Voluntary Muscles
In a normal individual, voluntary striated muscle structures responsible for fecal control are represented by a funnel-like muscle structure that inserts in the pubic bone, the lowest part of the sacrum, and the middle portion of the pelvis.
From there, this diaphragm-like muscle structure extends medially surrounding the rectum all the way down to the perianal skin.
The upper portion of this funnel-like structure is known as the "levator muscle" and the lowermost part as the "external sphincter." Different slings and subdivisions of these structures have been described, including the:
- Ischiococcygeus
- Ileococcygeus
- Pubococcygeus
- Puborectalis
- Deep external sphincter
- Superficial external sphincter
On the basis of the experience obtained through the posterior sagittal surgical exploration of normal individuals, for the purpose of removing pelvic tumors or repairing sequellae from trauma, the author has been unable to differentiate the different portions or slings already mentioned. Rather, there seems to be a continuum of striated muscle that runs around the rectum and parallel to it, down to the skin.
The external sphincter seems to be represented by a group of parasagittal muscle fibers that meet anterior and posterior to the anus.
Normal Male Anatomy | 
Normal Female Anatomy |
The muscle fibers that come from the upper insertion of the levator muscle run parallel to the rectum and, therefore, perpendicular to these parasagittal fibers.
Normal Anatomy Coronal View | 
Normal Anatomy Sagittal View
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The upper portion of the funnel is known as the levator muscle, and the lowest part is known as an external sphincter. We called the intermediate portion (vertical fibers) "muscle complex."
In reality, there is no separation between one group of fibers and another. Vertical fibers "muscle complex" and parasagittal fibers create two corners at the point where they meet, which represent the anterior and posterior limit of the anus.
The electrical stimulation of the upper portion of the levator reveals a contraction which pulls the rectum forward.
The same type of stimuli, when used at the level of the muscle complex, elicits an elevation of the anus, and the same stimuli, when applied to the parasagittal fibers, will provoke a contraction following the direction of those fibers that tend to close the anus, giving the false impression that these are circular fibers.
The striated muscle urethral sphincter is part of this funnel-type muscle structure and represents an anterior prolongation of it. In the female, this muscle extends along both sides of the entire urethra, as demonstrated by electrical stimulation in cases of persistent cloaca.
For more information or to request an appointment, please contact the Colorectal Center at Cincinnati Children's.
