874 A MANUAL OF ANATOMY 



the muscular and mucous coats in a very loose manner, so that the 

 latter is thrown into a number of temporary folds in the empty 

 condition of the gut. The internal hemorrhoidal plexus of veins 

 is embedded in this coat. 



The mucous coat is characterized by great thickness and vas- 

 cularity. In health, therefore, it has a reddish colour, and is 

 capable of free movement upon the muscular coat. The mucous 

 membrane is covered by columnar epithelium, and is provided 

 with crypts of Lieberkiihn and lymphoid nodules. It is thrown 

 into a number of folds in the empty state, but the majority of these 

 are temporary, and become effaced when the gut is distended. 

 There are, however, at least three permanent folds, which con- 

 stitute the rectal valves of Houston. The mucous membrane is 

 also studded over with a number of minute tubular depressions, 

 called rectal pits, which are surrounded at their deep ends by 

 lymphoid tissue (Birmingham). 



The rectal valves {valves of Hauston) are horizontal infoldings of 

 the wall of the rectum in certain situations. Each valve is crescentic 

 or semilunar, and consists of (i) the rectal mucous membrane, 

 (2) the submucous areolar tissue, and (3) a variable amount of the 

 circular muscular fibres. The valves are usually three in number — 

 first, second, and third— from above downwards, and their exact 

 positions are subject to variation. The flrst or highest valve is 

 usually situated on the right wall of the rectum at the level of the 

 third sacral vertebra, and therefore immediately below the lower 

 end of the pelvic colon. The second or intermediate valve, which 

 is the most indefinite, projects from the left wall about ij inches 

 below the first valve. The third or lowest valve, which may be 

 regarded as the principal one, usually projects from the anterior 

 wall of the rectum, but may also encroach upon the right wall. Its 

 distance from the anus is about 3 inches, and it is on a level with 

 the bottom of the recto-vesical pouch of the peritoneum. It is 

 therefore related to the fundus of the bladder. This valve is some- 

 times referred to as the plica transversalis recti, and it is occasion- 

 ally annular. The rectal muscular fibres (circular), which it con- 

 tains, constitute the so-called sphincter ani tertius, or sphincter of 

 Nelaton. 



The rectal valves are probably sustentacular in function, serving 

 to support the rectal contents. They may give rise to obstruction 

 during the introduction of instruments. 



Structure of the Anal Canal. 



The wall of the anal canal is composed of three coats — muscular, 

 submucous, and mucous. The muscular coat is composed of plain 

 muscular tissue, arranged as an external longitudinal and internal 

 circular layer. The longitudinal fibres are continuous with those 

 of the rectum, and, blending with them, there are fibres of the 

 levator ani. Inferiorly they pass between the internal and external 

 sphincters, to be attached to the anal integument. Associated 



