Urogenital triangle

• Boundaries

  The urogenital triangle sits anterior to the anal triangle and lies between the base of the triangle formed by an imaginary line between the ischial tuberosities and the apex of the triangle at the pubic symphysis. The urogenital diaphragm in found in the urogenital triangle and structures such as the urethra must pass through this diaphragm in both sexes.

  
• Fascia

  •Fatty and membranous

  •Fatty = Camper’s continuous with ischiorectal fossa and thigh

  •Males = scrotum Camper’s replaced by dartos smooth muscle

  •Membranous (Scarpa’s = Colles’) attached to posterior border of urogenital diaphragm, also into labia/scrotum

  
• Urogenital diaphragm

  The urogenital diaphragm fills the urogenital triangle and closes off the space between the pubic bones. This prevents structures passing out of the pelvic cavity – or prolapsing. It also provides an area of attachment for structures of the superficial perineal pouch – this will be described later. The descriptions of the urogenital diaphragm can be very complex as the fascial structures of the pelvis are interconnected but it can be described very simply as having 3 layers.

  
• Urogenital diaphragm

  The superior layer is composed of fascia that blends with the perineal body. The middle layer contains the voluntary sphincter that controls urination – sphincter urethrae and deep transverse perineal muscles. This layer is also described as containing an anatomical space called the deep perineal pouch. The third layer is the most superficial and is a layer of fascia called the perineal membrane. Structures that lie between this membrane and the skin are described as being in the superficial perineal pouch.

  
• Urogenital diaphragm

  •Layers fuse anteriorly, leaving a small gap near pubic symphysis

  •Posterior layers fuse with each other, membranous superficial fascia and the perineal body

  •‘Space’ between the layers of the urogenital membrane is called the deep perineal pouch

  
• Deep perineal pouch

  •Deep perineal pouch between the fascial layers of the urogenital diaphragm

  •Urethra, sphincter urethrae, deep transverse perineal muscles, internal pudendal vessels and branches

  •Pudendal (perineal) nerve

  •Female = vagina, dorsal nerves of clitoris

  •Male = membranous urethra, dorsal nerves of penis, bulbourethral glands, penile arteries

  
• Superficial perineal pouch

  •Structures between the perineal membrane (superficial fascia of the urogenital diaphragm) and the skin are in the superficial perineal pouch

  •Muscles = ischiocavernosus, bulbospongiosus, superficial transverse perineal

  •Urethra, vagina, clitoris, penis

  •Erectile tissue = crura, bulb of vestibule (female), bulb of penis (male)

  •Nerves and vessels

  
• MUSCLES

  These small skeletal muscles are supplied by perineal branches of the pudendal nerve. They overlie the erectile tissues of the perineum in both sexes and help to maintain erection of the tissues by contracting to impede venous return – retaining the blood inside the erectile tissue. Ischiocavernosus muscles overlie the crura and bulbospongiosus muscle overlie the bulb of the penis in males and the bulb of the vestibule in females.

  
• MUSCLES

  In females, the vagina opens into the perineum so the bulb of the vestibule is in two parts (one on either side), so the bulbospongiosus muscles in females also run either side of the opening of the vagina. In males, the bulbospongiosus muscles meet in the midline and surround the bulb of the penis.

  The superficial transverse perineal muscles shown by the blue block arrows support the edge of the urogenital diaphragm in both sexes (there are equivalent deep transverse perineal muscles in the deep pouch).

  
• Perineal body

  •Small fibrous structure

  •Posterior margin of the urogenital diaphragm

  •External anal sphincter, bulbospongiosus, superficial transverse perineal muscles all attach here

  Sitting at the posterior edge of the urogenital diaphragm is the perineal body. It provides attachment for the small muscles of the superficial perineal pouch and also the external anal sphincter.

  
• ERECTILE TISSUE

  The erectile tissues are similar in males and females, but much smaller in females. Lying next to the ischiopubic rami are the crura (singular is crus). In females they continue to form the body of the clitoris. In males they continue into the penis as the corpora cavernosa.

  The bulb of the penis continues as the corpus spongiosum of the penis which will also forms the glans penis. The bulb of the vestibule surrounds the opening of the vagina and also continues to form the glans of the clitoris.

  
• Penis

  The penis has 3 blocks of erectile tissue – 2 corpus cavernosum and 1 corpus spongiosum. The urethra lies within corpus spongiosum and is therefore called the spongy or penile urethra. The erectile tissue is surrounded by connective tissue that contains the vessels and nerves that supply the erectile tissue and skin.

  
• Branches of the internal pudendal artery supply the penis, as do sensory branches of the pudendal nerve

  
• The deeper arteries to the bulb of the penis and to the erectile tissues also branch from the internal pudendal artery

  
• Male urethra

  •Pre-prostatic

  •Prostatic

  •Membranous

  •Penile (spongy)

  •NB. Urinary and reproductive tract structure

  
• Male urethra

  There is a short pre-prostatic part between the trigone and the prostate gland, then the prostatic part passes through the prostate gland is is joined by the ejaculatory ducts. The third part is the membranous urethra that passes though the urogenital diaphragm and then the penile or spongy urethra that passes through corpus spongiosum to open at the glans of the penis.

  
• Male urethra

  The membranous urethra is normally the narrowest part of the urethra in a healthy individual, as the urethra is slightly compressed as it passes through the layers of the urogenital diaphragm. During prostatic enlargement, the prostatic part of the urethra may become compressed or distorted making this the narrowest part of the urethra and impeding the flow of urine.

  
• the branches of the pudendal nerve that supply sensory innervation to the penis and scrotum. The perineal branches of the pudendal nerve also contain motor fibres to the small muscles in the superficial perineal pouch.

  Note how the pudendal nerve passes through the greater sciatic notch, around the ischial spine and through the lesser sciatic notch to send branches to the structures of the anal and urogenital triangles.

  
• Clitoris

  In females, the equivalent structure to the penis is the clitoris. It is also composed of erectile tissue derived from the crura and the bulb (of the vestibule). The erectile tissue of the clitoris is also supplied by autonomic nerves and its blood supply is from branches of the internal pudendal artery and sensory nerve supply is from branches of the pudendal nerve.

  
• neurovascular structures that supply the female perineum.