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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 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 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 ...
Using horseradish peroxidase to stain the neurons, it has been determined that the external anal sphincter motoneurons are located in dorsomedial to the external urethral sphincter motoneuron in the cat, dog, monkey, golden hamster, as well as the man. However, the location of these motoneurons differs in the rat, Mongolian gerbil and domestic pig.
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 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 .
The middle fibers are inserted into the side of the rectum, blending with the fibers of the sphincter muscles; lastly in the male, the anterior fibers descend upon the side of the prostate to unite beneath it with the muscle of the opposite side, joining with the fibers of the external anal sphincter and transverse perineal muscles, at the ...
In females, there is no union, nor a tendinous perineal raphe; the parts are disjoint primarily and arise from the same central tendinous point of the perineum, which is the tendon that is formed at the point where the bulbospongiosus muscle, superficial transverse perineal muscle, and external anal sphincter muscle converge to form this major ...