Sphincter ani externus muscle
|Sphincter ani externus muscle|
|Coronal section through the anal canal. B. Cavity of urinary bladder V.D. Ductus deferens. S.V. Seminal vesicle. R. Second part of rectum. A.C. Anal canal. L.A. Levator ani. I.S. Sphincter ani internus. E.S. Sphinear ani externus.|
|Latin||Musculus sphincter ani externus|
|Gray's||subject #120 425|
|Nerve||Branch from the fourth sacral and twigs from the inferior hemorrhoidal branch of the pudendal nerve|
|Actions||Keep the anal canal and orifice closed|
It measures about 8 to 10 cm in length, from its anterior to its posterior extremity, and is about 2.5 cm opposite the anus, when defecation occurs the sphincter muscle retracts.
It consists of two strata, superficial and deep.
- The superficial, constituting the main portion of the muscle, arises from a narrow tendinous band, the anococcygeal raphe, which stretches from the tip of the coccyx to the posterior margin of the anus; it forms two flattened planes of muscular tissue, which encircle the anus and meet in front to be inserted into the central tendinous point of the perineum, joining with the Transversus perinæi superficialis, the Levator ani, and the Bulbocavernosus.
- The deeper portion forms a complete sphincter to the anal canal. Its fibers surround the canal, closely applied to the Sphincter ani internus muscle, and in front blend with the other muscles at the central point of the perineum.
Posteriorly, they are not attached to the coccyx, but are continuous with those of the opposite side behind the anal canal.
The upper edge of the muscle is ill-defined, since fibers are given off from it to join the Levator ani.
The action of this muscle is peculiar.
(2) It can be put into a condition of greater contraction under the influence of the will, so as more firmly to occlude the anal aperture, in expiratory efforts unconnected with defecation.
Anismus is a paradoxical contraction of the external anal sphincter, when the intent is relaxation, which can result in obstructed defecation and constipation. Abnormal function of this muscle is also seen in anorectal malformation and after certain surgeries, including coccygectomy.
Schematic demonstrating the anatomy of hemorrhoids.
- LUC spha
- SUNY Labs 42:13-0100 - "The Male Perineum and the Penis: The External Anal Sphincter"
- perineum at The Anatomy Lesson by Wesley Norman (Georgetown University) (analtriangle3)
- pelvis at The Anatomy Lesson by Wesley Norman (Georgetown University) (rectum)