The penis is a rod-shaped organ that is necessary for urination and sexual intercourse. It is located above the scrotum and it is made of erectile tissue (spongy tissue with blood vessels that is filled with blood to make an erection), connective tissue, blood vessels, nerves, lymphatic vessels, and covered with Dartos (a thin layer of vascular contractile tissue that contains smooth muscle fibers but no fat and is situated beneath the skin) and skin. In a simplified view, the penis is composed of three cylinders encased in a sheath called the Buck’s fascia, and covered with skin. These three cylinders are the corpus spongiosum and two corpora cavernosa. All these three cylinders are made of erectile tissue:
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Corpora cavernosa: The two cylinders of erectile tissue that form most of the penis. The corpus cavernosum contains spongy erectile tissue that can be filled with blood coming from the cavernosal arteries, which run along the middle of each corpus cavernosum. The function of the corpora cavernosa is to facilitate penile erections and provide rigidity.
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Corpus spongiosum: The single cylinder of erectile tissue that contains the urethra (the tube through which urine and sperm pass from the body)
The main vascular support is provided by the penile arteries, also known as the common penile arteries. They are terminal branches of the internal pudendal arteries, along with the scrotal arteries. They subdivide into three arteries: i) the bulbourethral arteries, which provide blood to the urethra and the corpus spongiosum; ii) the dorsal arteries of the penis, which run along the dorsal surface of the penis and provide blood to the glans and the two corpora cavernosa in a retrograde fashion; iii) the cavernosal arteries, which run inside the two corpora cavernosa.
The majority of venous drainage from the penis is by two main vessels, the superficial and deep dorsal veins of penis. These veins are located in the midline on the dorsal aspect of the penis and travel along the length of the penile shaft. The deep dorsal vein drains to the periprostatic plexus (Santorini plexus which is located behind the pubic bones and anteriorly to the prostate). The superficial dorsal vein drains to the external pudendal veins on both sides and from them to the saphenous veins of the legs.
Nerves of the penis control the erectile function and are key factors for intercourse. The penis is supplied by S2-S4 spinal cord segments and spinal ganglia. The sensory and sympathetic innervation to the skin and glans penis is supplied by the dorsal nerve of the penis, which is a branch of the pudendal nerve. The parasympathetic innervation is carried by cavernous nerves from the peri-prostatic nerve plexus, and is responsible for the vascular changes which control the erection.
The penis is covered by an elastic and extensible skin supported by the underline Dartos fascia. The redundant double-layered fold of smooth muscle tissue, blood vessels, neurons, skin, and mucous membrane that covers the head of the penis is called prepuce or foreskin. The prepuce provides adequate skin and mucosa to cover the entire penis during erection. Penile cancer usually arises from this part of the skin and progressively infiltrate the underline layers such as the Buck’s fascia, the glans, the corpus spongiosum, the urethra and the corpora cavernosa.
