Anatomy of the male urogenital triangle

Chapters:

Watch video only

Introduction0:00–0:23

The diamond-shaped perineum is divided into two triangles the eurogenital triangle anteriorly and the anal triangle posteriorly.
The eurogenital triangle differs between males and females. In males, it contains male external genitalia and the perineal muscles.
Let's start with the male external genitalia, which includes the membranous or intermediate urethra, the spongy urethra, the scrotum, and the penis.

External genitalia0:23–0:38

Membranous urethra0:38–2:03

The membranous urethra begins at the inferior surface of the prostate gland called the apex. Then it passes through the deep perineal pouch, which houses the deep perineal muscles, the deep transverse perineal muscles, and the external urethral sphincter.
The deep transverse perineal muscle extends from the medial surface of the ischium to the perineal body. The external urethral sphincter is a circular muscle that surrounds the membranous part of the urethra and compresses it to maintain urinary continence.
The deep perineal muscles are innervated by the deep branch of the perineal nerve. Now, in the deep perineal pouch, posterolateral to the membranous urethra, are a pair of P-shaped glands called the bulbo urethral or Cowper glands.
The ducts of these glands cross the deep perineal pouch with the membranous urethra and terminate by emptying into the proximal part of the spongy urethra.
During sexual arousal, these glands secrete mucus-like secretion into the urethra. Finally, the membranous part of the urethra extends through the bulb of the penis where it's continuous with the last part of the urethra called the spongy urethra.

Spongy urethra2:03–2:20

The spongy urethra is located within the corpus spongiosum and extends through the penis to the external urethral orifice.
The spongy urethra contains an expansion in the gland's penis called the navicular fossa. Now, let's discuss the scrotum, which is a fibromuscular sac that lies posteroinferior to the penis and inferior to the pubic symphysis.

Scrotum2:20–4:55

The scrotum contains the testes and associated structures. The scrotum develops as two masses that approach each other and fuse in the midline, forming the scrotal rafi.
The scrotal rafi is continuous anteriorly with the penile rafi on the ventral surface of the penis and posteriorly with the perineal rafie in the midline of the perineum.
All right, now, the arterial blood supply of the scrotum differs between the anterior and posterior surfaces. The anterior surface is supplied by the anterior scrotal arteries, which are the terminal branches of the deep external pudendal artery.
The posterior surface is supplied by the posterior scrotal arteries, which are the terminal branches of the perineal arteries, which are branches of the internal pudendal arteries.
Both the superficial external pudendal artery and the deep external pudendal artery anastomos with branches of the internal pudendal arteries and can also supply the skin of the penis.
Finally, both surfaces of the scrotum receive arterial blood supply from the cremasteric arteries, which are branches of the inferior epigastric arteries.
Next are the scrotal veins, which accompany the arteries and drain into the external pudendal veins. Finally, the innervation of the scrotum also differs between the anterior and posterior surfaces.
The anterior surface of the scrotum is supplied by the anterior scrotal nerves, which arise from the ilio inguinal nerve.
The anterior scrotum is also innervated by the genital branch of the genitofemoral nerve. The posterior surface is supplied by the posterior scrotal nerves, which arise from the superficial branch of the perineal nerve and the perineal branch of the posterior cutaneous nerve of the thigh.

Quiz4:55–5:10

OK, now let's take a deep breath and have a quick quiz. Can you name these structures?
Additionally, can you name each of these three lines? All right, now let's talk about the anatomy of the penis, which serves as a common passageway for semen during ejaculation and urine during mixturation.

Penis5:10–5:54

In the anatomical position, the penis is erect, meaning the dorsal surface lies near the abdomen, and this means the ventral surface lies near the scrotum when it's flaccid.
Within the penis are three bodies of erectile tissue two paired corpora cavernosa and a single corpus spongiosum. These erectile tissues are the main internal structures that compose the three parts of the penis, which are the root, the body, and the glands.
Let's start with the root, which is the attached part of the penis. The root lies in the superficial perineal pouch and consists of three erectile structures called the bulb and the paired crura.

Root of the penis5:54–6:44

The bulb of the penis lies in the midline and is the expanded proximal end of the corpus spongiosum. The posterior part of the bulb is penetrated superiorly by the membranous urethra, forming the spongy urethra.
Finally, the bulb of the penis is covered by the bulbospongiosis muscles. The right and left crura lie on each side of the bulb and are the proximal tapered ends of the paired corpora cavernosa.
The crura are covered by the ischio cavernosis muscles. Now, let's talk about the body of the penis, which is the free part of the penis hanging from the pubic symphysis.

Body of the penis6:44–8:55

As the erectile tissues continue into the body of the penis, they form 3 columns. A single corpus bungiosum, which lies ventrally, and the paired corpora cavernosa, which lies dorsally.
As we mentioned before, the corpus spongiosum contains the spongy urethra, which ends at the external urethral orifice or urethral meatus at the tip of the gland's penis.
The corpus spongiosum and the paired corpora cavernosa are covered by a continuation of deep perineal fascia called the deep fascia of the penis or buck fascia.
The corpora cavernosa are also covered by an extra layer of fibrous tissue called the tunica albuinia. Now, the corpus spongiosum courses through the body of the penis and expands distally, forming the gland's penis.
The lateral margins of the gland's penis expand to cover the distal ends of the corpora cavernosa. These extensions are called the corona of the penis.
The lower ends of the corona form a groove that connects the body of the gland's penis called the neck of the penis. Here, the skin and fascia of the penis extend to form a double layer that partially covers the gland's penis, called the prepu or foreskin.
This foreskin connects to the gland's penis on the ventral surface of the penis by a median fold of skin called the frenulum of the prepu.
The foreskin can be removed in a small procedure called circumcision. Males that undergo this procedure are called circumcised males, while males that have the foreskin are called uncircumcised males.

Ligaments of the penis8:55–9:37

All right, now all these erectile tissues are fixed by the ligaments of the penis which support the root of the penis and attach it to the surrounding structures.
There are two ligaments of the penis, the suspensory ligament and the fundiform ligament. Let's start with the suspensory ligament, which is a thickening of the deep fascia that anchors the erectile bodies of the penis to the anterior surface of the pubic symphysis.
Next is the fundiform ligament, which is a condensation of collagen and elastic fibers that arises from the linea alba and surrounds the penis like a sling.
Let's take a look at the arterial supply of the penis. The main arteries that supply the penis branch from the internal pudendal arteries.

Arterial supply of the penis9:37–11:19

These branches include the dorsal arteries of the penis, the arteries of the bulb of the penis, and the deep arteries of the penis.
Let's start with the dorsal arteries, which are paired arteries that run on the dorsal surface of the penis. Here the arteries lie on each side of a vein called the deep dorsal vein of the penis in a groove between the corpora cavernosa.
These arteries supply the fibrous tissues surrounding the corpora cavernosa, the corpus spongiosum, the membranous urethra, the spongy urethra, and the skin of the penis.
Next are the arteries of the bulb of the penis, which supply the bulb of the penis, the part of the spongy urethra within the bulb, and the bulbourethral glands.
Next are deep arteries of the penis, which are paired arteries that run in the center of each corpus cavernosum. These arteries give small branches that supply blood to the spaces of the corpora cavernosa called cavernous spaces.
Filling up these spaces with blood causes the penis to become erect. In a flaccid penis, these branches coil up to restrict blood flow, forming the helocene arteries of the penis.

Venous drainage of the penis11:19–11:58

Now venous drainage of the penis is mainly by the deep and superficial dorsal veins of the penis. Let's start with the deep dorsal vein, which receives blood from the cavernous spaces and drains into a plexus of veins that surround the prostate called the prostatic venous plexus.
Next is the superficial dorsal vein, which receives blood from the skin and the subcutaneous tissue of the penis and drains it into the superficial external pudendal veins.
Some venous blood may also drain into the internal pudendal veins. All right, now let's discuss the innervation of the penis.

Innervation of the penis11:58–12:59

The pudendal nerves, which arise from the anterior rami of S2 to S4, terminate as the dorsal nerves of the penis, which provide sensory and sympathetic innervation to the penis.
They run lateral to the dorsal arteries on the dorsal surface of the penis and innervate the skin of the penis and gland penis.
Parasympathetic fibers arise from the S2 to S4 spinal cord segments, but they pass through pelvic splennik nerves to the prostatic nerve plexus.
Here they form cavernous nerves which innervate the helocene arteries of the penis. Finally, the anterior scrotal nerves, which are branches of the ilio inguinal nerves, supply the skin of the root of the penis.

Quiz12:59–13:14

OK, now let's take a deep breath and have a quick quiz. Can you name these erectile tissues?
All right, now can you name these arteries? OK, now let's talk about the muscles that lie in the superficial pouch of the male eurogenital triangle.

Superficial perineal pouch13:14–13:32

These are the superficial transverse perineal, the bulbospongiosis, and the ischiocavernosis muscle. Let's start with the superficial transverse perineal muscles.

Superficial transverse perineal muscle13:32–14:02

The muscles originate from the internal surfaces of ischiopubic rami and ischial tuberosities and insert into a fibromuscular mass that lies in the middle of the perineum called the perineal body.
These muscles support and fix the perineal body, which in turn supports the pelvic viscera. Next are the bulbo spongiosis muscles.

Bulbospongiosus muscle14:02–14:54

They originate from the perineal body and each side of the median rafi that extends along the midline of the bulb of the penis.
They wrap around the bulb of the penis to insert into the perineal membrane and the corpus spongiosum and corpora cavernosa.
These muscles also support and fix the perineal body, which, as we said, supports the pelvic viscera. The contraction of the bulbospongiosis muscles pushes the residual urine or semen from the spongy urethra.
Finally, they aid in erection of the penis by compressing the outflow of blood through the deep dorsal vein of the penis and by pushing blood through the body of the penis.
Finally, there are the ischio cavernosis muscles, which also originate from the internal surfaces of the ischiopubic rami and ischial tuberosities.

Ischiocavernosus muscle14:54–15:34

These muscles wrap around the crura to insert onto their medial and inferior aspects and onto the perineal membrane. They maintain erection by compressing the outflow of blood through veins and by pushing blood through the body of the penis.
Finally, all of the superficial perineal pouch muscles are innervated by the deep branch of the perineal nerve. All right, now let's switch gears and discuss the lymphatic drainage of the male external genitalia.

Lymphatic drainage of male external genitalia 15:34–16:38

Starting with the lymphatic drainage of the membranous urethra, which goes with the lymphatic drainage of the cavernous bodies into the internal iliac lymph nodes.
Next is the lymphatic drainage of the spongy urethra, which accompanies the lymphatic drainage of the gland's penis into the deep inguinal lymph nodes, with some lymph vessels passing to the external iliac lymph nodes.
Now, lymph from the scrotum and the skin of the penis, except for the glands, drains into the superficial inguinal lymph nodes.
Finally, you may think that the lymphatic drainage of the testes is similar to the scrotum, right? Well, that's not the case.
The testes develop in the abdomen, so their lymph drains into the lumbar and pre-aortic lymph nodes. All right, now that we understand the anatomy of the penis, let's switch to physiology and discuss the male sexual response.

Male sexual response16:38–16:56

The sexual response has 4 stages, called erection, emission, ejaculation, and remission. Let's start with erection, which happens when the male is erotically stimulated, causing activation of the parasympathetic nervous system.

Erection stage16:56–17:40

This stimulation causes two changes. First, it relaxes the helocene arteries, making them straighter, which allows more blood to pass into the cavernous spaces.
Second, the bulbospongiosis and ischiocavernosis muscles also compress the veins that drain the cavernous spaces, which traps the blood in the cavernous spaces.
With more blood coming in but very little blood leaving, the erectile tissues swell up, causing the penis to become larger and more rigid.

Emission stage17:40–18:06

Next stage is emission, which is a response to sympathetic stimulation. This stimulation causes each ductus deferens and the seminal vesicles to contract.
This pushes seminal fluid into the prostatic urethra, where prostatic fluid is added. So, now the stage is set for ejaculation.

Ejaculation stage18:06–18:39

Ejaculation describes the process of expelling the seminal fluid from the urethra through the external urethral orifice.
Ejaculation involves both sympathetic and parasympathetic innervation. Sympathetic fibers cause the internal urethral sphincter to contract, and parasympathetic fibers cause the urethral muscle to contract.
Finally, the pudendal nerves cause the contraction of the bulbospongiosis muscles. After ejaculation, the penis gradually returns to its flaccid state in a process called remission.

Remission stage18:39–19:28

This process results from sympathetic stimulation, which causes the helocene arteries to contract and recoil and relaxes the bulbospongiosis and ischiocavernosis muscles.
These changes allow blood to empty from the cavernous spaces into the deep dorsal vein, and the penis finally returns to a flaccid state.
Now, here's a study tip for you. The mnemonic point and shoot can help you remember that point or initiating an erection is parasympathetic, and shoot or ejaculation is sympathetic.

Review19:28–22:24

All right, as a quick recap, the male urogenital triangle houses the perineal muscles and the male external genitalia. These genitalia include the membranous and the spongy urethra, the scrotum and the penis.
The membranous urethra starts at the apex of the prostate gland and joins the spongy urethra, which runs through the corpus spongiosum, terminating at the external urethra orifice.
These two parts of the urethra get their blood supply from branches of the dorsal arteries of the penis. Next up is the scrotum, which lies postero inferior to the penis and has a line called the scrotal rafi on its midline, which is continuous with the penile and perineal rapes.
The scrotum is supplied anteriorly by the anterior scrotal arteries and posteriorly by the posterior scrotal arteries and nerves.
The scrotal veins drain into the external pudendal veins. Next is the penis, which has 3 parts called the root, the body, and the glands.
The root contains 3 erectile structures, a single bulb, which is the proximal end of the corpus spongiosum, and the paired crura, which are the proximal ends of the corpora cavernosa.
In the body of the penis, these erectile tissues form 3 columns with the single corpus spongiosum lying ventrally, surrounding the spongy urethra and the paired corpora cavernosa lying dorsally.
These erectile tissues house tiny spaces called cavernous spaces which fill with blood during an erection. Finally, the corpus bungiosum extends distally to form the glands penis.
There are 2 ligaments that support the bulb of the penis, called the suspensary and the fundiform ligaments. The arterial blood supply of the penis is mainly by branches of the internal pudendal artery, including the dorsal arteries of the penis, the arteries of the bulb of the penis, and the deep arteries of the penis.
The male superficial perineal pouch houses three pairs of muscles, the superficial transverse perineal muscles, the bulbospongiosis muscles, and the ischiocavernosis muscles.
Finally, the male sexual response has 4 stages called erection, emission, ejaculation, and remission.