Sphincter ani internus muscle

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:
Artery:
Nerve:	pudendal nerve
Action:	keep the anal canal and orifice closed, aids in the expulsion of the feces
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.

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. It is innervated by parasympathetic nerves from S1, S2, and S3. It is not innervated by the pundendal nerve, which is a somatic nerve.

Additional images

Intestines

Schematic demonstrating the anatomy of hemorrhoids.

External links

• LUC spha
• Norman/Georgetown pelvis (rectum)

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 As such, some of the information contained herein may be outdated.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.

List of muscles of torso
BACK	splenius (capitis, cervicis) - erector spinae (iliocostalis, longissimus, spinalis) - latissimus dorsi transversospinales: (semispinalis dorsi, semispinalis cervicis, semispinalis capitis, multifidus, rotatores) - interspinales - intertransversarii
SUBOCCIPITAL	rectus capitis posterior (major, minor) - obliquus capitis (inferior, superior)
CHEST	intercostales (external, internal, innermost) - subcostales - transversus thoracis - levatores costarum - serratus posterior (inferior, superior) - diaphragm
ABDOMEN	anterior/lateral wall: obliques (external, internal) - transversus abdominis - rectus abdominis - pyramidalis posterior wall: quadratus lumborum - psoas major/psoas minor - iliacus cremaster
PELVIS/FLOOR	levator ani (iliococcygeus, pubococcygeus, puborectalis) - coccygeus
PERINEUM	anal triangle: sphincter ani (externus, internus) superficial perineal pouch (superficial transverse perineal, bulbospongiosus, ischiocavernosus) deep perineal pouch (deep transverse perineal, sphincter urethrae membranaceae)