6io ABDOMEN 



tain' 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 rectal valves. 



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 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. 



The submucous coat of the rectum and anal canal is composed 

 of lax areolar tissue, which allows the mucous coat to move 

 freely on the muscular coat. It contains vessels and nerves. 



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 this 

 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, the 

 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, the 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 Eecti (O.T. Valves of Houston). 

 These folds 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, consequently there are two 

 on the left side and one, the largest, on the right side. Each 

 is formed by an infolding of the mucous, submucous, and part 

 of the muscular coat. The positions of the folds are variable ; 

 but the right and largest is usually placed at the level of the 

 bottom of the recto-vesical excavation of peritoneum, whils 



