Content Reviewers:Viviana Popa, MD, Kaitlin Marshall,B.Sc. (Hons), M.Sc., David Clay, MSc, BSc (Hons), Andrew Horne, MSc, BSc (Hons)
The diamond shaped perineum is divided into two triangles, the urogential triangle anteriorly and the anal triangle posteriorly. The urogenital triangle differs between males and females. In males, it contains male external genitalia and the perineal muscles. Let’s start with the male external genitalia, which includes the membranous - or intermediate - urethra, the spongy urethra, the scrotum, and the penis.
The membranous urethra begins at the inferior surface of the prostate gland, called the apex, then it passes through the deep perineal pouch, which houses the deep perineal muscles: the deep transverse perineal muscles and the external urethral sphincter. The deep transverse perineal muscle extends from the medial surface of the ischium to the perineal body. The external urethral sphincter is a circular muscle that surrounds the membranous part of the urethra and compresses it to maintain urinary continence. The deep perineal muscles are innervated by the deep branch of the perineal nerve.
Now, in the deep perineal pouch, posterolateral to the membranous urethra are a pair of pea-shaped glands called the bulbourethral or Cowper glands. The ducts of these glands cross the deep perineal pouch with the membranous urethra, and terminate by emptying into the proximal part of the spongy urethra. During sexual arousal, these glands secrete mucus-like secretion into the urethra.
Finally, the membranous part of the urethra extends through the bulb of the penis where it’s continuous with the last part of the urethra, called the spongy urethra. The spongy urethra is located within the corpus spongiosum and extends through the penis to the external urethral orifice. The spongy urethra contains an expansion in the glans penis called the navicular fossa.
Now, let’s discuss the scrotum, which is a fibromuscular sac that lies posteroinferior to the penis, and inferior to the pubic symphysis. The scrotum contains the testes and associated structures.
The scrotum develops as two masses that approach each other and fuse in the midline forming the scrotal raphe. The scrotal raphe is continuous anteriorly with the penile raphe on the ventral surface of the penis, and posteriorly with the perineal raphe in the midline of the perineum.
All right, now the arterial blood supply of the scrotum differs between the anterior and posterior surfaces. The anterior surface is supplied by the anterior scrotal arteries, which are the terminal branches of the deep external pudendal artery.
The posterior surface is supplied by the posterior scrotal arteries, which are the terminal branches of the perineal arteries which are branches of the internal pudendal arteries. Both the superficial external pudendal artery, and the deep external pudendal artery anastomose with branches of the internal pudendal arteries and can also supply the skin of the penis. Finally, both surfaces of the scrotum receive arterial blood supply from the cremasteric arteries, which are branches of the inferior epigastric arteries.
Next are the scrotal veins, which accompany the arteries, and drain into the external pudendal veins. Finally, the innervation of the scrotum also differs between the anterior and posterior surfaces. The anterior surface of the scrotum is supplied by the anterior scrotal nerves, which arise from the ilioinguinal nerve.
The anterior scrotum is also innervated by the genital branch of the genitofemoral nerve. The posterior surface is supplied by the posterior scrotal nerves, which arise from the superficial branch of the perineal nerve, and the perineal branch of the posterior cutaneous nerve of the thigh.
Okay, now let’s take a deep breath and have a quick quiz! Can you name these structures? Additionally, can you name each of these three lines? All right, now let’s talk about the anatomy of the penis, which serves as a common passageway for semen during ejaculation, and urine during micturition. In the anatomical position, the penis is erect, meaning that the dorsal surface lies near the abdomen, and this means the ventral surface lies near the scrotum when it’s flaccid.
Within the penis are three bodies of erectile tissue: two paired corpora cavernosa and a single corpus spongiosum. These erectile tissues are the main internal structures that compose the three parts of the penis, which are the root, the body, and the glans.
Let’s start with the root, which is the attached part of the penis. The root lies in the superficial perineal pouch and consists of three erectile structures, called the bulb and the paired crura. The bulb of the penis lies in the midline and is the expanded proximal end of the corpus spongiosum.
The posterior part of the bulb is penetrated superiorly by the membranous urethra, forming the spongy urethra. Finally, the bulb of the penis is covered by the bulbospongiosus muscles. The right and left crura lie on each side of the bulb and are the proximal, tapered ends of the paired corpora cavernosa. The crura are covered by the ischiocavernosus muscles.
Now let’s talk about the body of the penis, which is the free part of the penis hanging from the pubic symphysis. As the erectile tissues continue into the body of the penis, they form three columns. A single corpus spongiosum which lies ventrally and the paired corpora cavernosa which lies dorsally. As we mentioned before, the corpus spongiosum contains the spongy urethra, which ends at the external urethral orifice - or urethral meatus - at the tip of the glans penis.
The corpus spongiosum and the paired corpora cavernosa are covered by a continuation of deep perineal fascia, called the deep fascia of the penis or Buck fascia. The corpora cavernosa are also covered by an extra layer of fibrous tissue called the tunica albuginea.
Now, the corpus spongiosum courses through the body of the penis and expands distally, forming the glans penis. The lateral margins of the glans penis expand to cover the distal ends of the corpora cavernosa. These extensions are called the corona of the penis. The lower ends of the corona form a groove that connects the body to the glans penis, called the neck of the penis.
Here, the skin and fascia of the penis extend to form a double layer that partially covers the glans penis, called the prepuce or foreskin. This foreskin connects to the glans penis on the ventral surface of the penis by a median fold of skin, called the frenulum of the prepuce.
The foreskin can be removed in a small procedure called circumcision. Males that undergo this procedure are called circumcised males, while males that have this foreskin are called uncircumcised males.
All right, now, all these erectile tissues are fixed by the ligaments of the penis, which support the root of the penis and attach it to the surrounding structures. There are two ligaments of the penis: the suspensory ligament and the fundiform ligament. Let’s start with the suspensory ligament, which is a thickening of the deep fascia that anchors the erectile bodies of the penis to the anterior surface of the pubic symphysis. Next is the fundiform ligament, which is a condensation of collagen and elastic fibers that arises from the linea alba, and surrounds the penis like a sling.
Let's take a look at the arterial supply of the penis. The main arteries that supply the penis branch from the internal pudendal arteries. These branches include the dorsal arteries of the penis, the arteries of the bulb of the penis, and the deep arteries of the penis.
Let’s start with the dorsal arteries, which are paired arteries that run on the dorsal surface of the penis. Here, the arteries lie on each side of a vein called the deep dorsal vein of the penis, in a groove between the corpora cavernosa. These arteries supply the fibrous tissue surrounding the corpora cavernosa, the corpus spongiosum, the membranous urethra, the spongy urethra, and the skin of the penis. Next are the arteries of the bulb of the penis, which supply the bulb of the penis, the part of the spongy urethra within the bulb, and the bulbourethral glands.
Next are deep arteries of the penis, which are paired arteries that run in the centre of each corpus cavernosum. These arteries give small branches that supply blood to the spaces of the corpora cavernosa, called cavernous spaces. Filling up these spaces with blood causes the penis to become erect. In a flaccid penis, these branches coil up to restrict blood flow, forming the helicine arteries of the penis.
Now, venous drainage of the penis is mainly by the deep and superficial dorsal veins of the penis. Let’s start with the deep dorsal vein, which receives blood from the cavernous spaces, and drains into a plexus of veins that surround the prostate, called the prostatic venous plexus. Next is the superficial dorsal vein, which receives blood from the skin and the subcutaneous tissue of the penis, and drains it into the superficial external pudendal veins. Some venous blood may also drain into the internal pudendal veins.
- "Anatomy and Physiology" Jones & Bartlett Learning (2004)
- "The Evolution of Organ Systems" Oxford University Press (2007)
- "Sexual selection and genital evolution" Austral Entomology (2013)
- "Hyman's Comparative Vertebrate Anatomy" University of Chicago Press (1992)
- "Sperm content of pre-ejaculatory fluid" Human Fertility (2010)
- "Genital asymmetry in men" Human Reproduction (1997)
- "A List of the Marine Mammals of the World" Academic Press (1967)
- "Cool sperm: why some placental mammals have a scrotum" Journal of Evolutionary Biology (2014)
- "Myofibroblasts and mechano-regulation of connective tissue remodelling" Nature Reviews Molecular Cell Biology (2002)