Male External Genitalia: Scrotum

• The male external genitalia consist of the scrotum and the penis.

Scrotum

• Is a pendulous sac of skin located below and behind the penis.
• Corresponds to the labia majora in females.
• Contains the testes, epididymis, and the lower part of the spermatic cord

External Features of the Scrotum

• Divided into right and left halves by a median ridge or raphe, which indicates the line of fusion of the two halves.
• The raphe continues on the undersurface of the penis in front and in the midline of the perineum to the anus behind.
• The skin is corrugated due to the presence of subcutaneous dartos muscle.
• Usually dark in color.
• The left half of the scrotum hangs lower than the right half because the left spermatic cord is longer.

Layers of Scrotum (from without inwards)

• Skin
• Dartos muscle, which replaces the superficial fascia
• External spermatic fascia
• Cremasteric muscle and fascia
• Internal spermatic fascia
• The dartos muscle helps regulate scrotal temperature by contraction or relaxation.

Blood Supply to Scrotum

• Superficial external pudendal artery
• Deep external pudendal artery
• Scrotal branches of the internal pudendal artery
• Cremasteric artery, a branch of the inferior epigastric artery

Nerve Supply to Scrotum

• Anterior one-third is supplied by the L1 segment through the ilioinguinal nerve and genital branch of the genitofemoral nerve.
• Posterior two-thirds is supplied by the S3 segment through the posterior scrotal branches of the perineal nerve and the perineal branch of the posterior cutaneous nerve of the thigh.
• The dartos muscle is supplied by sympathetic fibers from the genital branch of the genitofemoral nerve.
• Areas supplied by L1 and S3 spinal segments are separated by the ventral axial line.
• Lymphatics drain into the superficial inguinal group of lymph nodes

Development of Scrotum

• Develops from two labioscrotal swellings and urogenital folds.
• They fuse in the midline.

Clinical Correlation of the Scrotum

• Scrotal edema is a common site due to the laxity of the skin and dependent position.
• Sebaceous cysts often occur in the scrotum due to numerous sebaceous glands.
• Scrotal elephantiasis is a clinical condition marked by massive swelling and enlargement of the scrotum due to interstitial fluid accumulation, often caused by lymphatic vessel blockage due to filariasis.

Penis

• The male organ of copulation.
• Contains the penile urethra for the passage of urine and semen.

Parts of the Penis

• Root: The attached portion within the perineum.
• Body: The free pendulous portion outside the perineum.

Root of the Penis Details

• The root is attached to the inferior aspect of the urogenital diaphragm.
• It is comprised of three masses of erectile tissue: left and right crura, and the bulb of the penis.
• Each crus is attached to the pubic arch on the same side and continues as the corpora cavernosa.
• The bulb is attached to the inferior aspect of the perineal membrane stretching between the two crura.
• Anteriorly, the bulb narrows and continues as the corpus spongiosum. Its deep surface is pierced by the urethra to reach the corpus spongiosum.
• The urethra in the bulb shows a dilatation called the intrabulbar fossa.
• Each crus is covered by the ischiocavernosus muscle
• The bulb is covered by the bulbospongiosus muscle

Body of the Penis

• The free pendulous portion is suspended from the pubic symphysis and is continuous with the root.
• Divided into the body proper, the glans penis, and the neck.
• Body Proper: Cylindrical when flaccid, triangular/prism-like when erect.
• Faces ventral (backward and downward) and dorsal (forward and upward).
• Glans Penis: Enlarged conical structure at the distal end with the external urethral orifice.
• The projecting margin at the base of the glans is called the corona glandis.
• Neck of Penis: Obliquely grooved constriction at the base of the glans

Skin of the Penis

• Remarkably thin, delicate, and dark.
• Loosely attached to the fascial sheath of the organ, making it freely mobile.
• At the neck, the skin folds upon itself to form the prepuce (foreskin), covering the glans at a variable distance.
• The prepuce is retractable during coitus or manually.
• The internal layer of the prepuce is continuous with the thin skin covering the glans firmly.
• The skin covering the glans is continuous with the mucous membrane of the urethra at the external urethral orifice.
• On the urethral (ventral) surface, a small median fold passes from the inner prepuce aspect to the glans.
• Sensitivity of the frenulum of the prepuce plays an important role in orgasm.
• Prepuce of Skin: fold of skin that covers the glans for a variable extent and is attached to the neck of penis
• Frenulum of Prepuce: Median fold of skin on the ventral surface of the glans
• Preputial Sac: Potential space between the glans and prepuce.
• Numerous preputial sebaceous glands on the corona glandis and neck of the penis secrete smegma, which has a characteristic odor and collects in the preputial sac, which is medically significant and should be cleaned regularly to avoid inflammation or laceration.

Superficial Fascia of the Penis

• Consists of two layers: superficial and deep.
• Superficial Layer: devoid of fat and contains few muscle fibers (peripenic muscle)
• Deep Layer: condensed in lower anterior abdominal wall forming deep fascia (Buck's fascia) of penis
• Surrounds corpora cavernosa and corpus spongiosum, but stops beyond neck of penis.
• The superficial and deep dorsal veins of the penis are separated by Buck's fascia.

Ligaments of the Penis

• Support the weight of the free pendulous part (body).
• Fundiform Ligament: Springs from the lower part of linea alba, splits into two lamellae, encloses the proximal part of body, and unites on urethral aspect.
• Suspensory Ligament: Deep to fundiform ligament, triangular shaped, attached to pubic symphysis, blends with Buck's fascia on sides of body.

Arterial Supply of the Penis

• Deep arteries of the penis: branch of internal pudendal artery (branch of internal iliac artery).
• Dorsal arteries of the penis: branch of the internal pudendal artery.
• Arteries of the bulb: branch of the internal pudendal artery.
• Superficial external pudendal arteries: from the femoral artery.

Venous Drainage of the Penis

• The superficial dorsal vein drains the prepuce and skin, opening into the superficial external pudendal vein.
• The deep dorsal vein lies beneath Buck's fascia, receiving blood from glans and corpora and dividing into branches that enter vesical and prostatic plexuses.

Lymphatic Drainage of the Penis

• Lymphatics from the glans penis drain into the lymph node of Cloquet (deep inguinal group).
• Remaining lymphatics drain into superficial inguinal lymph nodes

Nerve Supply of the Penis

• Sensory: Dorsal nerve of the penis and ilioinguinal nerve.
• Motor: Perineal branch of the pudendal nerve.
• Autonomic: From inferior hypogastric plexus via prostatic plexus, reaching through pudendal nerve
• Sympathetic nerves are vasoconstrictors
• Parasympathetic fibers are vasodilators and derived from S2, S3, S4 spinal segments (nervi erigentes).

Mechanism of Erection

• Purely a vascular phenomenon in response to parasympathetic stimulation.
• Sexual arousal leads to rapid inflow of blood from helicine arteries into the cavernous space.
• Relaxation of smooth muscle; tumescence distends veins, causing erection.

Ejaculation

• Consists of emission and ejaculation.
• Emission: Transmission of seminal fluid from vasa deferentia, seminal vesicles, and prostate into the prostatic urethra.
• Ejaculation: Onward transmission of seminal fluid from prostatic urethra to exterior.
• Neural controls of erection and ejaculation are different.
• Erection is parasympathetic.
• Ejaculation is sympathetic and somatic.

Microanatomy of the Penis

• Three cylindrical bodies of erectile tissue: two corpora cavernosa (dorsal) and one corpus spongiosum (ventral, with penile urethra).
• Corpus cavernosum: network of trabeculae of connective tissue and smooth muscle fibers.
• Endothelial cells line spaces between trabeculae.
• Trabeculae are finer in corpus spongiosum.
• Each cavernous is surrounded by tunica albuginea made of collagen fibres.

Clinical Correlations of the Penis

• Impotence: The failure to achieve tumescence erection is called impotence.
• Common causes of erectile dysfunction include psychological disturbance, arterial insufficiency from disease, and involvement of nervi erigentes from diabetes
• Priapism: Failure of erection to detumesce .
• Phimosis: Narrowing of distal end of prepuce, making it non-retractable over the glans penis, may interfere with urination.
• Paraphimosis: Narrowing of prepuce insufficient to interfere with micturition but tight on glans, thus interfering with intercourse.
• Circumcision: Surgical removal of foreskin (prepuce), used for religious or for clinical purposes
• Bifid Penis/Double Penis: Very rare, associated with exstrophy of urinary bladder, two genital tubercles.
• Micropenis: A small penis, almost hidden by the suprapubic pad of fat, resulting from foetal failure or associated with hypopituitarism.