notably at the internal sphincter, which is merely a collection of these fibres, and at a point higher up where they are again gathered into a bundle either partly or completely surrounding the bowel, known as the third sphincter. This supposed muscle will be de- scribed more fully later. Submucous Coat.—The submucous tissue forming the bed upon which the mucous membrane rests is sufficiently lax to permit of considerable sliding of the mucous membrane on the muscular coat. In it the blood vessels ramify, and from it perpendicular processes are given off which perforate both the internal and external muscular layers and are finally lost in the sheaths of the muscular fibres, or go entirely through the muscular layer and blend with the fibrous stroma of the surrounding fatty tissue. These processes from the submucous tissue, together with the lymph and blood vessels, serve to bind the various layers of the rectal wall together.
Mucous Membrane.—The mucous membrane of the rectum corresponds in its general characters with that of the other parts of the bowel, being modified, however, in certain particulars to suit its location and function. Its thickness is about three-quarters of a mm.; it is redder and more vascular than that of other parts of the large intestine; it glides freely on the tissue beneath, and is so ample as to be gathered into folds at various points, which are of considerable surgical and anatomical interest. At its point of union with the skin of the anus, it is gathered into vertical folds which diminish when the bowel is distended, but do not entirely disappear, and hence are not due solely to the contraction of the sphincter. These vertical folds have received the name of colwmune recti, or columns of Morgagni; and Treitz states that they contain bands of muscular fibres running longitudinally and terminating above and below in elastic tissue. Kohlrausch* also describes a thin layer of longitudinal muscular fibres under the mucous membrane at this point, and has named it the sustentator tunicce mucose.
<Callout type="important" title="Important">These vertical folds are not just for appearance; they play a crucial role in maintaining the integrity of the rectal wall.</Callout>
Between the lower ends of the column recti little arches are sometimes stretched from one to the other, forming pouches (Fig. 5 and Fig. 6). These little pouches, or sacculi, have quite recently been brought into rather an undue prominence by the attempts of certain charlatans to locate in them many of the causes of rectal disease. They have always been known to anatomists, as Figures 5 and 6 will prove (Fig. 5 from Morgagni and Fig. 6 from Horner). But the fact is that both the more especially the first one, are diagrammatic exaggerations for the sake of clearness.
<Callout type="warning" title="Warning">These sacculi are often misdiagnosed as pathological conditions; be cautious when interpreting their presence.</Callout>
Muscles of the Rectum and Anus.—The muscles which may properly be included in a description of the rectum and anus are the external and internal sphincters, the levator ani, ischio-coccygeus, retractor recti or recto-coccygeus, and the transverrus perinei. External Sphincter.—The external sphincter muscle is a thin layer of voluntary fibres, about half an inch broad on each side of the anus, surrounding it in the form of an ellipse, and having a narrow, pointed insertion anteriorly and posteriorly. It is situated immediately beneath the skin, and extends about two centimetres up the bowel, where its upper limit may sometimes be seen by the white line between it and the internal sphincter already mentioned. It is divided into a superficial and a deep portion. The superficial is inserted, both in front and behind, into the subcutaneous cellular tissue. The deeper and thicker portion is inserted posteriorly by a narrow, flat tendon into the posterior surface of the fourth coccygeal vertebra. Between the tendon and the bone is a bursa about the size of a pea—bursa mucosa coceygea of Luschka. Anteriorly it is inserted into the central tendon of the perineum in common with the transversus perinei and bulbo-cavernosus, and in women with the sphincter vaginz.
<Callout type="tip" title="Tip">The external sphincter's deep portion plays a crucial role in maintaining continence.</Callout>
Internal Sphincter.—The internal sphincter is an involuntary muscle situated immediately above and partly within the deeper portion of the external sphincter, being separated from it by a layer of fatty connective tissue. Its thickness is about two lines; its vertical measurement from half an inch to an inch; and it is a direct continuation of the involuntary circular fibres of the bowel, growing thicker and stronger as they approach the anus. It also is supplied by the hemorrhoidal branch of the internal pudic. In dissecting this muscle for demonstration, it should be approached from the mucous surface of the bowel.
Recto-coccygeus (Retractor recti, Treitz;* Tensor Fascie Pel- , Kohlrausch).—This muscle consists of two flat lateral bands of unstriped fibres, each of which is about four mm. broad, which diverge at an acute angle from the anterior coccygeal ligament at the tip of the coc and, passing forward and down- ward, embrace the lower end of the rectum on each side like a fork. It is located directly under that portion of the levator ani which forms the floor of the pelvis between the tip of the coc- eyx and the anus: and blends partly with the longitudinal muscular fibres of the rectum, and partly with the pelvic fascia surrounding its end. Its function is to hold the end of the rectum against the coccyx and to give it a fixed point in defe- cation.
<Callout type="important" title="Important">The recto-coccygeus muscle is essential for maintaining proper defecation.</Callout>
Levator Ani.—The levator ani and ischio-coceygeus muscles form a true diaphragm to the pelvis by giving an uninterrupted muscular and tendinous plane from the lower border of the py- riformis, behind, to the arch of the pubes in front. That part which is named ischio-coccygeus is usually described as a sepa- rate muscle, though in no way differing in function from the larger portion, and only distinguishable from it by its more ten- dinous structure. It is situated just in front of the sacro-sciatic ligaments, and arises by aponeurotic fibres from the sides and tip of the spine of the hium, from the anterior surface of the lesser sacro-sciatic ligament, and often from the posterior part of the pelvic fascia. It is inserted, also by aponeurotie fibres, into the border of the coccyx and lower part of the border of th erum. Owing to its tendinous origin and insertion, the greater part of the muscle is composed of aponeurotic fibr It is in yelation superiorly, by its concave surface, with the rectum; in- feriorly, by its convex surface, with the sacro-sciatic ligaments and the gluteus maximus; posteriorly its border is in contact with the lower border of the pyriform and anteriorly it is directly continuous with the fibres of the levatorani. Its action is to draw the coccyx to its own side, or, when both muscles act together, to fix that bone and prevent its being thrown backward in defecation.
<Callout type="risk" title="Risk">Improper handling of the levator ani can lead to pelvic floor dysfunction.</Callout>
The levator ani proper, which constitutes the remaining portion of the pelvic diaphragm, is in its general shape an inverted cone, supporting the pelvic contents in its cavity and allowing the rectum and prostate to pass through its apex. Considering each lateral half of the muscle apart, we find it made up of a delicate layer of muscular fibres forming a thin, curved, and quadrilateral sheet, broader behind than in front. Its upper border is stretched across the pelvis from the pubes to the spine of the ischium, arising from both these bony points and from the tendinous line of union of the pelvic with the obturator fascia, which runs antero-posteriorly between them. Its attach- ment to the pubic bone is at a point on its inner surface near the middle of the descending ramus and a little to one side of the anus (Fig. 7). From this extensive though delicate and in great part membranous origin, the fibres proceed downward and inward toward the median line. Those most anterior unite with those of the opposite side beneath the neck of the bladder, the prostate, and the adjacent portion of the urethra. These fibres are concealed by the pubo-prostatic ligament or anterior fold of the recto-vesical fascia, from which they also sometimes take origin in part. They are in relation, in front, with the posterior surface of the triangular ligament. This portion is sometimes separated from the main body of the muscle by a cellular interval, similar to those often found in other parts of this thin muscular sheet.
<Callout type="beginner" title="Beginner">Understanding the anatomy and function of these muscles can help prevent pelvic floor issues.</Callout>
From the tip of the spine of the ischium, the fibres are inserted into the side of the tip of the coccyx, while the fibres immediately in front of these (precoceygeal) unite with those of the opposite side in the median line and form a raphé which extends from the point of the coccyx to the posterior border of the sphincter, and thus complete the floor of the pelvis. + The fibres which arise indirectly from the upper part of the obturator foramen and from the brim of the pelvis by means of the pelvic fascia, pass downward and inward, forming a curve with its concavity upward, and may be divided into vesical and anal. The vesical pass into the sides of the bladder. The anal fibres in part pass backward and meet behind the bowel, and in part blend with those of the external sphincter at its upper border, there being no distinct line of separation between the two muscles.
<Callout type="gear" title="Gear">Pelvic floor exercises can help strengthen these muscles for better continence.</Callout>
Key Takeaways
- The rectum and anus have complex muscular structures, including the internal and external sphincters.
- Vertical folds in the mucous membrane (columns of Morgagni) play a role in maintaining rectal integrity.
- The levator ani muscle is crucial for pelvic floor support during defecation.
Practical Tips
- Perform regular pelvic floor exercises to strengthen the muscles involved in continence.
- Be cautious when interpreting the presence of sacculi, as they are often misdiagnosed as pathological conditions.
- Understand the anatomy and function of the recto-coccygeus muscle for better defecation control.
Warnings & Risks
- Improper handling of the levator ani can lead to pelvic floor dysfunction.
- Misinterpreting sacculi as pathological conditions may result in unnecessary medical interventions.
- Reliance on outdated anatomical diagrams, such as those of sacculi, can be misleading.
Modern Application
While the detailed anatomy described in this chapter remains relevant for understanding rectal diseases and their treatment, modern techniques have improved diagnostic tools and surgical methods. The knowledge of these muscles' functions is still crucial for preventing and managing pelvic floor disorders, which are increasingly recognized as common issues in both men and women.
Frequently Asked Questions
Q: What are the columns of Morgagni and why are they important?
The columns of Morgagni are vertical folds in the mucous membrane at the point where it meets the skin of the anus. They play a crucial role in maintaining the integrity of the rectal wall, as described in the chapter.
Q: What is the function of the levator ani muscle?
The levator ani muscle supports the pelvic contents and helps prevent the backward movement of the coccyx during defecation. It forms part of the pelvic diaphragm and is essential for maintaining proper pelvic floor support.
Q: What are sacculi, and why should they be interpreted carefully?
Sacculi are small pouches in the rectal mucous membrane that can sometimes be misinterpreted as pathological conditions. The chapter warns against overdiagnosing these structures, emphasizing the need for careful interpretation.