Generality
The anus is the external opening which marks the end of the gastrointestinal tract and which allows the human being to eliminate feces.
According to some anatomical experts, the anus also includes the anal canal of the rectum. The rectum is the last section of the large intestine.
Located in the perineum, about 3 centimeters ahead of the coccyx, the anus has an internal anatomy that includes: an epithelium similar to that of the skin (but devoid of hair and sebaceous and sweat glands) and two circular muscles, known as the anal sphincter internal and external anal sphincter.
The internal anal sphincter and the external anal sphincter are fundamental in the process of defecation, which is the process of emission of fecal material.
Various pathologies and disorders can affect the anus and the anal canal of the rectum. The most common anal pathologies and disorders include: hemorrhoids, anal itching and anal fissure.
What is the anus?
The anus is the external opening where the gastrointestinal tract ends.
To be precise, it is the external opening that marks the end of the anal canal of the rectum, ie the terminal portion of the intestine.
In anatomy, external openings such as the anus are also known as "orifices" (singular "orifice").
ANOTHER DEFINITION OF ANO
Some human anatomy books treat the anus as the region of the gastrointestinal tract that includes the anal canal of the rectum and the external opening that marks the end of this canal.
In simpler words, the anus comprises the anal canal of the rectum with its orifice facing outwards.
WHAT IS THE RECTUM?
The rectum is the terminal tract of the large intestine or large intestine.
About 13 to 15 centimeters long and surrounded by several muscles and ligaments of the pelvic floor, the human rectum connects the colon-sigma intestinal tract with the anus.
Figure: portions of the large intestine.
Figure: anus and rectal canal.
Typically, anatomists divide the rectum into two portions: a pelvic portion and an anal (or perineal) portion.
Located in the pelvis, the pelvic portion represents the first portion of the rectum and includes a region called the rectal ampulla.
The rectal ampulla is used to accommodate the faeces ready for elimination and has considerable dilatation capacity.
The anal portion constitutes the second portion of the rectum and substantially corresponds to the aforementioned anal canal. Projecting posteriorly, the anal canal forms an angle of almost 90 ° with the pelvic portion and has an average length of about 3-4 centimeters.
Anatomy
The anus resides in an anatomical region known as the posterior perineum, about 3 centimeters in front of the coccyx, at the bottom of the groove between the two buttocks.
In resting conditions, its external appearance resembles that of a crevice with rippled edges, delimited by two lateral lips.
Always remaining outside the anus, numerous sweat glands, sebaceous glands and hair follicles are located around the orifice.
It is important to underline that the presence of anal hair is typical of men and rare in women.
Internally, the anus has a skin-like but hairless squamous epithelium, sebaceous glands and sweat glands.
Proceeding towards the rectum (to be precise towards the rectal canal), it is possible to recognize a particular circular area, which the anatomists called the combed line (or dentate line). The combed line essentially marks the point where the typical mucosa of the rectal tract ends and the point where the aforementioned skin-like squamous epithelium begins.
In correspondence with the combed line, they locate important anatomical structures, known as Morgagni's columns.
Two important circular muscles take place around the skin-like squamous epithelium and, in part, also around the mucosa of the rectal canal: the smooth anal sphincter (or internal anal sphincter) and the striated anal sphincter (or external anal sphincter).
INTERNAL ANAL SPHINTER
Made up of smooth muscle, the internal anal sphincter represents the thickened continuation of the smooth muscles surrounding the rectum.
It works involuntarily (like all smooth muscles) and is important, but not essential, for stool continence (fecal continence).
EXTERNAL ANAL SPHINTER
Formed by striated muscles, the external anal sphincter resides all around the internal anal sphincter.
It works voluntarily (like all striated muscles), represents a continuation of the levator ani muscle and is essential for stool continence.
BLOOD SPRAYING
The superior haemorrhoidal artery (which is a branch of the inferior mesenteric artery), the middle haemorrhoidal artery (which originates from the hypogastric artery) supply oxygenated blood to the anus and neighboring regions (such as the anal canal). and the inferior hemorrhoidal artery (which derives from the internal pudendal artery).
The drainage of venous blood, devoid of oxygen, involves the internal hemorrhoidal plexus and the external hemorrhoidal plexus. The internal hemorrhoidal plexus drains into the upper rectal veins, which in turn pour blood contents into the inferior mesenteric vein. The external hemorrhoidal plexus, on the other hand, drains into the middle rectal vein and the pudendal vein, which then flow into the internal iliac vein.
INNERVATION
The nerves that innervate the anus and adjacent areas come from the so-called superficial perineal nerve, which in turn derives from the pudendal nerve.
LYMPHATIC DRAINAGE
The lymphatic vessels of the anus and surrounding area drain their contents into the superficial inguinal lymph nodes.
Functions
The anus is the opening through which the human being eliminates faeces during the act of defecation.
Defecation - that is, the process of emission of fecal material - is a physiological reflex, which results from intestinal peristalsis.
The internal anal sphincter muscle and the external anal sphincter muscle play a fundamental role in the elimination of feces.
Both allow fecal matter to be released when relaxed.
The internal anal sphincter relaxes involuntarily, in response to the pressure exerted by the feces that reach the rectum (precisely in the "rectal ampulla).
The external anal sphincter, on the other hand, relaxes on the basis of a voluntary stimulus from the subject in need of defecating.
It is important to point out that the levator ani muscle also participates in the defecation process, supporting the anal sphincters in their action.
To learn more about the topic of defecation, readers can click here for a dedicated article.
