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The external anal sphincter (or sphincter ani externus) is an oval tube of skeletal muscle fibers. [1] Distally, it is adherent to the skin surrounding the margin of the anus. [2] It exhibits a resting state of tonical contraction [1] and also contracts during the bulbospongiosus reflex. [3] [4] [5] [6]
The internal anal sphincter, IAS, or sphincter ani internus is a ring of smooth muscle that surrounds about 2.5–4.0 cm of the anal canal. It is about 5 mm thick, and is formed by an aggregation of the smooth (involuntary) circular muscle fibers of the rectum.
The anal verge refers to the distal end of the anal canal, a transitional zone between the epithelium of the anal canal and the perianal skin. It should not be confused with the pectinate line between the upper and lower zones within the anal canal. The anal gland secretes lymphal discharge and built-up fecal matter from the colon lining.
The pudendal nerve that branches to supply the external anal sphincter also branches to the dorsal nerve of the clitoris and the dorsal nerve of the penis. [14] In addition to nerve endings, pleasure from anal intercourse may be aided by the close proximity between the anus and the prostate for males, and vagina, clitoral legs and anal area for ...
The inferior rectal nerves (inferior anal nerves, inferior hemorrhoidal nerve) usually branch from the pudendal nerve but occasionally arises directly from the sacral plexus; they cross the ischiorectal fossa along with the inferior rectal artery and veins, toward the anal canal and the lower end of the rectum, and is distributed to the sphincter ani externus (external anal sphincter, EAS) and ...
They meet with the corresponding fibers of the opposite side around the lower part of the rectum, and form for it a strong sling. Relaxation increases the angle between rectum and anus, allowing defecation in conjunction with relaxation of the internal and external anal sphincters. Levator ani relaxation and rectal emptying is facilitated by ...
The deep branch of the perineal nerve (or muscular branches) is a nerve of the perineum. It is a branch of the perineal nerve , from the pudendal nerve . It supplies the superficial transverse perineal muscle , bulbospongiosus muscle , ischiocavernosus muscle , the bulb of penis , levator ani , and the external anal sphincter .
Dogs have ear mobility that allows them to rapidly pinpoint the exact location of a sound. Eighteen or more muscles can tilt, rotate, raise, or lower a dog's ear. A dog can identify a sound's location much faster than a human can, as well as hear sounds at four times the distance. [41] Dogs can lose their hearing from age or an ear infection. [42]