THE LARGE INTESTINE 283 



examination. When the rectum becomes distended, the valves 

 form shelf-like ledges, which help to support the contents. 



Posteriorly, the rectum lies in contact with the sacrum and 

 coccyx, against which scybalous masses may be compressed 

 and broken down. The large nerve trunks which form the 

 left sacral plexus lie behind the rectum, before they leave the 

 pelvis to enter the gluteal region. When the rectum is greatly 

 distended, it may overlap and compress both sacral plexuses. 

 As a result of this pressure, painful symptoms are experienced 

 in the back of the thigh and the condition may be mistaken 

 for true sciatica (p. 182). Complete evacuation of the bowel, 

 however, effects a speedy cure in these cases. 



Anteriorly, the rectum is related to the posterior surface of 

 the bladder, the terminal parts of the ductus deferentes (vasa 

 deferentia), the seminal vesicles and the prostate. All of 

 these structures can be palpated on digital examination of the 

 anterior wall of the rectum. (The examination of the rectum in 

 the female is referred to on page 388.) 



The terminal part of the rectum is supported by the levatores 

 ani muscles (p. 184), which separate it, on each side, from the 

 ischio-rectal fossa. 



In rapid wasting conditions in childhood, the amount of fat 

 in the ischio-rectal fossae is much diminished, and the rectum 

 thus loses a certain amount of support. As the curvature of 

 the sacrum is less pronounced in the child than in the adult, 

 prolapse of the rectum may occur in these cases during violent 

 straining efforts to empty the bowel. 



The Anal Canal passes downwards and backwards through 

 the floor of the pelvis to open on the surface of the perineum. 

 The mucous membrane lining the upper part of the anal canal 

 is continuous with the mucous lining of the rectum and is 

 characterised by numerous longitudinal ridges, which are united 

 at their lower ends by transverse folds termed the anal valves. 

 During the passage of a scybalous mass one of the little 

 pockets formed by the valves may be torn, and this laceration 

 constitutes the condition which is termed anal fissure. 



