Last Updated on October 28, 2023
The penis is an external organ of the male reproductive system. It has two main functions. It acts as the intromittent organ that also serves as urinal duct. An intromittent organ is a general term for an external organ of a male organism that is specialized to deliver sperm during copulation
For sexual intercourse, the penis undergoes erection during erotic stimulation and becomes engorged with blood. Following ejaculation, the penis undergoes remission, returning to a flaccid state.
For micturition, the penis serves as a carrier of the urethra that comes from the bladder.
The article discusses the penis anatomy in detail.
Anatomical Position of Penis
In the anatomical position, the penis is erect. The surface that is closest to the abdomen is the dorsal side and the one which is closer to the testis is ventral side.
Structure of the Penis
The penis can be anatomically divided into three parts –
Root
Root is not visible externally. It is the most proximal, fixed part of the penis located in the superficial perineal pouch of the pelvic floor.
The root contains three erectile tissues (two crura and bulb of the penis), and two muscles (ischiocavernosus and bulbospongiosus).
The root of the penis is supported by two ligaments, which attach it to the surrounding structures:
Suspensory ligament
It is a condensation of deep fascia which connects the erectile bodies of the penis to the pubic symphysis.
Fundiform ligament
It is a condensation of abdominal subcutaneous tissue that runs down from the linea alba, surrounding the penis like a sling, and attaching to the pubic symphysis.
Body or shaft
It is the free part of the penis between the root and glans and is suspended from the pubic symphysis. It is composed of three cylinders of erectile tissue – two corpora cavernosa, and one corpus spongiosum.
The erectile tissue within the corpora contains arteries, nerves, muscle fibers, and venous sinuses lined with flat endothelial cells. In the cut section, the corpora cavernosa looks like a sponge. A thin layer of areolar tissue that separates this tissue from the tunica albuginea.
Tunica albuginea is a dense fibrous sheath of connective tissue with relatively few elastic fibers that surrounds the two cavernosa.
The tunica albuginea consists of 2 layers, the outer longitudinal and the inner circular.
Ventrally, the tunica albugenia is thicker and forms a groove to accommodate the corpus spongiosum. The tunica albuginea of the corpus spongiosum is considerably thinner though.
Flattened columns or sinusoidal trabeculae made of fibrous tissue and smooth muscle surround the endothelial-lined sinusoids (cavernous spaces) on the inner side of tunica albugenia.
A row of structural trabeculae arises near the junction of the 3 corporal bodies inserts in the walls of the corpora about the midplane of the circumference.
There is an incomplete midline septum between two cavernosa though it is more complete proximally at the base where both corpora diverge and attach to the ischiopubic rami
The single corpus spongiosum lies in the ventral groove between the 2 corpora cavernous with urethra passing through it.
The corpus spongiosum possesses a much thinner and more elastic tunica albuginea. This keeps spongiosum elastic enough to distend or passage of the ejaculate through the urethra.
Thin tunica also causes less rigid corpus during erection.
Therefore, the glans, distal extension of the penis, cover the tips of the corpora cavernosa to provide a cushioning effect.
The edge of the glans overhangs the shaft of the penis and the rim formed is called the corona.
Glans
Glans is the conical shaped most distal part of the of the penis. It is formed by the distal expansion of the corpus spongiosum. It contains the external urethral orifice.
Fasciae
The three erectile bodies are surrounded by from inside out by deep penile or Buck fascia, the dartos fascia, and the penile skin.
The deep penile or Buck fascia is immediately superficial to the tunica albuginea. It is continuous with the deep fascia of the muscles covering the crura and bulb of the penis, the ischiocavernosus, and bulbospongiosus. It is a strong fascia that adheres these structures to the pubis, ischium, and the urogenital diaphragm.
On the dorsal aspect of the corpora cavernosa, Buck fascia contains
- Deep dorsal vein
- paired dorsal arteries
- branches of the dorsal nerves
The dartos fascia or simply dartos is a layer of connective tissue found in the penile shaft, foreskin, and scrotum.
It is superficial to deep fascia and is also called as the superficial fascia of the penis or the subcutaneous tissue of the penis.
It is also continuous with Colles fascia of the perineum and Scarpa’s fascia of the abdomen.
Skin
The skin of the penis is more pigmented than that of the rest of the body. A loose connective tissue connects it to the underlying fascia.
Proximally the skin of the penis is continuous with that of the lower abdominal wall and distally, the penile skin is confluent with skin covering the glans. It folds on itself at the corona to form the prepuce which is also called foreskin. Foreskin overlies the glans.
Foreskin or prepuce is a double layer of skin and fascia, located at the neck of the glans. It covers the glans to a variable extent.
The frenulum is the median fold of skin ventrally that connects prepuce to the surface of the glans.
The potential space between the glans and prepuce is termed the preputial sac.
Erectile Tissues
The erectile tissues are responsible for the erection of the penis during sexual arousal. The erection is produced by filling of blood in these erectile tissues.
There are three masses of erectile tissues spanning root and body of the penis.
In the root, these are known as the left and right crura, and the bulb of the penis. The bulb is situated in the midline of the penile root and is traversed by the urethra. The left and right crura are located laterally.
Each crus [singular of crura] is attached to the ipsilateral ischial ramus and covered by the paired ischiocavernosus muscles.
The left and right crura continue anteriorly into the dorsal part of the penis – they form the two corpora cavernosa. They are separated by the septum of the penis, although often incompletely. The bulb of the penis continues as the corpus spongiosum, which lies ventrally.
Glans is a continuation of corpus spongiosum. The male urethra runs through the corpus spongiosum.
Apart from these erectile tissues, the shaft contains enveloping fascial layers, nerves, lymphatics, and blood vessels, all covered by skin.
Blood Supply of Penis
The pudendal artery is responsible for blood supply of the penis.
Penile skin is supplied by external pudendal arteries whereas internal parts are supplied by the internal pudendal artery.
The superficial external pudendal arteries branch into dorsolateral and ventrolateral branches. These branches send collaterals across the midline
There is a subdermal plexus present too.
The skin on the ventral aspect of the penis is by the posterior scrotal artery, a superficial branch of the deep internal pudendal artery.
The pudendal artery becomes the penile artery at the root of the penis. This artery then branches into
- The artery of the bulb (bulbourethral artery)
- passes through the deep Buck fascia
- supplies the bulb of the penis and penile urethra
- Dorsal artery
- travels along the dorsum of the penis between the dorsal nerve and deep dorsal vein
- gives off circumflex branches that accompany the circumflex veins
- gives terminal branches to the glans penis
- The deep penile artery or Cavernosal artery
- one artery on each side
- enters the corpus cavernosum at the crus
- runs the length of the penile shaft, giving off the helicine arteries
- helicine arteries are an integral component of the erectile process
Veins
The penis is drained by 3 venous systems, the superficial, intermediate, and deep.
Superficial
- Contained in the dartos fascia on the dorsolateral surface of the penis
- Coalesce at the base to form a single superficial dorsal vein
- Usually drains into the great saphenous veins via the superficial external pudendal veins.
Intermediate
- Deep dorsal and the circumflex veins
- Lie within and beneath the deep penile or Buck fascia.
- Drainage is by emissary veins
- Begin within the erectile tissue of the corpora cavernosa
- course through the tunica albuginea
- Drain into the circumflex or deep dorsal veins.
- The circumflex veins arise from the spongiosum, on ventral aspect of the penis
- The circumflex veins drain into the deep dorsal vein.
- The deep dorsal vein
- lies in the midline groove between the 2 corpora cavernosa and is formed from 5-8 veins emerging from the glans penis [the retrocoronal plexus]
- Receives blood from the emissary and circumflex veins
- Passes underneath the symphysis pubis
- Drains into the prostatic plexus.
Deep venous drainage
- Crural and cavernosal veins.
- Crural veins
- Arise in the midline, in the space between the crura.
- Cavernosal veins
- Consolidations of the emissary veins
- Join to form a large venous channel that drains into the internal pudendal vein.
Lymphatics
- From the glans penis drains into large trunks in the area of the frenulum.
- These lymphatic vessels then circle to the dorsum of the corona and unite
- Course proximally beneath the deep penile
- Terminate mostly in the deep inguinal nodes of the femoral triangle.
- Some lymphatic drainage is to
- Presymphyseal lymph nodes
- lateral lymph nodes of the external iliac lymphatics.
Nerve Supply
The penis is supplied by S2-S4 spinal cord segments and spinal ganglia via the pudendal and cavernous nerves.
Sensory and sympathetic innervation to the skin and glans penis is supplied by the dorsal nerve of the penis, a branch of the pudendal nerve.
The pudendal nerves also supply somatic motor innervation to the penis.
Cavernous nerves carry parasympathetic transmission received from the peri-prostatic nerve plexus and are responsible for the vascular changes which cause an erection.
In addition to parasympathetic fibers, cavernous nerves also carry visceral afferent fibers and provide the nerve supply to the erectile tissue. The cavernous nerves run in the crus and corpora of the penis, primarily dorsomedial to the deep penile arteries.
Microscopic Anatomy of Different Structures of Penis
Tunica albuginea
- Irregular, latticed network of elastic fibers on which the collagen fibers rest.
- Inner circular layer and an outer longitudinal layer
- Emissary veins travel between the two layers
- often exit the outer layer in an oblique manner
- Outer layer of the tunica compresses the emissary veins during engorgement [erection]
Corpora cavernosa
- Two spongy cylinders
- Interconnected sinusoids separated by smooth muscle trabeculae
- Surrounded by elastic fibers, collagen, and loose areolar tissue
- The sinusoids are larger in the center and smaller in the periphery
- Terminal cavernous nerves and helicine arteries intimately associated with smooth muscle.
Corpus spongiosum
- Similar structure to cavernosa except
- Sinusoids are larger
- Thin tunica albuginea
- Glans has no tunical covering.
Erectile tissue vessels
Helicine arteries
- Branches of the deep penile artery
- Supply the trabecular tissue and sinusoids.
- Contracted and tortuous in the flaccid state
- Dilated and straight in the erect state.
Venous Drainage
- Starts in the venules at the peripheral sinusoids beneath the tunica albuginea
- Travel in the trabeculae between the tunica and the peripheral sinusoids
- Form subtunical venular plexus before exiting as the emissary veins
Microscopic Anatomy of Different Structures of Penis
Tunica albuginea
- Irregular, latticed network of elastic fibers on which the collagen fibers rest.
- Inner circular layer and an outer longitudinal layer
- Emissary veins travel between the two layers
- often exit the outer layer in an oblique manner
- Outer layer of the tunica compresses the emissary veins during engorgement [erection]
Corpora cavernosa
- Two spongy cylinders
- Interconnected sinusoids separated by smooth muscle trabeculae
- Surrounded by elastic fibers, collagen, and loose areolar tissue
- The sinusoids are larger in the center and smaller in the periphery
- Terminal cavernous nerves and helicine arteries intimately associated with smooth muscle.
Corpus spongiosum
- Similar structure to cavernosa except
- Sinusoids are larger
- Thin tunica albuginea
- Glans has no tunical covering.
Erectile tissue vessels
Helicine arteries
- Branches of the deep penile artery
- Supply the trabecular tissue and sinusoids.
- Contracted and tortuous in the flaccid state
- Dilated and straight in the erect state.
Venous Drainage
- Starts in the venules at the peripheral sinusoids beneath the tunica albuginea
- Travel in the trabeculae between the tunica and the peripheral sinusoids
- Form subtunical venular plexus before exiting as the emissary veins
Physiology of Erection
Erection is triggered by the parasympathetic division of the autonomic nervous system which released nitric oxide levels to rise in the trabecular arteries and smooth muscle of the penis.
Other neurotransmitters, including vasoactive intestinal peptide, calcitonin gene-related peptide, prostaglandins, and other peptides, may also be involved
With the relaxation of the smooth muscles in the trabeculae and the arterial wall, the following events occur, which lead to an erection:
- Increase in arterial inflow as a result of dilatation of the arterioles and arteries
- Distension of the sinusoids within the corpora cavernosa
- Subtunical venular plexuses are compressed between the tunica albuginea and the distended sinusoids, leading to decreased venous outflow
- Tunica albuginea is further stretched and cause compression of emissary veins and thus further decreasing venous outflow
- Intracavernous pressure increases
- Contraction of the ischiocavernous and bulbospongiosus muscles further results in full rigidity
