Faecal incontinence has been a disabling, difficult-to-treat disorder for years. The overall incidence of faecal incontinence is estimated at 2-7% of the adult population (1), however, these data may be significantly underestimated due to the fact that patients are often ashamed of their disorder and therefore do not seek medical attention. Damage to the anal sphincter apparatus is one of the factors that may lead to faecal incontinence. It is most often observed in females after vaginal delivery, with an increased risk in those with a history of instrumental (e.g. forceps) childbirth (2). It may seem trivial to say that high-quality surgical reconstruction of anal sphincters requires a good anatomical knowledge, but even experienced clinicians and anatomists may be surprised at the complexity of anal sphincter anatomy.

Two anal sphincters are classically distinguished: the internal anal sphincter (IAS) and the external anal sphincter (EAS). IAS is about 30 mm high and about 3 mm thick continuation of the smooth circular muscle layer at the rectum at the level of the anal canal (3-5). As a component of gastrointestinal musculature, it is composed of smooth muscle cells and controlled by the autonomic nervous system, independently of our will. The external anal sphincter, on the other hand, is a skeletal (striated) muscle. It is supplied by somatic nerves, mainly by the motor branches of the pudendal nerve (known as the inferior rectal nerves), reaching the muscle through the ischioanal fossa. Although its activity depends on our will, it remains in a certain constant tone, even if not contracted. Its exact morphology is much more complex, which will be discussed later in the article. Between both sphincters, there is also the longitudinal layer, consisting mainly of muscles of the longitudinal layer of the rectum musculature. It is referred to as the conjoint longitudinal muscle layer (CLML) or conjoint tendon by some authors due to the fact that it receives additional fibres from the levator ani muscle (3, 6, 7). However, research indicates that it consists mainly of smooth muscle cells, and therefore it should be treated as an extension of the rectum musculature. Its fibre arrangement significantly differs from that in sphincters (the fibres run vertically instead of circularly), hence it is clearly separated from these muscles, which can be easily observed during dissection. Some of its fibres end within both sphincters, while some anchor in the perianal skin, forming the so-called corrugator cutis ani together with the EAS (lat. musculus corrugator cutis ani). In addition to the common muscle layer, some authors also distinguish the so-called intersphincteric space, which is believed to contain a small amount of fat and connective tissue. However, most authors do not use this concept, but instead refer to this structure as CLML.

The anatomy of EAS, although investigated since ancient times, still holds many mysteries. EAS was already described by Galen, and was depicted for the first time by Vesalius in the famous “De Humani Corporis Fabrica” (8). It was initially treated as a homogeneous muscle. Santorini was the first to describe its three parts (subcutaneous, superficial and deep) in 1715, and it was not until 1897 that Holl included this structure in an anatomical textbook, although he was also the first to notice some ambiguity in the connection of the EAS with the levator ani muscle. Interestingly, the first editions of Gray’s Anatomy showed the EAS as undivided. Contemporary works usually cite the 1934 paper by Milligan and Morgan, attributing the authorship of this EAS model to these authors, although in fact the paper was devoted only to clinical aspects, and did not offer any new anatomical data (9-11).

Modern anatomical textbooks offer a description of EAS anatomy, such as that proposed by Santorini and confirmed by Milligan and Morgan: this muscle is divided into three parts: subcutaneous, superficial and deep, located one above the other, encircling the anal canal (12). The subcutaneous and at the same time the most superficial part has no skeletal attachment, but instead attaches only to the subcutaneous tissue, anteriorly and posteriorly from the anus in the midline. The slightly deeper superficial part makes up the main mass of the muscle, it also has its attachment points anteriorly and posteriorly from the anus, within the perineal body and in the anococcygeal ligament, respectively, this way indirectly attaching to the coccyx. The third and at the same time the deepest part of the EAS, i.e. the so-called the deep part, shows a very circular fibre arrangement and, like the superficial part, has no skeletal attachment; however, its exact attachment locations are not described.

Figure 1 shows a specimen from the Department of Descriptive and Clinical Anatomy, Centre for Biostructure Research, Medical University of Warsaw. Visible layered anatomy of the anal canal wall in cross-section (held with tweezers): circular IAS fibres are visible closest to the lumen of the rectum filled with faecal masses, then vertically running CLML fibres, and the outermost circular EAS fibres, in which three layers can be distinguished: subcutaneous (Sc-EAS), superficial (Su-EAS) and deep (P-EAS).

When discussing EAS anatomy, it is also necessary to mention a handful of structures that are either anatomically or functionally closely related to this muscle. These include the already mentioned levator ani muscle, the perineal body and the anococcygeal ligament.