Sphincter ani internus muscle

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Sphincter ani internus 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.
Coronal section of rectum and anal canal.
Latin	musculus sphincter ani internus
Gray's	subject #120 426
Origin:
Insertion:
Blood:
Nerve:	pudendal nerve
Action:
Dorlands/Elsevier	m_22/12550809

The Sphincter ani internus (or internal anal sphincter) is a muscular ring which surrounds about 2.5 cm. of the anal canal; its inferior border is in contact with, but quite separate from, the Sphincter ani externus.

It is about 5 mm. thick, and is formed by an aggregation of the involuntary circular fibers of the intestine. Its lower border is about 6 mm. from the orifice of the anus.

[edit] Actions

Its action is entirely involuntary. It helps the Sphincter ani externus to occlude the anal aperture and aids in the expulsion of the feces.

[edit] Additional images

Intestines

Schematic demonstrating the anatomy of hemorrhoids.

[edit] External links

• LUC spha

This article was originally based on an entry from a public domain edition of Gray's Anatomy. As such, some of the information contained herein may be outdated. Please edit the article if this is the case, and feel free to remove this notice when it is no longer relevant.

Muscles of the Head — Neck — Trunk — Upper limb — Lower limb— LIST OF ALL MUSCLES

BACK: splenius: (capitis | cervicis) | erector spinae (iliocostalis | longissimus | spinalis) | latissimus dorsi| transversospinales: (semispinalis dorsi | semispinalis cervicis | semispinalis capitis | multifidus | rotatores) | interspinales | intertransversarii | (Gray's s115) SUBOCCIPITAL: rectus capitis posterior (major, minor) | obliquus capitis (inferior, superior) | (Gray's s116) CHEST: intercostales (external, internal, innermost) | subcostales | transversus thoracis | levatores costarum | serratus posterior (inferior, superior) | diaphragm | (Gray's s117) ABDOMEN: obliques (external, internal) | transversus abdominis | rectus abdominis | pyramidalis | cremaster | quadratus lumborum | (Gray's s118) PELVIS: levator ani (iliococcygeus, pubococcygeus, puborectalis) | coccygeus | (Gray's s119) PERINEUM: sphincter ani (externus, internus) | superficial perineal pouch (transversus perinei superficialis | bulbospongiosus | ischiocavernosus) | deep perineal pouch (transversus perinei profundus, sphincter urethrae membranaceae) | (Gray's s120)