1042 THE DIGESTIVE SYSTEM. 
seen on each side, indicating the upper limit of the canal. It is said that these 
ridges can also be felt during lite by the finger introduced into the rectum (Cripps). 
Below, the anal canal ends at the anus, or anal orifice, by eee on the exterior. 
Its Jength is usually from 1 to 13 inches (2°5 to 3°7 cm.), being greater when 
the bowel is empty, and less when it is distended. Its antero- posterior diameter 
when closed varies between 3 and # inch (12 to 19 mm.) 
The direction of the anal canal, as already pointed out, is downwards and back- 
wards, often forming an angle of nearly 45° with the horizontal, although it 1s 
usually somewhat nearer to the vertical. 
Relations.—It 1s surrounded by both the external and internal sphincters, 
and above also by the borders of the levatores ani, these muscles forming a 
muscular cylinder around it (Fig. 701). On each side is situated the ischio- 
rectal fossa with its contained fat, which allows of the distension of the canal 
during the passage of feces. Behind is placed a mass of mixed connective and 
muscular tissue, known as the ano-coccygeal body (Symington), which intervenes 
between it and the eoccyx. Finally, in front, it lies close behind the bulb of the 
urethra and the hase of the triang ular ligament i in the male, and a sound in the urethra 
can be easily felt by the finger introduced into the anal canal, particularly in thin 
bodies. In the female it is separated from the vagina by the wedge- shaped mass of 
fatty and muscular tissue known as the “perineal body.” 
Structure of the Rectum and Anal Canal.—The wall of the rectum is made up 
of four coats, viz.:—1l. The outer coat, formed in part of peritoneum (already described), 
and, where the peritoneum is absent, of connective tissue which can be dissected off in 
several layers. In this connective tissue the hemorrhoidal vessels run until they pierce 
the wall of the tube. In it also, at the back and sides of the rectum, are found embedded 
a number of rectal lymphatic glands. 
2. The muscular coat, which is much thicker than in any other portion of the intes- 
tine, is composed of two stout layers of unstriped muscle—an outer longitudinal and an 
inner circular—like that of the intestine generally. The Jongitudinal fibres, although 
present all round, are chiefly accumulated on the front and back of the tube (see p. 1038), 
where they form two broad bands ; at the sides they are reduced to a thin layer, the 
deepest fibres of which are folded in and take part in the formation of the rectal valves. 
Where the rectum pierces the floor of the pelvis, the outer layer of longitudinal fibres is united 
to the deeper portion of the levator ani, partly by tendinous fibres and partly by an inter- 
change of muscular fibres, between the levatores and the muscular coat of the rectum. This 
interchange of fibres, however, is denied by Peter Thompson and Browning. Below, the longi- 
tudinal fibres pass betw een the external and internal sphincter muscles, or through the latter 
to join the skin around the anus. The folding of the rectum from side to side, described above, 
is brought about and maintained by the shor tness of the fibres of this coat on the front and back 
of the bow el. 
In saggital sections of the pelvis near the mesial plane there can generally be seen a distinct 
band of red, longitudinally-arranged, muscular fibres, which descends ¢ on each side from the front 
of the coceyx to ‘blend with the longitudinal fibres on the back of the rectum. This band is the 
rectococcygeus muscle. It is composed of striped fibres above, but becomes unstriped below. 
Some unstriped muscular fibres which are found descending in the subcutaneous tissue of the 
lower part of the anal canal, to join the skin around the anus, have been described by Ellis as the 
corrugator cutis ani. Acc ording to Roux, they are some of fhe longitudinal fibres of the rectum 
which have passed through the internal ‘sphincter to the submucous tissue, and then descended 
to the skin. 
The circular fibres form along the whole length of the tube a continuous layer, which 
is doubled inwards to assist in the formation of each rectal valve, and is thickened 
below to form the internal sphincter of the anus. The internal sphincter (sphincter ani 
internus), as Just pointed out, is formed by a great, and rather sudden, increase of the 
circular muscular fibres, which begins at the upper end of the anal canal. It surrounds 
the canal for about an inch (2°5 to 3-0 em.), and terminates } or } inch (6 or 7 mm.) 
above its lower aperture (Fig. 705). 
3. The submucous coat 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 hemorrhoidal plexus of veins is contained in this layer. 
The mucous coat must be considered separately in the rectum and anal canal. 
That of the rectum is redder in colour than the mucous membrane of the colon, as a result 
