eS a ee 
THE ANAL CANAL. 1043 
of its greater vascularity. It is also thicker, and owing to the looseness of the underlying 
submucosa, is thrown into numerous irregular rugze 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 rectal valves. Lymphoid nodules and 
glands of Lieberkiihn are present ; but these latter are not so numerous as in the colon, 
although their calibre is greater. 
The mucous membrane of the anal canal presents in its upper half a number of 
vertical ridges known as the columns of Morgagni ; between the lower ends of these are 
found a series of small semilunar folds which are disposed horizontally around the 
passage and are called the anal valves (Fig. 705). 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 (7.e. the lower 12 to 16 mm. of the canal) is covered by modified 
skin, continuous with that around the anus. 
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 fmely-pointed pencil against the membrane. These rectal pits, of which we 
can find no previous description, are tubular in form, and have an accumulation of lymphoid 
tissue at the bottom of each; the whole appearance being such as might be produced if a small 
solitary gland were drawn down from the surface into the intestinal wall. 
Rectal Valves, or Valves of Houston (plicee transversales recti).—These, some- 
times termed the valves of Kohlrausch, are crescentic shelf-like folds which project 
into the cavity of the rectum on its lateral aspects (Fig. 704). They are composed 
of an infolding of the mucous, submucous, and 
ereater part of the muscular coats, and their 
form is preserved by the relative shortness of 
the anterior and posterior bands of longi- 
tudinal 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 inflec- 
tions 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 mesial 
sections of the empty rectum are examined, the 
valves are not easily seen (Fig. 737), as they 
then project but slightly, and are almost com- 2 
pletely hidden amongst the numerous ruge of es Neier SS ft naa 
the mucous coat. They are most evident in 
a distended rectum which has been hardened 
The diagram is a reduced tracing of Fig. 702. 
After removal of the prostate, ete., an 
in situ ; they can also be seen during life, per aperture was made in the anterior wall 
anum. with the aid of a rectal speculum. of the rectum, through which two rectal 
? - valves can be seen corresponding to 
As a rule two valves are found on the left and rae eee: . the ae ae 
“c . 2 Oe a *s - evator anl 1s also Shown, and its relation 
one on the right side; this latter is generally the fo the Béghininglof ‘the antell teansl as 
largest, and is situated a little above the level of suggested, 
the peritoneal reflection, viz. 3 or 34 inches (7°5 to 
8:7 cm.) above the anus ; the other two valves are found about 1 to 14 inches (2°5 to 3-7 em.) 
higher up and lower down respectively. The valves are distinctly marked in the foetus 
(see Fig. 705), 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 (see p. 1038). They are said to interfere sometimes with the 
introduction of an enema tube. 
The rectum in animals generally is free from the lateral inflexions described above, and the 
condition found in man is evidently an adaptation to the erect attitude. In quadrupeds the 
contents of the rectum do not press unduly on the sphincter, owing to the horizontal position of 
the body. With the assumption of the erect attitude, on the other hand, the whole weight of 
the contents would be thrown on the sphincters, were it not for the lateral foldings and the 
resulting shelves. 
