29.2 The Penis

The penis has three elements: the two corpora cavernosa and the corpus spongiosum, which expands distally to form the glans penis (Figure 29.1). Each corpus consists of a sponge of intercommunicating venous sinuses. Each corpus cavernosum is attached to the medial aspect of the ischiopubic ramus (Figure 29.2). They join under the symphysis pubis to form the hilum of the penis, which is attached by the suspensory ligament.

Each corpus is enclosed in the specialised tunica albuginea made of strong fibroelastic tissue and is predominantly collagenous (Figure 29.3). Between the corpora, the tunicae fuse to form the pectinate septum that is perforated by endothelium lined cavernous sinuses that form the spongy‐like appearance of the tissue on macroscopic examination. These sinuses allow intracorporeal communication between the two erectile bodies; hence, one corpus instantly fills the other. The corpus spongiosum runs ventral to the corpus cavernosa and is traversed by the anterior urethra throughout its entire length. The urethra opens at the external urethral meatus at the tip of the glans penis, which is an expansion of the corpus spongiosum capping all three corpora.

Buck fascia surrounds both corpus cavernosa and splits at the ventral aspect to enclose to encircle the corpus spongiosum. Proximally, Buck fascia fuses with tunica albuginea and distally fuses with the base of the glans at the corona [2]. As a result of this fascial fusion, bleeding within Buck fascia is limited to the penile shaft. In Buck fascia run the deep dorsal vein and the sensory nerves supplying the glans penis. Between Buck fascia and the skin is a loose layer of vascular connective tissue. The skin continues over the glans as the prepuce [3].

29.2.1 Blood Supply and Lymphatic Drainage

29.2.1.1 Arterial Supply

The arteries of the penis are terminal branches of the internal pudendal artery – last branch of the internal iliac artery. Each internal pudendal artery splits into three (Figure 29.4):

1. A dorsal artery enters between the crus of the penis and the pubis and runs in Buck fascia on either side of the deep dorsal vein of the penis. It gives off circumferential branches, supplying the corpus spongiosum and urethra.
   
2. A second larger cavernous artery runs in the middle of each corpus cavernosum and is the main artery responsible for erection.
   
3. A pair of bulbourethral arteries run in the corpus spongiosum on either side of the urethra [4, 5], supplying the urethra and the glans.

The corpus spongiosum and its contained urethra receive dual blood supply from bulbourethral and circumferential branches of the dorsal arteries.

The cavernous arteries supply blood for tumescence of the corpora cavernosa in the first and second stages of erection: the dorsal and bulbar arteries are responsible for distension of the glans penis in the third stage.

The arteries of the corpora cavernosa and spongiosum give off short branches, which open directly into the venous sinuses. In the flaccid state, these arteries are constricted; in the first and second phases of erection they dilate (Figure 29.5).

29.2.1.2 Venous Drainage

The venous sinuses of the spongy tissue drain into efferent veins. Those of the glans enter a coronal plexus, which flows into the deep dorsal vein. Emissary veins that run obliquely through the tunica albuginea drain the middle part of the penis into the deep dorsal vein directly or via circumflex veins (Figure 29.6).

The veins of the bulb and proximal corpora cavernosa join up to form large cavernosal veins which flow into the deep dorsal vein which itself runs under the symphysis into the pre‐prostatic plexus of Santorini.

A superficial venous system drains the subcutaneous tissues of the penis via the superficial dorsal vein of the penis into the saphenous vein. There are many communications between these two venous systems [4, 5].

29.2.1.3 Lymphatics

Most of the lymphatics of the penis join dorsally and then drain into the medial group of superficial inguinal lymph nodes, superficial to the deep fascia of the thigh (i.e. fascia lata) in both sides; some flow through the ‘sentinel’ node lying over the fossa ovalis, and others accompany the deep dorsal vein under the pubis into the lymphatics of the pelvis [6].

29.2.2 Nerve Supply

29.2.2.1 Autonomic

A rich autonomic plexus forms a sleeve around the cavernous and bulbar arteries. This is linked to primitive centres in the brain – the hippocampus, cingulate gyrus, and thalamus [7, 8]. There are two spinal centres concerned with erection, one at T₁₂–L₃, the other at S₂–S₄ [9].

The main neurovascular bundle lies posterolateral to the prostate gland and can sometimes be preserved during prostatectomy or cystectomy by displacing the bundle laterally in its layer of pelvic fascia [10].

29.2.2.2 Somatic Afferent and Efferent Nerves

The pudendal nerve (i.e. S₂, S₃) terminates with two branches, the dorsal nerve of the penis and the perineal nerve. Sensory branches run with the two dorsal nerves of the penis in Buck fascia to the skin and glans penis and branches to the corpus cavernosum. Care must be taken to safeguard these nerves when operating for Peyronie disease. The perineal nerve supplies the perineal muscles, urethral sphincter, ischiocavernosus and bulbospongiosus, penile urethra sensation, and posterior scrotal branches. Small branches of the perineal nerve supply the ventral surface of the penis nearing the urethra and glans.

Efferent fibres supply the bulbospongiosus and ischiocavernosus muscles to raise the blood pressure within the corpora above the systolic pressure during the third phase of erection.

29.3 The Scrotum

The skin of the scrotum is hairy and rich in sebaceous glands. There is a distinct plane of cleavage between the scrotal skin and the dartos that has a rich vascular supply and is innervated by sympathetic fibres of S₄. The midline raphe defines the line of fusion of the genital tubercle, and it runs from the external urinary meatus to the anus. The scrotum has two compartments separated by a septum and contains a testicle in each compartment.

The dartos is part of the panniculus carnosus of the body and continues as a distinct layer over the penis. It has a profuse blood supply and is sensitive to temperature; when cold it contracts, converting the scrotum into a compact lump. When warm, the scrotum becomes a loose dependent bag.

29.3.1 Fascia

The fascia of Colles is attached behind to the perineal membrane, each side to the pubis and continues upwards as the fascia of Scarpa onto the abdominal wall. Blood and extravasated urine will collect in this space and sharply define its limits (Figure 29.7).

29.3.2 Blood Supply and Lymphatic Drainage

29.3.2.1 Arterial Supply

The anterior scrotal wall derives its arterial supply from the external pudendal artery branch of the femoral artery. Posterior branches of the perineal arteries supply the posterior aspect of the scrotum. Arteries normally course parallel to the scrotal rugae and do not cross the median scrotal raphe.

29.3.2.2 Venous Drainage

The venous drainage of the scrotum is via the saphenous to the femoral vein. The scrotal veins anastomose freely with those of the penis.

29.3.2.3 Lymphatic Drainage

The scrotal lymphatics drain into the ipsilateral superficial group of the inguinal lymph nodes, and from there, along the course of the external iliac artery.

29.3.2.4 Nerves

The anterior third of the scrotum is innervated by the ilio‐inguinal nerve (L₁): the posterior two‐thirds by the scrotal branches of the perineal (S₃) and posterior cutaneous nerve of the thigh (S₂) (Figure 29.8) [11].

29.4 The Urethra

29.4.1 Male Urethra

The male urethra is an elastic tube capable of doubling in length during erection. It is lined with transitional epithelium as far as the bulb where it is squamous for the next 5 cm and cuboidal for the remainder of its length. Paraurethral glands enter the urethra along its length, being most numerous in the bulb and near the external meatus (Figure 29.9). Cowper’s glands lie within the levator ani muscle and send their ducts down beside the bulbar urethra to open into it. Paired glands of Littré open on either side of the external urinary meatus.

A sleeve of spongy tissue – the corpus spongiosum – with a structure similar to that of the corpora cavernosa, surrounds the urethra and is continuous with the glans penis. Like the corpora cavernosa, the nerves of the corpus spongiosum and urethra come from the neurovascular bundles just posterolateral to the prostate.

In small boys, the urethra is extremely narrow and does not enlarge until puberty. The calibre of the adult male urethra is very variable, but at its most narrow is about 8 mm in diameter (24 Charrière).

29.4.2 Female Urethra

The female urethra measures about 4 cm in length and is lined with transitional epithelium above and squamous epithelium below; the junction between the two types of epithelia is variable, and it is normal for the squamous epithelium to extend up onto the trigone [12] (Figure 29.10).

The urethra is surrounded by erectile spongy tissue that is anatomically continuous with the glans of the clitoris [11] (Figure 29.11).

Surrounding the spongy tissue is a sleeve of smooth and striated muscle fibres, entirely distinct from the levator ani sheet. These muscles contain fast‐ and slow‐twitch fibres, similar to those of the intramural sphincter of the male membranous urethra.

The lumen of the female urethra forms a crescent, with a marked crest on the posterior wall (Figure 29.12). Numerous paraurethral glands of unknown function open into the urethra.

29.5 Erection

Erection is a complex neurovascular process involving relaxation of endothelial smooth muscle in the corpus cavernosa resulting in increased intracavernosal arterial blood volume and pressure and restricted venous drainage. This process is mediated through neurotransmitters.

When the penis is flaccid its arterioles are constricted, and there is a very low blood flow. Parasympathetic activity from conscious erotic stimulation or local contact, releases neurotransmitter substances, which relax the branches of the deep artery of the corpora cavernosa.

Acetylcholine (Ach) and nitric oxide (NO) play a critical role in the physiology of penile erection. L‐arginine, an amino acid, is converted to NO. NO synthase (NOS) cleaves nitrogen for the amino acid and combines it with oxygen to for NO. Ach stimulates endothelial cells and neuronal endings to produce NO. Three types of NOS exist depending on the tissue producing them, neuronal NOS, endothelial NOS, and cytokine‐inducible NOS. Increase in NO causes activates guanylate cyclase, which causes an influx of intracellular calcium into the endoplasmic reticulum ergo reducing intracellular calcium, leading to smooth muscle relaxation. Phosphodiesterase (PDE) catalyse the conversion of cGMP to its inactive form GMP leading to the termination of this process. Other mediators may release a factor from endothelium causing the smooth muscle to relax, or may act with vasoactive intestinal polypeptide (VIP) [3]. There are alpha and beta‐adrenergic receptors in the muscle of the penile blood vessels. Injections of alpha‐blocking agents produce erection.

There are five phases in erection:

Phase 1 (Tumescence): As the first stimuli reach the penis, there is an increased blood flow. The spaces of the corpora fill with blood. At first there is no increase in pressure whereby the penis remains soft (Phase 1A) and then there will be a progressive increase in pressure causing some increase in length and girth (Phase 1B).

Phase 2 (Erection): When the vascular sinusoids are filled, there is an increase in pressure inside the corpora, which makes the penis stiff. At the end of this phase, the blood flow into the penis slows down.

Phase 3 (Full erection): Intracavernosal pressure rises, causing occlusion of the venous flow through the emissary veins caused by impediment against the noncompliant tunica albuginea, but the pressure inside the corpora is still only l0 mm Hg below the systolic pressure. The inflow of blood has almost stopped.

Phase 4 (Rigid erection): The intracavernous pressure rises to several times that of the systolic blood pressure. The change from ‘full’ to ‘rigid’ erection requires closure of the emissary veins, which are still under the effect of the ‘veno‐occlusive mechanism’ (Figure 29.13). In addition, the bulbospongiosus muscles contract and blood ceases to flow in or out of the penis.

Phase 5 (Detumescence): Here, the venous outflow starts again, the arterial inflow remaining very small; the penis shrinks and gradually returns to its flaccid state [3].