THE PERINEUM 381 



The Urachus is a fibrous cord which extends in the adult from the apex 

 of the bladder to the umbilicus. It lies between the fascia transversalis 

 and the parietal peritoneum and represents the embryonic connection between 

 the cloaca and the allantois, after the formation of the umbilical cord (p. 287). 

 This connection normally disappears quite early, but it may persist and give 

 rise to cysts or umbilical urinary fistultz. 



Congenital Anomalies of the Rectum and Anal Canal. 



The anal membrane (p. 380) lies at the bottom of a surface 

 depression, which is termed the proctodceum. During the third 

 month ; the anal membrane breaks down and the alimentary 

 canal opens on the perineum. The upper half of the anal canal 

 is entodermal in origin, but its lower half is derived from the 

 ectoderm lining the proctodaeum. 



Should the anal membrane fail to break down, the condition 

 of imperforale anus results. In some cases, the condition is 

 very simple, and a small incision through the anal depression 

 at once opens the lower end of the rectum. In other cases, 

 however, the rectum ends blindly two or three inches or more 

 from the surface. It is probable that, in these cases, the 

 mesoderm, which separates the urogenital and anal membranes, 

 has invaded the latter and, by rapid proliferation, has widely 

 separated its two layers from one another. 



Connections of the rectum with the bladder or prostatic 

 urethra are due to failure of the mesodermic septum to effect 

 a complete subdivision of the cloaca into ventral, urinary, and 

 dorsal, intestinal segments. 



In Excision of the Rectum, the method adopted depends 

 entirely on the extent of the disease. The operation which is 

 described here is chosen when a permanent colostomy has been 

 previously established ; and it is carried out with the patient 

 in the exaggerated lithotomy position. In order to prevent 

 any leakage during the subsequent manipulations, the anus is 

 stitched up and an incision is made through the skin and fascia 

 around it. This is continued backwards in the median plane 

 up over the coccyx and lower part of the sacrum, if considered 

 necessary. The posterior part of the incision is deepened and 

 the median raphe of the levatores ani is split as far back as the 

 coccyx. In the removal of the coccyx, which constitutes the 

 next step, the glutseus maximus must be separated from its 

 dorsal surface and the attachments of the coccygeus and the 

 sacro-spinous and sacro-tuberous ligaments (small and great 

 sacro-sciatic ligaments) to its sides require to be cut through. 

 The bone may then be disarticulated from the sacrum or, if 



