Overview
The penis
along with the scrotum forms the external male genitalia. It develops from
tissue in the urogenital sinus and the same structure becomes the clitoris in
females. It is located at the anterior
base of the perineum. It forms part of the organs of reproduction in men along
with the testes, prostate and bulbourethral gland. The functions of the penis
are for the performance of sexual intercourse, a route for ejaculation and
micturation. Here we will consider the anatomy of the penis and bulbourethral
glands.
Gross Anatomy
In the
anatomical position the penis lies erect. It begins in a fixed position in the
urogenital triangle and then becomes free anteriorly. It can be divided into the root (fixed) and body (free) and is formed of three distinct structures : The Corpus
Cavernosa, the Corpus Spongiosum and the Glans Penis.
The Root of the penis lies within the urogenital triangle
and is formed from the crura of each cavernosa and the bulb of the penis which
is fixed to the perineal membrane.
The body of the penis is the external part of the penis
which is entirely covered with skin.
Suspensory ligament: from pubic symphysis to the base
of the penis
Fundiform ligament: from the linea alba and anterior
abdominal wall and runs around each side of the penis to rejoin below it.
Muscles
There are
two pairs of muscles at the root of the penis
- Bulbospongiosus – contracts around the bulb of
the penis to empty residue present.
- Ischiocavernosus – covers the crura of the
penis. It can contract to force blood upward into the distal corpus
cavernosa to maintain erections.
Corpus Cavernosa
· Begins as two “crura” (legs), each
of which is attached to the inferior surface of the pubic arch.
· The crura converge at the pubic symphysis
where they become part of the body of the penis. They form the dorsal side and
main bulk of the body of the penis and erectile tissues.
· Each cavernosa contains an artery
(cavernous artery) from which blood flows into each cavernosa during erection.
· The insides are formed of
interconnected sinusoids separated by smooth muscle trabeculae.
· The function of the cavernosa is to
erect the penis for intercourse.
Corpus Spongiosum
· Begins as the bulb of the penis. It
is anchored to the perineal membrane.
· Forms the ventral side of the penis.
· Contains the bulbar and penile
urethra.
· The main action is to maintain
patency of the urethra whilst erect and fills to a lesser pressure than the
cavernosa.
Glans
· Forms the distal end of the penis
and is contiguous with the corpus spongiosum.
· Distal to the base it bulges outward
to form two “corona”. These converge to form the midline raphe and the urethral
meatus.
· The skin which was tightly adherent to the penis now loosely covers the
glans as a double fold i.e. the prepuce (foreskin).
Fascial Layers (superficial to deep)
1.
Skin.
2.
Dartos
Fascia (superficial penile fascia) – continuous with Scarpa’s Fascia.
3.
Bucks
Fascia – Continuous with the Suspensory ligament and external spermatic fascia
of scrotum.
4.
Tunica
Albuginea.
Mnemonic
Fasical
Layers - Silent Diseases Batter Testicles / Skin, Dartos, Bucks, Tunica.
Blood
supply and Innervation of the Penis
Arterial
supply: Internal pudendal artery via the
internal iliac. It separates into two dorsal vessels (deep and
superficial) in the cavernosa, and the bulbourethral artery in the spongiosum.
Venous
Supply – The Skin is drained by the superficial dorsal vein, whilst the deep
veins drains the erectile tissues.
Somatic
innervations: Via The dorsal nerve of the penis , a branch of the
internal pudendal nerve (S2-4).
Parasympathetic
innervations: Via the prostatic plexus. This transmits the changes
in vascular tone to create erections
Lymphatics – Via the
deep inguinal nodes.
Bulbourethral Glands
Derived
from the urogenital sinus (along with the bladder and prostate) they are also
known as Cowper’s Glands and lie within the fibers of the external urethral sphincter.
They lie posteriolateral to the membranous urethra just above the bulb of the
penis (the root).
The ducts
of the bulbourethral glands open up into the spongey urethra just below the membranos urethra.
The
bulbourethral glands are lined with columnar
epithelium and produce a mucinus substance that acts as a lubricant for sexual
arousal. It has a high pH and is also thought to neutralise the acidity of the
vaginal mucosa.
Blood
supply – Small branches from the internal pudendal artery
Innervation
: Via the prostatic plexus (autonomic) – See physiology of erection and
ejaculation below
Lymphatic
drainage - internal and external iliac
lymph nodes.
Erection
· Erection is a vascular event which
is triggered by parasympathetic nerve fibres from S2-4.
· Cavernous arteries relax allowing blood to fill each cavernosa
· Arteries are branches of the
internal pudendal artery
· The caverns are drained by the deep dorsal vein into the prostatic plexus
Erection and ejaculation are two
distinct actions . Erection is controlled by the parasympathetic nervous system
whilst ejacutation is sympathetically modulated.
Mnemonic – Parasympathetic Points (erection) , Sympathetic Shoots (ejaculation)
Clinical Anatomy
·
Erectile
dysfunction
A Disorder of the partial or complete inability to
initiate or maintain an erection. It may be secondary to vascular disease
(atherosclerosis) or nervous dysfunction (diabetes, Parkinson’s disease,
multiple sclerosis or post prostatectomy ).
Management of erectile dysfunction is complex but
initially involves lifestyle modification followed by pharmacological measures.
Surgical intervention is not commonly used and only carried out in specialist
centres.
·
Peyronies
Disease – A Fibro-inflammatory condition of the tunica
albuginea which leads to abnormal curvature of the penis and consequently, painful
erections and difficulty in intercourse.
·
Squamous
cell carcinoma of the penis - A rare but serious malignancy of the penis,
treated by partial or complete penectomy.
·
Phimosis–
the inability to retract the foreskin over the glans
·
Paraphimosis
– Inability replace the foreskin back over the glans, can commonly occur
iatrogenically after catheterisation but can also occur as a result of phimosis
after intercourse or masturbation.
· Balanitis
Xerotica Obliterans (BXO) - A chronic sclerotic disease of the
foreskin leading to phimosis and later meatal stenosis. Manageed by
Circumcision
· Hypospadias - Congenital defect
where the urethra enters on the ventral side of the penis instead of the glans.
· Penile fracture – traumatic rupture
of the corpus cavernosum. An uncommon but true urological emergency, most cases
occur during intercourse.
Quick Anatomy
Key Facts
|
Embrological
origin |
Urogenital
sinus |
|
Arterial
supply |
Via a
branch of the internal pudendal artery |
|
Venous
supply |
Deep and
superficial dorsal vein complexes |
|
Somatic innervations |
Pudendal
nerves |
|
Autonomic
innervations |
Branches
of S2-4 via the cavernous nerves |
Aide-Memoire
Summary
Disorders of the penis constitute only a small proportion of casework
in clinical practice but they can be highly distressing and/or
embarrassing for the patient.