The internal anal sphincter is formed by:
Correct Answer: Circular muscle fibers
Description: Ans: B (Longitudinal muscle fibers) Ref: Grays Anatomy 40th editionExplanation:Internal anal sphincterThe internal anal sphincter is a well-defined ring of obliquely orientated smooth muscle fibers that is continuous with the circular muscle of the rectum, and which terminates at the junction of the superficial and subcutaneous components of the external sphincter.Its thickness varies between 1.5 and 3.5 mm, depending upon the height within the anal canal and whether the canal Is distended.It is usually thinner in females and becomes thicker with age.It may also be thickened in disease processes such as rectal prolapse and chronic constipation.The lower portion of the sphincter is crossed by fibers from the conjoint longitudinal coat which passes into the submucosa of the lower canal.External anal sphincterThe externa] anal sphincter is an oval tube-shaped complex of striated muscle, composed mainly of type 1 (slow twitch) skeletal muscle fibers, which are well suited to prolonged contraction.Endoanal ultrasound and magnetic resonance imaging revealed that the uppermost fibers blend with the lowest fibers of puborectalis.In the upper third, some of these upper fibers decussate anteriorly into the superficial transverse perineal muscles and posteriorly some fibers are attached to the anococcygeal raphe.The majority of the fibers of the middle third of the external anal sphincter surround the lower part of the internal sphincter.The middle third is attached anteriorly to the perineal body and posteriorly to the coccyx via the anococcygeal ligament: some fibers from each side of the sphincter decussate in these areas to form a commissure in the anterior and posterior midline.The fibers of the low er third lie below the level of the internal anal sphincter and are separated from the lowest anal epithelium by submucosa.
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