466 ABDOMEN 



The layer is not, however, uniformly thick on all aspects of 

 the gut, for in front and on the back the fibres are massed 

 to form two broad bands, which maintain the flexures and 

 prevent the rectum from elongating as it becomes loaded. 

 The circular muscle-fibres form a more or less uniform layer, 

 internal to the longitudinal fibres, and they are prolonged 

 into the bases of the plicae transversales. 



Muscular Coat of the Anal Canal. The muscular wall of 

 the anal canal is very thick and powerful. The internal circular 

 layer of muscle-fibres, prolonged down from the rectum, is greatly 

 thickened to form a muscular cylinder, the internal sphincter, 

 which embraces the whole length of the canal, except the 

 lower 12.5 mm. (half-inch). The longitudinal fibres from the 

 rectum are also prolonged downwards, outside the internal 

 sphincter, and they blend with the fibres of the levator ani, 

 which are inserted into the wall of the canal between the 

 internal and the external sphincters. The external sphincter 

 surrounds the lower part of the canal outside the levator ani. 



Submucous Coat of the Rectum and Anal Canal. The sub- 

 mucous coat is composed of lax areolar tissue, which allows the 

 mucous coat to move freely on the muscular coat. It contains 

 vessels and nerves. 



Mucous Membrane of the Rectum and Anal Canal. The 

 mucous membrane of the rectum and anal canal is thicker 

 and more movable upon the muscular tunic than the mucous 

 membrane of the colon, and, in consequence of its mobility, 

 it is thrown into irregular folds or rugae when the gut is 

 empty. In the upper part of the anal canal the mucous 

 membrane is thrown into a series of longitudinal folds, called 

 columns rectales (Morgagni). A short distance above the 

 anal orifice the columns are connected by a number of 

 irregular semilunar folds, called the anal valves. In the con- 

 cavity of each valve is a pocket-like recess, termed a sinus 

 rectalis. The folds are of importance in connection with the 

 condition known as fissured anus, and they indicate the level 

 at which the scaly epithelium of the integument merges 

 into the columnar epithelium of^the gut. 



Plicae Transversales Recti (O.T. Valves of Houston). 

 The transverse folds of the rectum are not always visible, 

 and are usually seen best in a rectum which has been fixed 

 with formalin when in a state of distension. They are three 

 in number, in conformity with the inflections of the gut; 



