PENIS

The penis is composed of 3 spongy cylinders. The three cylinders consist of paired corpora cavernosa and a single corpus spongiosum. The crural (roots) of the corpora cavernosa attach at the under surface of the ischiopubic rami as two separate structures. Such anatomy prevents the erect penis from sinking into the perineum when faced with an axially-oriented vaginal compressive load during intercourse. This unique anatomic arrangement, however, unfortunately places the penile crus at great danger from crush injuries during blunt perineal trauma.

The tunica albuginea consists of layers of collagen which can accommodate a considerable degree of intracavernosal pressure prior to rupture. To function effectively, these fascial layers must provide the penis with a wall container capable of withstanding a high degree of rigidity and axial strength when erect, yet be supple when flaccid. The tunica must be able to elongate symmetrically and increase in girth with tumescence, assuring a straight erection. The tensile strength of the tunica is approximately 1200 - 1500 mmHg making this fascia one of the most strong in the body. Approximately 5% of the tunica is elastin which enables the penis to develop elongation. The average volume increase of the erect penis from the flaccid volume is 3-fold with a range from 1.7 - 5 fold. The mechanical properties of the tunica which allow for maximum volume changes of the erect penis are called tunica dispensability. Regions of the tunica with focal poor dispensability cause the erect penis to bend. This focal tunical abnormality in dispensability is called Peyronie’s disease.
 

The substance of the corpora cavernosa (erectile tissue) consists of numerous sinusoids (lacunar spaces) among interwoven trabeculae of smooth muscles and supporting connective tissue. The corpora cavernosa sinusoids are widely communicative and larger in the center of the corpora, having a Swiss-cheese appearance. This fact enables the blood within the penis to transfer easily from the top to the bottom of the corpora. This also enable the penis to have a common intracavernosal pressure and a common penile rigidity. The sinusoids are smaller in the periphery and have a grape-like appearance. Peripheral sinusoids have a greater individual surface area than central sinusoids. These characteristics aid in the passive process of corporal veno-occlusion by sub-tunical venule compression against the tunica albuginea. All lacunar spaces are lined with endothelial cells, thought previously to have only a slippery surface preventing blood clotting. Recent research has revealed that endothelial cells have secretory function and synthesize factors involved in the regulation of corporal smooth muscle tone.
 

The paired internal pudendal artery, a branch of the hypogastric artery is the main source of arterial blood supply to the penis.
 

The internal pudendal artery terminates when the artery divides into the scrotal and common penile artery.
 

The common penile artery defines the condition whereby all red blood cells in the artery somehow end up in the penis. The common penile artery branches into 3 arteries, the bulbourethral, the dorsal and the cavernosal arteries. The common penile artery has direct apposition to the ischiopubic ramus. This artery is therefore commonly injured during blunt perineal traumatic events such as falling onto the top tube of a bicycle.
 

The penis is innervated by autonomic (parasympathetic and sympathetic) and somatic (sensory and motor) nerves.
 

The cavernosal nerves are branches of the pelvic plexus that innervate the corpora cavernosa of the penis. Injury to this branch may occur during radical prostatectomy, during urethral surgery, such as internal urethrotomy and from electrocautery injury during transurethral surgery.

The common output tract for urine and sperm. A strange structure that is under the control of a complex series of reflexes, neuronal and humoral control. Contains several aggregations of "cavernous" tissue that under certain conditions can become engorged with blood, causing the penis t become rigid. In this state the penis is capable of delivering the genetic material contained in the sperm during
coitus.
 

The penile erectile apparatus consists of paired vascular spongy organs (corpora cavernosa) that are closely attached to each other except in the proximal third. The corpus spongiosum with the urethra is related to the ventral aspect of the penile shaft and expands distally to from the glans penis. The pendulous part of the penis if 4-6 inches (?10.2-15.2 cm) long. The penile skin is continuous with that of the lower abdominal wall and continues over the glans penis to form the prepuce; it then folds itself to reattach at the coronal sulcus. The penile skin envelopes the shaft and can be moved freely over the erect organ. The underlying fascial layer or dartos fascia (Colles’ fascia) is continuous with Scarpa’s fascia of the lower abdominal wall; inferiorly, it continues as the dartos fascia of the scrotum and Colles’ fascia of the perineum and attaches to the posterior border of the perineal membrane. The superficial dorsal vein is seen in this layer of the fascia. Buck’s fascia is the deep layer of the penile fascia that covers both the corpora cavernosa and the corpus spongiosum in separate fascial compartments. Proximally, Buck;s fascia is attached to the perineal membrane; distally, it is tightly attached to the base of the glans penis at the coronal sulcus, where it fuses with the ends of the corpora. Th ischiocavernosus and the bulbospongiosus muscles lie beneath Colles’ fascia, but superficial to Buck’s fascia, to which their intrinsic fascia is loosely attached. Buck’s fascia has a dense structure and is composed of longitudinally running fibers; it is firmly attached to the underlying tunica albuginea and encloses the deep dorsal vein, dorsal arteries and dorsal nerves.
 

The fundiform ligament is a thickening of the superficial penile fascia, deep to which is the suspensory ligament which is a continuity with Buck’s fascia. The attachment of the ligament to the pubic symphysis maintains the penile position during erection. Severance of this ligament will lead to a lower angulation of the penile shaft during erection.
 

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The tunica albuginea forms a thick fibrous coat to the spongy tissue of the corpora cavernosa and corpus spongiosum. It consists of two layers, the outer longitudinal and the inner circular. The tunica albuginea becomes thicker centrally where it forms a groove to accommodate the corpus spongiosum. As the crura diverge proximally, the circular layer provides the support. The corpora are separated in the center by an intercavernous septum. The septum is incomplete distally, perforated on its dorsal margin by vertically orientated openings in the pectiniform septum that provides communication between the corpora. Along the inner aspect of the tunica albuginea, numerous flattened columns or sinusoidal trabeculae composed of fibrous tissue, elastin fibers and smooth muscle surround the endothelium-lined sinusoids or cavernous spaces. In addition, a row of structural trabeculae arises near the junction of the three corporal bodies and inserts in the wall of the corpora about the midplane of the circumference. The tunical albuginea provides a tough uniform backing for the engorged sinusoidal spaces. The tunical albuginea of the corpus spongiosum is thinner and contains smooth muscles that aid ejaculation. The glans is devoid of tunica albuginea. The corpus spongiosum becomes bulbous where it is covered by the bulbospongiosus to form the urethral bulb.
 

The ischiocavernosus is a paired muscle that arises from the inner surface of the inschial tuberosity and inserts into the medial and inferior surface of the corpora. These muscles increase penile turgor during erection beyond that attainable by arterial pressure alone. They are supplied by the perineal branch of the perineal nerve (S3-4).
 

The bulbospongiosus muscle invests the bulb of the urethra and distal corpus spongiosum. It arises from the central tendon of the perineum. The fibres run obliquely upwards and laterally on each side of the bulb and insert in the midline dorsally. The muscle is supplied by a deep branch of the perineal nerve and helps to empty the last few drops of urine and to ejaculate semen.

ARTERIAL SUPPLY

The arterial supply to the erectile apparatus originates from superficial and deep arterial systems. The superficial arterial system arises as two symmetrically arranged vessels arising from the inferior external pudendal artery (a branch of the femoral artery). Each of these vessels divides inito a dorsolateral and ventrolateral branch, which supply the skin o fhte shaft and prepuce. At the coronal sulcus there is a communication with the deep arterial system. The deep arterial system arises from the internal pudendal artery, which is the final branch of the anterior trunk of the internal iliac artery. This passes dorsal to the sacrospinous ligament at the level of the ischial spine and passes through Alcock’s canal. As it emerges, it divides into the perineal and penile arteries, running deep to the superficial transverse perineal muscle and pubic symphysis. It pierces the urogenital diaphragm meddial to the inferior ramus fo the ischium close to the bulb of the urethra and then divides into three branches—the bulbourethral artery, the urethral artery and the cavernous artery or deep artery of the penis; it terminates as the deep dorsal artery of the penis. An accessory internal pudendal artery may arise from the obturator, inferior vesical or superior vesical and may be damaged during radical prostatectomy in as many as 50% of patients. The bulbo-urethral artery supplies the bulb of the urethra, the corpus spongiosum and the glans penis. It may arise from the cavernous, dorsal or acessory pudendal arteries. The urethral artery commonly arises as a separate branch form the penile artery, but may arise from the artery to the bulb, the cavernous or the dorsal artery. It runs on the ventral surface of the corpus spongiosum beneath the tunica albuginea.
 

The cavernous artery (deep artery fo the penis) usually arises form the penile artery, but may originate from the accessory pudendal. It runs lateral to the cavernous vein along he dorsomedial surface of the crura to enter the erectile tissue where the two corpora fuse; it then continues in the center of the corpora cavernosa.
 

The dorsal artery of the penis is the termination of the penile artery; it runs over the resepctive crus and then along the dorsolateral surface of the penis as far as the glans between the dorsal vein medially and dorsal nerve of the penis laterally. This artery has tortuous configuration to accommodate for elongation during erection. It may arise from the accessory internal pudendal artery within the pelvis, and thus may be at risk during radical pelvic surgery. On its way to the glans, it gives off circumflex arteries to supply the corpus spongiosum. Distally, the dorsal artery runs in a ventrolateral position near the sulcus prior to entering the glans. The frenular branch of the dorsal artery curves around each side of the distal shaft to enter the frenulum and glans ventrally.

INTRACORPORAL CIRCULATION
 

Arterial blood is conveyed to the erectile tissues in the deep arterial system by means of dorsal, cavernous and bulbo-urethral arteries. The cavernous artery (deep artery of the penis) gives off multiple helicine arteries among the cavernous spaces within the center of the erectile tissue. Most of these open directly into the sinusoids bounded by trabecular, but a few helicine arteries terminate in capillaries that supply the trabeculae. The petiniform septum distally provides communication between the two corpora. The emissory veins at the periphery collect the blood from the sinusoids through the subalbugineal venous plexuses and empty it into the circumflex veins which drain into the deep dorsal vein. With erection, the arteriolar and sinusoidal walls relax secondary to neurotransmitters and the cavernous spaces dilate, enlarging the corporal bodies and stretching the tunica albuginea. The venous tributaries between the sinusoids and the subabugineal venous plexus are compressed by the dilating sinusoids and the stretched tunica albuginea. The direction of blood flow could be summarized as follows: cavernous artery -> helicine arteries -> sinusoids -> post-cavernous venules -> subalbugineal venous plexuses -> emissary vein.

VENOUS DRAINAGE

The venous drainage system consists of three distinct groups of veins—superficial, intermediate and deep. The superficial drainage system consists of venous drainage from the penile skin and prepuce which drain into the superficial dorsal vein that runs under the superficial penile fascia (Colles’) and joins the saphenous vein via the external pudendal vein. The intermediate system consists of the deep dorsal vein and circumflex veins that drain the glans, corpus spongiosum and distal two-thirds of the corpora cavernosa. The veins leave the glans via a retrocoronal plexus to join the deep dorsal vein that runs in the groove between the corpora. Emissary veins from the corpora join the circumflex veins; the latter communicate with each other at the side by lateral veins and corresponding veins from the opposite side, and run under Buck’s fascia before emptying obliquely into the deep dorsal vein. The latter passes through a psace in the suspensory igament and between the puboprostatic ligament and drains into the internal iliac veins. The deep drainage system consists of the cavernous vein, bulbar vein and crural veins. Blood from the sinusoids from the proximal third of the penis, carried by emissary veins, drains directly into the cavernous veins at the periphery of the corpora cavernosa. The two cavernous veins join to form the main cavernous vein that lies under the cavernous artery and nerves. The cavernous vein runs between the bulb and the crus to drain into the internal pudendal vein; it forms the main venous drainage of the corpora cavernosa. The crural veins arise from the dorsolateral surface of each crus and unite to drain into the internal pudendal vein. The bulb is drained by the bulbar vein, which drains into the prostatic plexus.

LYMPHATIC DRAINAGE

The lymphatics from the penile skin and prepuce run proximally towards the presymphyseal plexus and then divide to right and left trunks to join the lymphatics from the scrotum and perineum. They run along superficial external pudendal vessels into the superficial inguinal nodes, especially the superomedial group. Some drainage occurs through the femoral canal into Cloquet’s node. The lymphatics from the glans and penile urethra drain into deep inguinal nodes, presymphyseal nodes and, occasionally, into external iliac nodes.

NERVES

Somatic innervation arises from sacral spinal segments S2-4 via the pudendal nerve. The perineal branch of the pudendal nerve supplies the posterior part of the scrotum and the rectal nerve to the inferior rectal area. The pudendal nerve continues as the dorsal nerve of the penis, which runs over the surface of the obturator internus under the levator, runs deep to the urogenital diaphragm, and passes through the deep transverse perineal muscle to run along the dorsum of the penis accompanied by the dorsal vein and dorsal artery. In epispadia and exstrophy the dorsal nerves are displaced laterally in the middle and distal portion of the penile shaft. Cultaneous nerves to the penis and scrotum arise form the dorsal and posterior branch of the pudendal nerve. The anterior part of the scrotum and proximal penis is supplied by the ilioinguinal nerve after it leaves the superficial inguinal ring. The pudendal nerve supplies the ischiocavernous and bulbocavernous muscles. It branches into the inferior rectal nerve and the scrotal nerve and continues as the dorsal nerve of the penis.
 

Autonomic nerves consist of sympathetics that arise from lumbar segments L1 and L2 and parasympathetics from S2-4 (nervi erigentes or pelvic nerve). Lumbar splanchnic nerves join the superior hypogastric plexus over the aortic bifurcation, left common vein and sacral promontory. From this plexus, right and left hypogastric nerves travel medial to the internal iliac artery to the inferior hypogastric plexus. The pelvic plexus adjacent to the base of the bladder, prostate, seminal vesicles and rectum contain parasympathetic fibers as well. Nerves from the inferior pelvic plexus supply the prostate, seminal vesicles, epididymis, membranous and penile urethra and bulbo-urethral gland.

CAVERNOUS NERVE NEUROVASCULAR BUNDLE

The cavernous nerves arise from the pelvic plexus from the lateral surface of the rectum. These nerves run posterolateral to the apex, mid-portion and base of the prostate anterior to Denonvilliers’ fascia between the posterolateral surface of the prostate and the rectum to lie between the lateral pelvic fascia and the prostatic fascia. The branches from the cavernous nerve accompany the branches of the prostatovesicular artery and provide a macroscopic landmark for nerve-sparing radical prostatectomy. The cavernous nerve leaves the pelvis between the transverse perineal muscles and membranous urethra before passing beneath the pubic arch to supply each corpus cavernosum; it also supplies the corpus cavernosum and penile urethra, and terminates in a delicate network around the erectile tissue.