The anus ( Latin ānus '(foot) ring' , ancient Greek πρωκτός prōktos ), German after ( mhd. After , ahd. Aftero , actually "rear"; substantiated from "behind, below"), colloquially also asshole , rosette or poperze , is the outlet opening of the intestinal canal in humans and multicellular animals . The feces leave the intestines through the anus .
Origin of the anus
The anus evolved together with the intestine as its outlet and appeared for the first time in bilateria , which consist of three cotyledons . In contrast, phylogenetically more primordial organisms such as the cnidarians (Cnidaria) and the comb jellyfish (Ctenophora) have a gastric space with a mouth opening, but no anus separated from it. A directed transport of food is enabled through the anus, in which indigestible food residues can be excreted separately from the mouth opening. Within the Bilateria, only very few groups do not have an anus, such as the flatworms (platelet worms) and the jaw mouths (Gnathostomulida).
Unlike the intestine, the anus is not formed by the endoderm , which forms a so-called gastrula with the invagination of a primal mouth (blastopore) into the blastula , but by the ectoderm . The cavity ( blastocoel ) created during gastrulation develops into the intestine, which forms another opening at the other end of the body, which is lined with an ectoderm. In this way, an intestinal canal is created with an entry and an exit opening, which later become the mouth and anus. In the case of the original mouths (protostomia), the original mouth becomes the final mouth and the opening becomes the anus. The actual formation of the intestine can proceed very differently, with the annelid worms the blastopore arises as a slit-like indentation which is closed on the abdomen and from which the mouth and anus emerge without a new breakthrough.
In the case of the new mouths (deuterostomia), which also includes vertebrates and thus also humans, the original mouth becomes the later anus and the secondary breakthrough on the front of the embryo becomes the mouth opening.
In its simplest form, the anus represents a simple opening of the intestine to the outside through which the indigestible food residue can leave it. As a rule, it is surrounded by muscles that continue the intestinal peristalsis and thus move the feces out. In vertebrates there are also sphincters at the anus , through which bowel evacuation can take place in a controlled manner.
In numerous invertebrates and their larvae, the anus fulfills other tasks in addition to defecating. For example, in the anus of the dragonfly larvae there is a tissue with a strong blood supply that is used for intestinal respiration . In insects, the anal area is surrounded by a chitinous end ring, the periproct , as well as the anal valves consisting of an unpaired epiproct and the paired paraproct .
Anal glands are formed in the anal region of many mammalian species . In the amphibians , the reptiles grouped together, and the birds, there is a common body outlet for the digestive, sexual and excretory organs with the cloaca . It is a section of the rectum into which the outlet ducts of the genital organs ( gonoducts ) and the ureters open, whose products ( sperm and egg cells ) and excreta such as excrement are released via the anus.
- Zona columnalis : with longitudinal folds (Columnae anales) and indentations in between (anal crypts) with the mouths of the proctodeal glands (Glandulae anales)
- Zona intermedia : with a multilayered squamous epithelium
- Zona cutanea : with cornified, multilayered squamous epithelium, sweat and sebum glands and hair .
The dentate line marks the boundary between the squamous epithelium of the anal canal and the epithelium of the rectum.
- Musculus sphincter ani internus (internal anal sphincter ): It represents a strengthening of the smooth muscles of the intestinal wall.
- Musculus sphincter ani externus (external anal sphincter ): It consists of striated muscles , so it can be influenced at will.
The peristalsis of the anal canal is stimulated by parasympathetic nerve fibers from the cross section of the spinal cord (Nervi pelvini) . These also cause the internal anal sphincter to relax. In interaction with the abdominal muscles (abdominal press), this leads to an emptying of the rectum via the defecation reflex (excrement, defecation ). The column of feces is pushed out of the intestine. If the abdominal muscles are not used for elimination, defecation will take longer.
The sympathetic nerve fibers of the hypogastric nerve reduce peristalsis and increase the tone of the internal anal sphincter. This enables stool retention (Continentia alvi) . By arbitrarily influencing the external anal sphincter, the excrement can be suppressed. It is innervated by the pudendal nerve (or its caudal rectal nerve ) .
The sensory innervation of the anus takes place via the nervi anococcygei and the nervus perinealis superficialis ("superficial dam nerve") of the nervus pudendus. Since the anus has a large number of nerve endings , it is very sensitive and is also regarded as an erogenous zone , especially the external anus muscle and the perineal muscles that separate from it ( see also anal reflex , anal intercourse ).
Examination and diseases of the anus
- External inspection
- Rectal digital examination (as one of the most important and inexpensive methods)
- Proctoscopy as a so-called “reflection” of the rectum with a special endoscope or a rigid proctoscope; possibly extended examination as rectoscopy or rectosigmoidoscopy
The anus can have various malformations and diseases, for example
- Atresia ani , a congenital malformation with a closed anus (due to the failure of the anal membrane to open )
- Anal eczema , acute or chronic intertrigo in the anal area
- Anal fissure , painful tear in the skin in the anal canal
- Anal fistula , usually as a result of inflammation
- Anal carcinoma , a cancer that often metastasizes prematurely
- Anal polyp , abnormally enlarged anal papilla
- Anal prolapse , a prolapse of the anal mucosa
- Perineal tear, tearing of the tissue between anus and vagina or scrotum. In severe cases (DR III °) the anal sphincter can also tear.
- Hemorrhoid disease , a nodular enlargement of the corpus plexus cavernosum
- Cryptitis , inflammation of the glandular crypts of the anal canal
- perianal abscess , an abscess in or around the anus
- Periproctitis , inflammation of the tissues near but outside the mucosal lined lumen of the rectum.
- Perianal vein thrombosis
- Pruritus ani , itching in the anal area as a result of various diseases
- Proctalgia fugax , spasmodic pain in the internal sphincter muscle that occurs in spasms
Also rectal foreign body may occur.
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