The perineal membrane and its interconnected structures form a complex apparatus that attach many structures (). It is a 3-dimensional mass of tissue with several attachments. There are has two distinct regions; a dorsal portion lateral to the perineal body and a ventral portion lateral to the urethra. The dorsal portion consists of bilateral transverse fibrous bands of connective tissue that attach the perineal body and lateral wall of the vagina to the ischiopubic rami. This portion is bounded above by ischiorectal fossa fat and below by the structures of the perineum including the vestibular bulb, clitoral crus and their investing muscles the bulbospongiosus and ischiocavernosus muscles. In the ventral region, the membrane is part of a solid 3-dimensional tissue mass in which several structures are embedded. It is continuous with the paraurethral and paravaginal connective tissues and contains the compressor urethrae and urethrovaginal sphincter muscles of the distal urethra. The ventral margin of this mass is continuous with the insertion of the arcus tendineus fascia pelvis into the pubic bone. The levator ani muscles are attached to the cranial surface of the perineal membrane complex, while the vestibular bulbs and clitoral crus are fused with the caudal surface. Medially, these structures fuse with the walls of the urethra and vagina.
Figure 1 Drawing of dissection revealing the perineal membrane (PM) showing its lateral attachment to the inferior pubic ramus. A window in the perineal membrane has been cut to reveal the attachment of the levator ani muscle (LA) and its fusion with the vestibular (more ...)
demonstrates the changing nature of the perineal membrane in coronal section. Panel A shows the arcuate pubic ligament that spans the distance between the ischiopubic rami ventral to the first appearance of the perineal membrane complex seen in Panel B. Here, the ventral portion of the perineal membrane complex is defined by the location of its surrounding structures. It lies cephalad to the vestibular bulb and the clitoral crus, and caudal to the levator ani muscles. Medially a portion of the wall of the urethra can be seen as well as the paraurethral portion of the vagina. The fact that the ventral portion of the perineal membrane is one component of this an integral part of these inter-related structures –and not a freestanding structure– is evident here.
Figure 2 Contiguous coronal sections of a 33-year-old female cadaver demonstrating the changing nature of the perineal membrane from its ventral margin (B) to its dorsal-most extent (E). The left side of each image is the midline portion of the specimen. Abbreviations: (more ...)
Panel C lies at the level of the vaginal lumen where the transition from the ventral to the dorsal portion of the perineal membrane complex occurs. At this point the structure becomes a distinct layer of connective tissue between the lateral vagina and the ischiopubic ramus though still maintaining its close relationship to the levator ani muscle. Like the traditional paradigm, this dorsal portion is distinct from its surrounding structures creating a “deep space”; i.e., ischio-anal fossa. The ischio-anal fossa defines the upper margin of this dorsal portion, and the erectile structures (VB and CC) its caudal margin.
Panels D and E show the dorsal portion of the perineal membrane complex. The fibromuscular layer of the posterior vaginal wall (D) and the perineal body (E) are attached to the ischiopubic ramus via this fibrous dorsal portion of the perineal membrane. The distance between the midline viscera and the lateral ischiopubic rami has widened in this region compared with panels A – C so that the perineal membrane is a distinct structure with less interconnectedness between surrounding structures.
Reviewing the Panels from B to E reveals the constant anatomic relationships. The viscera are medial; the ischiopubic rami are lateral; the erectile structures are caudal; and the levator ani muscles cranial in the ventral sections and the ischio-anal fossa cranial in the dorsal sections.
demonstrates the histologic detail in coronal section of the ventral portion of the perineal membrane. Panel A shows the ventral-most portion of the striated urethral sphincter muscle and the compressor urethrae/urethrovaginal sphincter muscle complex. Panel A indicates, via shading, the location of the pubic bone that was removed to facilitate cutting the histological sections. Note the attachment of the arcus tendineus fascia pelvis to the pubic bone indicated by the asterisk. There is no evidence of any perineal membrane structures in this section.
Figure 3 Histologic (trichrome) sections of an adult female cadaver in coronal plane demonstrating the complex, 3-dimensional nature of the ventral portion and its relationship with the superior fascia of the levator ani muscles and the endopelvic fascia of the (more ...)
Panels B and C show the ventral margin of the perineal membrane complex developing. Notice that the superior fascia of the levator ani muscles is continuous with the insertion of the arcus tendineus fascia pelvis (B), which is continuous with the paraurethral connective tissue supports (C), which is in turn continuous with the ventral portion of the perineal membrane complex (D & E). As the distance between the midline viscera and the pubic bone increases (C,D,E) the connective tissue mass of this ventral portion of the perineal membrane increases. Also, notice the pudendal neurovascular bundle embedded in this tissue (C).
Panels D and E show the continued development of the 3-dimensional mass of tissue; the perineal membrane. As seen in previous ventral panels, this mass of tissue is continuous with the superior fascia of the levator ani muscles and with the paraurethral connective tissue or endopelvic fascia. The compressor urethrae and urethrovaginal sphincter muscles are embedded into the medial portion of this tissue mass, while the pudendal neurovascular bundle is embedded into the lateral portion near the periosteum of the pubic bone. Also notice the clitoral crus and the vestibular bulb and their respective muscles fused to the caudal surface of the ventral portion of the perineal membrane complex.
Panel F is a section through the vaginal lumen at which point the transition from the ventral to the dorsal perineal membrane complex occurs. The distance between the midline viscera and the lateral pubic bone has increased, and therefore, the connective tissue mass has increased. It is continuous with the superior fascia of the levator ani muscles, the paravaginal connective tissue, and likely the periosteum of the pubic bone as well. The levator ani muscles are inserting into this connective tissue mass cranially, while the clitoral crura and vestibular bulbs, with their respective muscles, are inserting into or are fused with the tissue mass caudally.
demonstrates in sagittal section the insertion of the levator ani muscle fibers into the portion of the perineal membrane lateral to the vagina. Panel A is a parasagittal section through the vaginal lumen, perineal body, and the ventral portion of the internal anal sphincter muscle. Ventral to the vagina, the lateral portion of the striated urethral sphincter and the ventral midline portion of the compressor urethrae and urethrovaginal sphincter muscle complex can be seen.
Figure 4 Histologic (trichrome) sections of an adult female cadaver in sagittal plane beginning in the parasagittal plane just lateral to the urethral lumen (A) and progressing laterally to the region of the perineal membrane demonstrating the insertion of levator (more ...)
Panels B and C are sections lateral to A, and show the vaginal lumen becoming the lateral vaginal wall and its connective tissue (C). Similarly, the perineal body also becomes connective tissue as the sections move laterally (C). This connective tissue lateral to the vagina and perineal body, within which the compressor urethrae and urethrovaginal sphincter muscles are contained, is the perineal membrane structure.
Panels C and D show the levator ani muscle fibers inserting into the cranial surface of the perineal membrane complex. These sections demonstrate that the structures in this region are not separate entities, but are interconnected parts of a single tissue mass.
shows the axial sections of the same cadaver whose coronal sections are shown in . A comparison of the coronal and axial sections from the same cadaver demonstrates that axial section is not favorable to defining the relationships of the complex 3-dimensional structure of the perineal membrane, its attachments to the pubic bone, perineal body and pelvic viscera. Note the lower vagina, mid-urethra, levator ani and obturator internus muscles. Panel B, shows the ventral portions of the perineal membrane lateral to the urethra and vagina. The superficial transverse perineal muscle marks the dorsal edge of the perineal membrane. Although this muscle is not part of the complex, it lies on the caudal surface of the dorsal edge of the perineal membrane where fibrous connective tissue bands connect the perineal body to the ischia. The section in Panel C is caudal to the perineal membrane.
Figure 5 Contiguous axial sections of the lower urogenital tract, including portions of the perineal membrane (PM), from the other side of the cadaver shown in . Cross sections run from cephalic (A) to caudal (C). Abbreviations: anus (A); ischiopubic ramus (more ...)