New Insights into the Anatomy of the Bulbospongiosus Muscle and Its Role in the Composition of the External Anal Sphincter in Humans

Abstract

Traditional descriptions of the bulbospongiosus muscle in males and females describe a striated muscle overlying the most proximal portion of the corpus spongiosus, the bulb of the penis in males, and the bulbs of the vestibule in females. Likewise, in both sexes the bulbospongiosus muscle and the superficial transverse perineal muscles are described as inserting at the midline on the perineal body. In dissections of forty (40) human cadavers we found both of these descriptions do not accurately characterize the anatomy. In all forty cadavers examined, as suggested by the embryology literature, the adult bulbospongiosus continues from the urogenital triangle to the anal triangle as an uninterrupted ring of muscle that forms the superficial portion of the external anal sphincter and passes deep and lateral to the perineal body without insertion. Additionally, the superficial transverse perineal muscle passes anteriorly and superiorly to insert onto the bulbospongiosus muscle, and does not follow traditional descriptions asserting that this muscle continues medially from the ischial tuberosities to the perineal body. The perineal body is inferior to the rest of the perineum and in the great majority of cadavers we did not observe any muscle fibers inserting on the perineal body, again contradicting the published literature. Dissections of the external anal sphincter demonstrate that the subcutaneous external anal sphincter has a different fiber direction and is not continuous with the superficial external anal sphincter. Also, the deep external anal sphincter is continuous with the puborectalis muscle and is located deep to and separate from the superficial external anal sphincter. Therefore, we suggest that the traditionally conceived external anal sphincter, with three parts of a single muscle, is more usefully described as three individual muscles.Support or Funding InformationMidwestern University Intramural Funds