1230 THE DIGESTIVE SYSTEM. 



3. The tela submucosa is composed of loose areolar tissue, which allows of a free 

 movement of the mucous layer on the muscular coat, and which also admits, under 

 certain abnormal conditions, of a prolapse of the mucous membrane through the anal 

 orifice. The hsemorrhoidal plexus of veins is contained in this layer. 



4. The tunica mucosa must be considered separately in the rectum and pars analis recti. 

 That of the rectum is redder in colour than the mucous membrane of the colon, as a result 

 of its greater vascularity. It is also thicker, and owing to the looseness of the underlying 

 submucosa, is thrown into numerous irregular rugae when the rectum is empty; these 

 disappear when the bowel is distended, and there then become evident three (sometimes 

 more, sometimes less) crescentic folds, which are much less noticeable in the empty state, 

 and which have been already referred to as the plicee transversales recti. Lymph nodules 

 and intestinal glands are present; but these latter are not so numerous as in the colon, 

 although their calibre is greater. 



The mucous membrane of the pars analis recti presents a number of vertical ridges 

 known as the columns rectales (Morgagnii) ; between the lower ends of these are 

 found a series of small semilunar folds which connect the lower ends of these columns 

 together and are called the anal valves (Fig. 962). Above the level of the anal valves 

 the canal is lined by a modified mucous membrane resembling that of the rectum ; the 

 portion below the valves (i.e. the lower 12 to 16 mm. of the canal) is covered by modified 

 skin, continuous with that around the anus. 



The term annulus hcemorrhoidalis is applied to the segment enclosed by the sphincter 

 externus muscle. It extends from the region of the rectal sinuses to the anal orifice, 

 and in its wall is found the large venous plexus, termed the plexus hcemorrhoidalis. 



The mucous membrane of the rectum presents a characteristic punctated appearance, which 

 is due to the presence of a considerable number of rounded depressions, such as might be made 

 by firmly pressing a finely pointed pencil against the membrane. These rectal pits are tubular 

 in form, and have an accumulation of lymph tissue at the bottom of each, the whole appear- 

 ance being such as might be produced if a small solitary nodule were drawn down from the 

 surface into the intestinal wall. 



Plicae Transversales Recti (O.T. Rectal Valves, or Valves of Houston). These 

 are crescentic shelf-like folds which project into the cavity of the rectum from its 

 sides (Fig. 960). They are composed of an infolding of the mucous, submucous, 

 and greater part of the circular muscular coats, and their form is preserved by the 

 relative shortness of the anterior and posterior bands of longitudinal muscular 

 fibres. They are produced, as pointed out above, by the projection, into the 

 interior of the bowel, of the creases on the exterior which result from the lateral 

 inflections of the rectum. In the majority of cases three are present (there may 

 be four, five, or, it is said, even more), but often the lowest of the three is small 

 or absent ; or all the valves may be ill-developed and indistinct. When median 

 sections of the empty rectum are examined, the valves are not easily seen, as they 

 then project but slightly, and are almost completely hidden amongst the numerous 

 rugse of the mucous coat. They are most evident in a distended rectum which 

 has been hardened in situ ; they can also be seen during life, per anum, with the 

 aid of a rectal speculum. 



As a rule two valves are found on the left and one on the right side ; this latter is 

 generally the largest, and is situated a little above the level of the peritoneal reflection, 

 viz., 3 or 3J inches (7*5 to 8'7 cm.) above the anus; the other two valves are found about 

 1 to 1 J inches (2-5 to 3 '7 cm.) higher up and lower down respectively. The valves are 

 distinctly marked in the foetus, and seem to constitute an essential part of the human 

 rectum, their use being to support the contents of the rectum, which they break up into 

 segments, each supported by a valve. They are said to interfere sometimes with the 

 introduction of an enema tube. 



Column* Rectales [Morgagnii]. The mucous membrane of the pars analis 

 recti, or lower portion of the anal canal, presents a number (5 to 10) of permanent 

 vertical folds, separated by grooves, and known as the columnse rectales (O.T. columns 

 of Morgagni) (Fig. 962). They are usually J to J inch (8 to 12 mm.) in length, J- to 

 J inch (3 to 6 mm.) in width, and they extend down to within J or f inch 

 (12 to 20 mm.) of the anal aperture. They are formed by infoldings of the mucous 

 membrane, containing in their interior some bundles of longitudinal muscle and 

 also, as a rule, an artery and a vein. 



