The penile shaft is composed of 3 erectile columns, the 2 corpora cavernosa and the corpus spongiosum, as well as the columns' enveloping fascial layers, nerves, lymphatics, and blood vessels, all covered by skin (see the following images). The 2 suspensory ligaments, composed of primarily elastic fibers, support the penis at its base. 
The paired corpora cavernosa contain erectile tissue and are each surrounded by the tunica albuginea, a dense fibrous sheath of connective tissue with relatively few elastic fibers. The corpora cavernosa communicate freely through an incomplete midline septum. Proximally, at the base of the penis, the septum is more complete; ultimately, the corpora diverge, forming the crura, which attach to the ischiopubic rami.
The tunica albuginea consists of 2 layers, the outer longitudinal and the inner circular (see the image below). The tunica albuginea becomes thicker ventrally where it forms a groove to accommodate the corpus spongiosum. The tunica albuginea of the corpus spongiosum is considerably thinner (< 0.5 mm) than that of the corpora cavernosa (approximately 2 mm). Along the inner aspect of the tunica albuginea, flattened columns or sinusoidal trabeculae composed of fibrous tissue and smooth muscle surround the endothelial-lined sinusoids (cavernous spaces). In addition, a row of structural trabeculae arises near the junction of the 3 corporal bodies and inserts in the walls of the corpora about the midplane of the circumference. 
The erectile tissue within the corpora contains arteries, nerves, muscle fibers, and venous sinuses lined with flat endothelial cells, and it fills the space of the corpora cavernosa. The cut surface of the corpora cavernosa looks like a sponge. There is a thin layer of areolar tissue that separates this tissue from the tunica albuginea.
Blood flow to the corpora cavernosa is via the paired deep arteries of the penis (cavernosal arteries), which run near the center of each corpora cavernosa (see the following image).
The single corpus spongiosum lies in the ventral groove between the 2 corpora cavernosa. The urethra passes through the corpus spongiosum. The corpus spongiosum possesses a much thinner and more elastic tunica albuginea to allow for distention of the corpus spongiosum for passage of the ejaculate through the urethra. The thinner tunica albuginea of the corpus spongiosum also allows the corpus to become less rigid during erection. Hence, the distal extension of the spongiosum, the glans penis, covers the tips of the corpora cavernosa to provide a cushioning effect. The urethral meatus is positioned just slightly on the ventral surface of the glans and is slitlike. The edge of the glans overhangs the shaft of the penis, forming a rim called the corona.
The 3 erectile bodies are surrounded by deep penile (Buck) fascia, the dartos fascia, and the penile skin. The deep penile (Buck) fascia is a strong, deep, fascial layer that 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.
On the dorsal aspect of the corpora cavernosa, the deep dorsal vein and paired dorsal arteries and branches of the dorsal nerves are contained within the deep penile (Buck) fascia. This fascia splits to surround the corpus spongiosum, and it extends into the perineum as the deep fascia of the ischiocavernosus and bulbospongiosus muscles. The deep penile (Buck) fascia encloses these muscles and each crus of the corpora cavernosa and the bulb of the corpus spongiosum, adhering these structures to the pubis, ischium, and the urogenital diaphragm.
The penile skin is continuous with that of the lower abdominal wall. Distally, the penile skin is confluent with the smooth, hairless skin covering the glans. At the corona, it is folded on itself to form the prepuce (foreskin), which overlies the glans. The subcutaneous connective tissue of the penis and scrotum has abundant smooth muscle and is called the dartos fascia, which continues into the perineum and fuses with the superficial perineal (Colle) fascia. In the penis, the dartos fascia is loosely attached to the skin and deep penile (Buck) fascia and contains the superficial arteries, veins, and nerves of the penis.
Blood supply to the skin of the penis is from the left and right superficial external pudendal arteries, which arise from the femoral artery (see the image below). The superficial external pudendal arteries branch into dorsolateral and ventrolateral branches, which collateralize across the midline. In addition, branches in the skin form an extensive subdermal vascular plexus. The blood supply to the ventral penile skin is based on the posterior scrotal artery, a superficial branch of the deep internal pudendal artery.
The blood supply to deep structures of the penis is derived from a continuation of the internal pudendal artery, after it gives off the perineal branch. Three branches of the internal pudendal artery flow to the penis, as follows:
The artery of the bulb (bulbourethral artery) passes through the deep penile (Buck) fascia to enter and supply the bulb of the penis and penile (spongy) urethra
The dorsal artery travels along the dorsum of the penis between the dorsal nerve and deep dorsal vein and gives off circumflex branches that accompany the circumflex veins; the terminal branches are in the glans penis
The deep penile (cavernosal) artery is usually a single artery that arises on each side and enters the corpus cavernosum at the crus and runs the length of the penile shaft, giving off the helicine arteries, which are an integral component of the erectile process
The penis is drained by 3 venous systems, the superficial, intermediate, and deep (see image below).
Superficial veins are contained in the dartos fascia on the dorsolateral surface of the penis and coalesce at the base to form a single superficial dorsal vein, which usually drains into the great saphenous veins via the superficial external pudendal veins.
The intermediate system contains the deep dorsal and the circumflex veins, lying within and beneath the deep penile (Buck) fascia. Emissary veins begin within the erectile tissue of the corpora cavernosa and course through the tunica albuginea and drain into the circumflex or deep dorsal veins. The circumflex veins arise from the spongiosum, ventrum of the penis, and often, the emissary veins drain into them.
The circumflex veins course laterally around the cavernosa, passing beneath the dorsal arteries and nerves and 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, forming the retrocoronal plexus. It receives blood from the emissary and circumflex veins and passes underneath the symphysis pubis at the level of the suspensory ligament, leaving the shaft of the penis at the crus and draining into the prostatic plexus.
Deep venous drainage is via the crural and cavernosal veins. The crural veins arise in the midline, in the space between the crura. The cavernosal veins are consolidations of the emissary veins, which join to form a large venous channel that drains into the internal pudendal vein. Three or 4 small cavernosal veins course laterally between the corpus spongiosum and the crus of the penis for 2-3 cm before draining into the internal pudendal veins.
Lymphatics and Nerve Supply
Lymphatic drainage 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, coursing proximally beneath the deep penile (Buck) fascia, terminating mostly in the deep inguinal nodes of the femoral triangle. Some lymphatic drainage is to the presymphyseal lymph nodes and to the lateral lymph nodes of the external iliac lymphatics.
The nerves to the penis are derived from the pudendal and cavernous nerves. The pudendal nerves supply somatic motor and sensory innervation to the penis. The cavernous nerves are a combination of parasympathetic and 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.
The tunica is composed of elastic fibers that form an irregular, latticed network on which the collagen fibers rest. The tunica albuginea is composed of an inner circular layer and an outer longitudinal layer. Emissary veins travel between the inner and outer layers of the tunica and often exit the outer layer in an oblique manner. The outer layer of the tunica compresses the emissary veins when the penis becomes engorged with blood.
The corpora cavernosa are 2 spongy cylinders. Within the tunica albuginea are the interconnected sinusoids separated by smooth muscle trabeculae and surrounded by elastic fibers, collagen, and loose areolar tissue. The terminal cavernous nerves and helicine arteries are intimately associated with smooth muscle. The sinusoids are larger in the center and smaller in the periphery.
The structure of the corpus spongiosum is similar to that of the corpora cavernosa, except that the sinusoids are larger and a much thinner outer layer of the tunica albuginea is present. The glans has no tunical covering.
Erectile tissue vessels
The helicine arteries, branches of the deep penile artery, supply the trabecular tissue and sinusoids. They are contracted and tortuous in the flaccid state and dilated and straight in the erect state. The venous drainage from the erectile tissue originates in the venules starting at the peripheral sinusoids beneath the tunica albuginea. They travel in the trabeculae between the tunica and the peripheral sinusoids, forming the subtunical venular plexus before exiting as the emissary veins.
Neurotransmitters and Receptors
Adrenergic nerve fibers and receptors are present in the cavernous trabeculae, and they surround the deep penile arteries. Noradrenaline is the major neurotransmitter controlling penile flaccidity and tumescence. [3, 4] Sympathetic contraction is thought to be mediated by activation of postsynaptic alpha-adrenergic receptors and modulated by presynaptic alpha-adrenergic receptors. Acetylcholine is required for vascular smooth muscle relaxation, and cholinergic nerves have been demonstrated within the cavernosal smooth muscle and surrounding penile arties.
Nitric oxide (NO) appears to be the principal neurotransmitter causing penile erection. Nonadrenergic, noncholinergic (NANC) neurons release NO. The release of NO increases the production of cyclic guanosine monophosphate (cGMP), which relaxes cavernosal smooth muscle. [5, 6, 7] Other neurotransmitters, including vasoactive intestinal peptide (VIP), calcitonin gene-related peptide (CGRP), prostaglandins, and other peptides, may also be involved in the erectile process.  With relaxation of the smooth muscles in the trabeculae and the arterial wall, the following events occur, which lead to an erection:
- Arterial inflow increases as a result of dilatation of the arterioles and arteries
- The sinusoids within the corpora cavernosa distend with blood
- Subtunical venular plexuses are compressed between the tunica albuginea and the distended sinusoids, leading to decreased venous outflow
- The tunica albuginea is stretched to its capacity, compressing emissary veins and thus further decreasing venous outflow; as a result, intracavernous pressure increases and is further increased by contraction of the ischiocavernous and bulbospongiosus muscles, resulting in full rigidity 
Congenital absence of the penis, or aphallia, is a rare anomaly caused by developmental failure of the genital tubercle. The approximate incidence of this condition is 1 case per 30 million population. The phallus is completely absent, including the corpora cavernosa and corpus spongiosum; however, some children have been reported to have small portions of corpora cavernosa. The urethra opens at any point of the perineal midline from over the pubis to, most frequently, the anus or anterior wall of the rectum.
See also Genital Anomalies.
Duplication of the penis, or diphallia, is another rare anomaly resulting from incomplete fusion of the genital tubercle. Two distinct forms of penile duplication are recognized. The most common form is associated with bladder-exstrophy complex. The patient exhibits a bifid penis, which consists of 2 separated corpora cavernosa that are associated with 2 separate hemiglans.
The second form, or true diphallia, is an extremely rare congenital condition. It appears in many ways, ranging from duplication of the glans alone to duplication of the entire lower genitourinary tract. The urethral opening can be in normal position or in a hypospadiac or epispadiac position.
The term microphallus, or micropenis, is applicable only to a normally formed yet abnormally short penis. Specifically, the term applies to a penis with a stretched length more than 2.5 standard deviations (SD) less than the mean for age.
Penile torsion is a rotational abnormality of the penis. The embryologic abnormality is often an isolated skin and dartos, but it may also be related to abnormalities in the orientation of the cavernosal bodies.
Webbed penis is a common congenital abnormality in which a web or fold of scrotal skin obscures the penoscrotal angle (see the image below).
In hidden (buried, concealed) penis, the penile shaft is buried below the surface of the prepubic skin. This happens in obese children and adults, because the prepubic fat is very abundant and hides the penis. The condition may also derive from poor anchorage of penile skin to deep fascia or be acquired when the shaft of the penis is entrapped in scarred prepubic skin following an extreme circumcision or other trauma.
Absence of the corpora cavernosa and corpora cavernosa plus corpus spongiosum
Congenital absence of the corpora cavernosa and all 3 corporal bodies result in dilatation of the posterior urethra, megalourethra. Scaphoid megalourethra is related to absence of the corpus spongiosum and is more common than fusiform megalourethra, which is the result of absence of all 3 corporal bodies.
Curvature of the penis
Curvature of the penis may be congenital or acquired. Congenital curvature may be classified as chordee without hypospadias or true congenital curvature of the penis. Chordee without hypospadias is a term implying that although the meatal location is normal, curvature is present due to inappropriate fetal development of the ventral penile structures. With congenital curvature of the penis, although the urethra, corpus spongiosum, and fascial layers are normally developed, one aspect of the tunica albuginea of the corpora cavernosa has a relative shortness or inelasticity.
Typically a ventral curvature has been present throughout life. Abnormalities of the ventral penile skin may also exist. In the most of these patients, the penis is curved because of inelasticity of the ventral aspect of the corpora cavernosa. In some patients, the corpus spongiosum may become atretic distal on the shaft, with no coverage around the distal urethra.
Individuals with congenital curvature of the penis can have ventral, lateral, or less commonly, dorsal curvature. The curvature tends to involve the entire pendulous portion of the penile shaft.
Peyronie disease is an inflammatory condition that is characterized by the formation of fibrous, noncompliant nodules within the tunica albuginea. [10, 11, 12, 13, 14, 15] One of the most likely causes of Peyronie disease may be repeated tunical mechanical stress and microvascular trauma as well as abnormal wound healing.  The tunica albuginea is a multilayered structure consisting of inner circular and outer longitudinal layers of connective tissue encompassing the corpora cavernosa (see the following image). [16, 17]
The tunica albuginea is composed of fibrillar (mainly type I but also types III and V) collagen in organized arrays interlaced with elastic fibers. [17, 18] Although collagen has a great tensile strength, it is unyielding. Indeed, it is the elastin content that provides the compliance of the tunica albuginea. The fibrotic plaques (composed of collagen but not elastin) that form in Peyronie disease are produced most likely by tunical fibroblasts in response to cytokine stimulation. 
Erectile dysfunction may be from a psychogenic or organic component. The erectile process is a neurovascular event, requiring functioning cavernous nerves, arteries, and veins. Injury to the cavernous nerves — such as that which occurs during radical prostatectomy and certain colorectal surgeries (abdominal perineal resection [APR] and low anterior resection) — may result in erectile dysfunction. Cardiovascular disease may contribute to arterial insufficiency. Lastly, diseases such as Peyronie disease, which affect the tunica albuginea, may lead to inadequate compression of the emissary veins and a resultant venous leak. The penile curvature associated with Peyronie disease may also make sexual penetration difficult.