentia, however, lying first on the inner borders of the vesiculi seminales, and then lying together for a short distance above the base of the prostate, may be made out. Nearer the anus, and also in front, the prostate gland can be felt, and, when enlarged, it projects markedly into the rectum, giving the patient the feeling that he cannot empty the bowel, and sometimes even leading to partial obstruction. It, too, when enlarged, may be pressed on during defalcation, causing pain if inflamed, and sometimes also producing a so-called 'spermatorrhoea' from the discharge of its secretion by the penis. In the female this portion of the rectum is in relation to the vagina, and advantage may be taken of this in rectal examination to evaginate the lower rectal wall through the anus by the finger introduced into the vagina. At the upper part, the os and cervix uteri may be felt, sometimes projecting markedly into the rectum, and suggesting the presence of tumour. Laterally also, particularly in the child, the lateral pelvic wall over the acetabulum may be explored, and disease of the acetabulum ,with extension of tubercular pus to the inner side of the pelvis, detected. Through the posterior rectal wall the coccyx may be felt, and possibly part of the sacrum, and this wall is also in relation to the coccygei and levatores ani muscles, sacro - sciatic ligaments and haemorrhoidal vessels and lymphatics. Laterally, below the peritoneal reflection, the rectum is in relation to the vasa deferentia and upper extremity of the vesiculi seminales, and receives the middle haemorrhoidal vessels, enclosed in a layer of connective tissue, derived from the pelvic wall, and called the lateral ligament of the rectum. The rectum may also be examined for tumours such as polypi, which are frequently situated not very far from the anal orifice, or for carcinoma, which is frequently only to be felt by a long finger. An excellent view of the interior of the rectum may be obtained by inverting the patient and stretching the sphincter, the rectum then distending with air.