Honored everywhere from ancient statues and Renaissance paintings to modern-day graffiti, the human genitalia may be the most memorable human organs on the planet. However, there’s no piece of phallic art that can describe the penis as scientifically accurate as human anatomy can.
The penis is a copulatory organ of the external genitalia of the males. It consists of three parts; root (radix), body (shaft) and glans. The core of the penis contains three erectile tissues, namely the two corpora cavernosa and the corpus spongiosum. Furthermore, the penis transmits the penile neurovascular bundle, as well as the terminal part of the urethra, which opens at its tip. Therefore, the penis is a meeting point between the urinary and reproductive systems, and its functions are defined within the spectrum of functions of these two systems;
- The penis is responsible for reproduction as it conveys the seminal fluid and the sperm, as well as for the sensations of sexual arousal and pleasure. Out of the state of sexual arousal, the penis is described as flaccid. Whilst during sexual arousal and consequential arterial congestion in the erectile tissues, it is described as erect. The process of seminal fluid discharge is called the ejaculation.
- The penis enables the micturition (urination), since it conveys the urine away from the urinary bladder to the external urethral orifice to be expelled from the body.
This article will discuss the anatomy and functions of the penis.
|Definition||A copulatory organ of the external genitalia of males whose function is to enable the reproduction, sexual pleasure and micturition|
|Parts||Root (radix), body (shaft), glans|
|Contents||Corpora cavernosa, corpus spongiosum, urethra|
Bulbourethral artery, dorsal artery of penis, cavernous (deep/cavernosal) artery of penis;
Superficial dorsal, deep dorsal, circumflex, crural, and cavernous (deep/cavernosal) veins of penis
Sensory: Pudendal nerve (via dorsal nerve of penis)
Autonomic: Cavernous (deep/cavernosal) nerve of penis
- Shaft and glans
- Blood supply
- Erection and ejaculation
The root of the penis is the most proximal part of the penis. It is located in the urogenital triangle of the perineum, where it is fixed to the pubic symphysis via the two suspensory ligaments of the penis. The root consists of the two muscles (ischiocavernosus and bulbospongiosus muscles) and proximal expansions of the erectile tissues; the two crura of penis and the bulb of penis.
The crura of penis are the proximal projections of the corpora cavernosa. The crura diverge laterally, with each crus attaching to the ipsilateral ischiopubic ramus.
The bulb of penis is a proximal expansion of the corpus spongiosum. It is located in the interval between the crura of penis, and is proximally continued by the bulbospongiosus muscle. The bulb is pierced by the penile urethra, which after passing through the bulb, continues through the entire length of the corpus spongiosum until reaching the tip of the glans.
Shaft and glans
The body of penis is the free pendulous part entirely enveloped in skin. Deep to the skin, there are three fasciae that envelop the contents of the penis. From superficial to deep, they are the superficial fascia of penis (dartos fascia of penis), deep fascia of penis (Buck’s fascia) and tunica albuginea.
The shaft of the penis contains three erectile tissues; the two corpora cavernosa and the corpus spongiosum. These tissues extend through the entire body of penis. The corpora cavernosa lie one next to another in the dorsal compartment of the penis, while the corpus spongiosum lies in the ventral groove between them. In addition, the body of penis transmits the urethra and the penile neurovasculature.
The corpora cavernosa are the two erectile masses found within the dorsal part of the penis. Each begins within the root of penis as the crus of penis, traverses the shaft and terminates within the glans.
The corpora cavernosa are enveloped by the tunica albuginea, which is a layer of dense fibroelastic connective tissue. The tunica albuginea consists of an inner (circular) and an outer (longitudinal) fascial layer. Each corpus cavernosum is wrapped by its own circular layer, while a single longitudinal layer encloses them both. The circular layers form an incomplete fibrous septum between the adjoining sides of the corpora cavernosa. The ventral groove between the corpora cavernosa is occupied by the corpus spongiosum. The dorsal groove is traversed by the neurovascular bundle of penis which consists of the dorsal arteries of penis, dorsal veins of penis and the dorsal nerves of penis.
The corpora cavernosa contain a network of interconnected irregularly shaped sinusoid vascular spaces lined with endothelial cells. These sinusoid spaces are separated by the septae that pass from the tunica albuginea. The septae consist of the fibrous connective tissue rich with numerous smooth muscle cells.
Once filled with arterial blood from the helicine arteries, the cavernous sinuses compress the venous drainage from the penis, which manifests as the penile erection. The fibrous septum between the corpora cavernosa is incomplete proximally, allowing the blood exchange between the cavernous sinuses of the corpora.
The corpus spongiosum is a spongy mass of the erectile tissue found within the ventral compartment of the penis. It has a proximal dilation that projects into the root of penis, i.e. the bulb of penis. The corpus spongiosum features a distal bulbous expansion at the end of the penile shaft as well, which forms the glans penis. The glans shows a rounded base called the corona which demarcates it from the penile shaft. The glans is covered by the foreskin (prepuce), which is a double layer of loose retractable skin that attaches to the ventral surface of glans. The attaching site of the deep layer of the foreskin to the urethral surface lies under the corona and is called the frenulum.
The corpus spongiosum is enveloped by a thin layer of tunica albuginea and contains less erectile tissue than the corpora cavernosa. It is traversed by the penile urethra, which opens at the tip of the glans. The corona of glans contains many small preputial glands that secrete the sebaceous product called the smegma.
The penis is supplied by the internal pudendal artery, a branch of the internal iliac artery. This artery enters the penis via Alcock’s canal after which it divides into the perineal and common penile arteries. The former supplies the ischiocavernosus and bulbospongiosus muscles, while the common penile artery divides into three branches to supply the deep structures of the penis. These branches are the:
- Artery of bulb of penis. It enters the bulb of penis and travels as far as the glans. It supplies the bulb and the penile urethra.
- Dorsal artery of penis. It traverses the dorsal groove between the corpora cavernosa, running deep to deep penile (Buck’s) fascia. It gives off many circumferential branches along its course with which it supplies the corpora cavernosa.
- Deep (cavernosus) artery of the penis . It’s a paired vessel that travels through the center of each corpus spongiosum. It gives off straight and helicine arteries along its course, which open directly into the sinusoids of the corpora cavernosa.
The venous drainage of the penis happens via one of the three systems;
- Superficial venous system. It consists of the several superficial veins of penis, which traverse the dartos fascia of penis. They drain the skin of the penile shaft and prepuce and merge into a single superficial dorsal vein of penis at the base of the penile shaft. This vein empties into the great saphenous vein.
- Intermediate venous system. It gathers the circumflex and deep dorsal veins of penis which course beneath Buck’s fascia. They drain the glans, corpus spongiosum and the distal two-thirds of the penis into the prostatic venous plexus.
- Deep venous system. It consists of the crural and cavernous (deep/cavernosal) veins, which arise between the crura of penis and the corpora cavernosa, respectively. They drain the proximal one-third of the penis into the internal pudendal vein.
The sensory innervation to the penis comes from a terminal branch of the pudendal nerve; the dorsal nerve of penis. This nerve traverses the dorsal groove between the corpora cavernosa together with the same-named artery and vein. Along its course, it gives off many sensory branches that supply the skin of the penile shaft, as well as the prepuce of glans. The glans receives most of the sensory nerve endings, which is why it is the most sensitive area of the penis. The root of the penis receives the sensory innervation from the branches of the ilioinguinal nerve.
- The parasympathetic input comes from S1-S4 segments of the spinal cord. These fibers travel within the pelvic splanchnic nerves (nervi erigentes), which synapse within the ganglia of the pelvic plexus. The postganglionic parasympathetic fibers leave the pelvic plexus via the cavernous nerve, which traverses the corpora cavernosa together with the corresponding artery and vein. Prior to entering the corpora cavernosa, the cavernous nerve gives off a branch that supplies the corpus spongiosum as well.
- The sympathetic innervation for the penis comes from the T11-L1 segments of the spinal cord. These nerves synapse with the sympathetic trunk, which gives off the postganglionic sympathetic fibers that pass through the pelvic plexus and join the cavernous nerve.
Erection and ejaculation
Erection and ejaculaton are regulated by the autonomic innervation of the penis. The parasympathetic stimulation is excitatory for the penis, as it causes the erection. The parasympathetic stimuli relax the smooth musculature of the corpora cavernosa and produce vasodilation in the helicine arteries. The helicine arteries then fill the corpora cavernosa, compressing the venous outflow from the penis. This process is called the veno-occlusive mechanism and it results with penile erection.
On the other hand, the inhibitory sympathetic stimuli cause the ejaculation when the critical level of sexual excitement has been reached. The ejaculation process can be divided into two phases. In the first phase, the sympathetic stimuli cause the vasoconstriction of helicine arteries, contraction of the smooth muscle cells within the septa of corpus spongiosum and contraction seminal vesicles and prostate. This results in accumulation of seminal fluid into the proximal part of the urethra. During the second phase, the bulbospongiosus muscle contracts and expels the seminal fluid through the urethra (ejaculation). When the ejaculation is finished the penis returns into the flaccid state. An individual normally produces 3-5 ml of semen per one ejaculation which contains about 300 million sperm cells.
Learn more about the anatomy and function of the male reproductive system with our articles, video tutorials, quizzes and labeled diagrams.
Phimosis is a condition in which a person has difficulties to retract the prepuce (foreskin). It is recognized as a ring of foreskin around the tip of the glans. Phimosis is classified as physiological and pathological.
- Early after birth, the prepuce is physiologically non-retractable. The period of this physiological phimosis varies among individuals, with most of the cases resolving until the age 5-7.
- Pathological phimosis is caused by scarring after an infection or inflammation. Here, the scarring process results in forming a sclerotic constricting band around the tip of the glans. Pathological phimosis cannot resolve by itself, and any attempted retraction of the prepuce causes pain and bleeding. Often, it is further complicated by painful urination, painful erection, recurrent infections of the prepuce (balanoposthitis) and paraphimosis.
The treatment of pathological phimosis depends on its severity and conjoined complications. Usually there are three therapeutic approaches; smooth and gentle manual daily retraction, application of topical corticosteroids, or surgical circumcision.
Paraphimosis is a condition in which the retracted prepuce cannot be put back in its normal position. This often causes the swelling of the foreskin that can lead to the blockage of the blood flow in the glans. If not urgently treated this condition can lead to serious complications.Once surgically resolved, it does not require any further treatment.
Recurring paraphimosis is rare and it is an indication for circumcision.