Boundaries and External Landmarks
The perineum is separated from the thigh by the genitofemoral crease, and posteriorly it is outlined by the lower borders of the gluteus maximus muscles. The bony boundaries form a rhombus:
- Anteriorly: inferior margin of the pubic symphysis.
- Posteriorly: tip of the coccyx.
- Laterally: ischial tuberosities.
An imaginary line connecting the ischial tuberosities divides this rhombus into two triangles: anterior (urogenital) and posterior (anal).
Important: in medicine, the obstetric perineum is distinguished—this is merely a narrow strip of tissue between the posterior commissure of the labia majora and the anus. The anatomical definition encompasses the entire rhombus.
Urogenital region (Regio urogenitalis)
The anterior triangle is bounded anteriorly by the subpubic angle (angulus subpubicus). Its base is formed by the urogenital diaphragm of the pelvis (diaphragma urogenitale). The urethra passes through it in men, and the urethra and vagina in women.
Superficial muscle layer includes three structures enclosed in fascial sheaths formed by the superficial fascia:
- Medially: bulbospongiosus muscle (m. bulbospongiosus).
- Laterally: ischiocavernosus muscle (m. ischiocavernosus).
- Posteriorly: superficial transverse perineal muscle (m. transversus perinei superficialis).
Deep layer is represented by the deep transverse perineal muscle (m. transversus perinei profundus). Its fibers encompass the membranous urethra, forming a sphincter. This muscle is sandwiched between the superior and inferior fasciae of the urogenital diaphragm. By fusing at the edges, the fasciae form a closed space. Anteriorly, they form the transverse perineal ligament and the arcuate ligament of the pubis, between which passes the deep dorsal vein of the penis (or clitoris).
Anal region (Regio analis)
The posterior triangle is bounded posteriorly by the coccyx and laterally by the sacrotuberous ligaments. Its base is formed by the pelvic diaphragm (diaphragma pelvis), through which the rectum passes.
In the center of the region lies the anal orifice, surrounded by the external anal sphincter (m. sphincter ani externus). The sphincter fibers blend into the skin, forming characteristic radial folds around the anus.
Lateral to the rectum lies the ischiorectal fossa (fossa ischiorectalis). It is filled with abundant adipose tissue, which is a direct continuation of the subcutaneous fat layer (there are no distinct boundaries between them).
Soft tissues, blood supply, and innervation
The skin of the perineum is thin laterally and thickens toward the center. It is rich in sebaceous and sweat glands and covered with hair. In men, the perineal raphe (raphe perinei) runs along the midline from the root of the scrotum to the anus.
A crucial landmark is the perineal body (centrum tendineum perinei). This is the central fascial node where the fibers of the bulbospongiosus, superficial transverse perineal, and external anal sphincter muscles intersect. It ensures the functional interplay of these muscles.
- Blood supply: internal pudendal artery (a. pudenda interna).
- Venous drainage: internal pudendal vein (v. pudenda interna).
- Lymphatic drainage: inguinal lymph nodes.
- Innervation: pudendal nerve (n. pudendus), branches of the ilioinguinal nerve (n. ilioinguinalis), and the perineal branch of the posterior cutaneous nerve of the thigh.
Briefly: answers to topical questions
Which muscles form the pelvic diaphragm (diaphragma pelvis)?
The pelvic diaphragm consists of paired muscles covered by fasciae. The following take part in forming the pelvic diaphragm:
- Levator ani muscle (m. levator ani) — a paired muscle comprising the pubococcygeus (m. pubococcygeus), puborectalis (m. puborectalis), and iliococcygeus (m. iliococcygeus) muscles.
- Coccygeus muscle (m. coccygeus).
- External anal sphincter (m. sphincter ani externus) — an unpaired muscle of the superficial layer of the pelvic diaphragm.
What forms the walls of the ischiorectal fossa?
The walls of the ischiorectal fossa are formed by adjacent muscles and the pelvic fascia covering them:
- Lateral wall — formed by the lower two-thirds of the obturator internus muscle (m. obturatorius internus), which is covered by the parietal pelvic fascia.
- Medial wall — formed by the inferior surface of the levator ani muscle (m. levator ani), covered by the inferior fascia of the pelvic diaphragm (fascia diaphragmatis pelvis inferior).
Which vessels and nerves pass through the pudendal canal (Alcock's canal)?
The pudendal canal (Alcock's canal), which is a split in the obturator fascia, transmits the internal pudendal neurovascular bundle. This bundle includes:
- Internal pudendal artery and vein (a. et v. pudendae internae).
- Pudendal nerve (n. pudendus).
To memorize the three superficial muscles of the urogenital triangle, imagine the letter "P" (or an arch): the ischiocavernosus muscles form the sides (pillars), the bulbospongiosus is in the center, and the superficial transverse perineal muscle closes the base from below.
Class and exam questions
How does the obstetric perineum differ from the anatomical one?
The anatomical perineum is the entire rhombus closing the pelvic outlet. The obstetric perineum is merely a small area of tissue between the posterior commissure of the labia majora and the anus.
Why are purulent infections in the urogenital triangle dangerous for the urethra?
The superior and inferior fasciae of the urogenital diaphragm fuse at the edges of the deep transverse muscle, forming a closed space. When pus accumulates, it has nowhere to escape, which can lead to its rupture directly into the urethral lumen.
What is the perineal body?
This is a dense central fascial node where the fibers of several muscles intersect simultaneously (bulbospongiosus, superficial transverse perineal, and external anal sphincter). It coordinates their joint function and serves as an important surgical landmark.
What else to study on this topic
- Topography of the ischiorectal fossa (fossa ischiorectalis)
- Pudendal canal (Alcock's canal) and its contents
- Differences between male and female urogenital diaphragms
- Structure of the vestibular bulb and greater vestibular glands in women
- Anatomy of the crura of the corpora cavernosa and corpus spongiosum in men
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