860 A MANUAL OF ANATOMY 



Rectum. — The rectum succeeds to the pelvic colon, and extends 

 from the front of the third sacral vertebra to a point i| inchej 

 in front of and below the tip of the coccyx, where it pierces the 

 pelvic diaphragm, and terminates in the anal canal. Its direction 

 is at first downwards and slightly backwards, then vertically 

 downwards, and finally downwards and forwards. It is fully 

 5 inches in length, and its diameter, for the most part, is about 

 1 1 inches in the empty state, but it becomes enlarged above the 

 anal canal, this dilated part being called the ampulla. When empty, 

 its anterior and posterior walls are in contact, and in transverse 

 section it appears as a transverse slit. The rectum is destitute of 

 a mesentery. The peritoneum covers the upper third laterally 

 and anteriorly, but not posteriorly, the middle third being covered 

 by peritoneum only anteriorly, whilst the lower third is free from 

 serous covering. The point at which the peritoneum leaves the 

 bowel is usually about 3 inches above the anus. 



The rectum, so far from being straight in man, presents both 

 antero-posterior and lateral curves. The antero-posterior curves are 

 two in number, upper and lower. The upper curve extends from 

 the third sacral vertebra to the posterior or rectal surface of the 

 prostate gland, and its concavity is directed forwards. The lower 

 curve, which is abrupt, corresponds with the rectal surface of the 

 prostate, its concavity looking backwards, and containing the ano- 

 coccygeal body. The lateral curves are of importance, because they 

 tend to obstruct the passage of instruments by giving rise to the 

 valves of Houston. These curves are usually three in number — 

 upper, middle, and lower. The upper and louer curves have their 

 convexities directed towards the right, whilst the middle curve has 

 its convexity directed towards the left. In the concavities of these 

 curves the rectal wall becomes inflected, and so gives rise to more or 

 less prominent shelves of mucous membrane, which are known as 

 the rectal valves of Houston. The lateral inflections and resultant 

 valves are associated with the erect posture of man, which posture 

 throws considerable pressure upon the anal aperture and its sphincter 

 muscles. They are best marked in the distended rectum, and the 

 lateral curves are brought about in the following manner : The 

 longitudinal muscular fibres of the rectum are rather shorter than 

 the gut to which they are applied. Moreover, they are principally 

 disposed in two stout broad sheets, one on the anterior and the 

 other on the posterior wall, there being very few longitudinal fibres 

 along the lateral walls. The shortness of these longitudinal fibres, 

 therefore, throws the rectum into lateral curves on account of the 

 sparseness of the longitudinal fibres at the sides. These lateral 

 curves may be taken as representing the sacculations of the other 

 parts of the colon. 



Relations — Anterior. — ^The recto-vesical pouch of peritoneum for 

 a short distance ; the base of the bladder, vesiculae seminales, and 

 vasa deferentia, with the intervention of the recto-vesical lamina 

 of the visceral pelvic fascia ; and the posterior or rectal surface of 

 the prostate gland, with the intervention of the recto-prostatic 



