The pelvic outlet is bounded caudally by a bilateral fasciomuscular closure, the pelvic diaphragm. It supports the retroperitoneal pelvic organs to prevent their perineal prolapse. The perineum extends deeply to the level of imaginary lines from the apex of the sacrum dorsally, ischial arch and tuberosities ventrally and sacroischiatic ligament laterally, as well as superficially along the region which extends ventrally from the anus to the level of the caudal end of pelvic symphysis. It consists of two anatomical triangular areas; dorsal and ventral, anal and urogenital respectively figure 2. The anal triangle is the area that is included in the lines between the sacral apex dorsally, sacroischiatic ligament laterally and ischial tuberosities ventrally. The region is composed of a bilateral ischiorectal fossae, each of them extends from the sacral apex dorsally, anal opening ventromedially, ischial tuberosities ventrolaterally and sacroischiatic ligament laterally figure 3a. The urogenital triangle extends ventrally to the former and its base facing dorsally between the ischial tuberosities while its apex reaches the level of the pubic symphysis. The pelvic diaphragm lodges in the deep and superficial aspects of the perineum and is categorized in three parts; dorsal, intermediate and ventral. The former, the ischiorectal part includes the coccygeus muscle group and the anal part which forms the suspensory apparatus of the anus. The latter, consists of the rectococcygeus and the retractor penis muscle.
Figure 2: A photograph showing the areas of dissection (caudal view).
The intermediate perineal part represents the perineal body and the longitudinal cutaneous perineal muscle and the ventral one is composed of the urogenital diaphragm. The perineal fasciae represent the latter and is composed of superficial and deep parts.
The dorsal part
The Ischiorectal part (Coccygeus muscle group) comprises the lateral coccygeus muscle and the medial coccygeus group (corresponding to levator ani muscle). The lateral coccygeus muscle (Figures 3a,3b,4-7) is a thin flat quadrilateral in shape, arising along the ischiatic spine and its fibers directed caudo- dorsally and medially. The muscle is inserted in the lateral aspects of the first three caudal vertebrae. It guards the caudolateral aspect of the sacroischatic angle and is bounded laterally by the ischiorectal fat bad and sacrotuberous ligament, while medially it is related to the medial coccygeus group. The muscle has ventral, cranial, dorsal and caudal borders; the former, measures about 1.8-2 cm length, 2.7-2.9cm for the cranial, 3.6-3.8cm for the dorsal and 4.5-4.7cm for the caudal one. The medial coccygeus muscle group (Levator ani muscle) (Figures 3a,3b,4,7-10,11a,11b) (3a,3b,4,is the main closure of the pelvic outlet and is considered the chief muscle of the pelvic diaphragm. The group consists of iliococcygeus, pubococcygeus and ischiococcygeus muscle. The first, (Figures 8-10) is a flat triangular muscle attached from the cranial border of the ilio-pubic junction and its fibers directed caudo- dorsally and medially to be inserted in the lateral aspect of 4th caudal vertebrae. Its base facing ventrally and measures about 2.1-2.3cm, 5.4-5.6 cm for the cranial border as well as 4.6-4.8 cm for the caudal one. The muscle is related laterally to the shaft of ilium and medially to the pubococcygeus muscle and obturator nerve. It overlaps the craniolateral aspect of the latter muscle, where its caudal fibers join that of pubococcygeus forming single muscle bundle. The pubococcygeus muscle (Figures 3a,3b,4,7-10) is the largest part of the group, the thin and flat muscle fibers arise from the dorsal and lateral aspect of the pubis and pelvic symphysis. The muscle is fan shaped and its fibers ascend dorsally around the terminal part of the rectum and medially directed to be attached to the lateral aspect of 4th caudal vertebra. It is bounded laterally to the ilium and medially to a fat bad separating it from the prostate gland. The Ischiococcygeus muscle (Figures 3a,3b,4,7-10) is the ischial part of the medial coccygeus muscle group. The thin flat muscle arises from the caudo dorsal aspect of the pelvic symphysis and covers the caudolateral aspect of the pubococcygeus muscle. It extends dorsally in nearly vertical direction, related to the obturator muscle laterally and the rectum medially. The muscle curves on the dorsal aspect of the latter where attached to the lateral sides of 4th caudal vertebra. The last described muscle and the pubococcygeus one, form the levator window where the anal canal with the suspensory apparatus is pass through.
A photograph the perineal region (lateral view).
A photograph showing the ischiorectal fossa (Caudolateral view).
1- Sacroischiatic ligament
2- Lateral coccygeus muscle
3- Pubococcygeus muscle
4- Ischiococcygeus muscle
5- External sphincter ani muscle
6- Internal obturator muscle
The dotted triangular region indicates the right ischiorectal fossa
A photograph showing the perineal region (lateral view).
1- Lateral coccygeus muscle (reflected)
2- Pubococcygeus muscle
3- Ischiococcygeus muscle
4- External phincter ani muscle
Figure 5: A photograph showing the perineal region, superficial dissection (caudal view).
1- Lateral coccygeus M
2- External phincter ani M
3- Ischial tuberosity
4- Colle’s fascia
5- Penie part of retractor penis M
6- Superficial perineal pouch
A photograph showing the perineal region (caudal view).
1- Lateral coccygeus M
2- External sphincter ani M
3- Ischial tuberosity
4- Colle’s fascia
5- Penile part of retractor penis M
6- Ischiocavernosus M
7- Bulbospongiosus M
8- Scrotal septum
9- Deep penile fascia
10- Superficial penile fascia
Figure 7: A photograph showing the right ischiorectal fossa (deep dissection).
1- External sphinter ani M
2- Ishiococcygeus M
3- Pubococcygeus M
4- Lateral coccygeus M
5- Ischiocavernosus M
6- Bulbospongiosus M
7- Penile part of retractor penis M
8- Perineal M
9- Pelvic urethra
10- Deep (dorsal) layer of the deep perineal fascia
11- Ischial tuberosity
12- Internal obturator M
The black arrow indicates the ischiourethralis M (covered by 10)
The blue arrow indicates superficial perineal fascia (septum between 5 and 6)
A photograph showing the perineal region (removed left half of pelvic bone).
1- Iliococcygeus M
2- Pubococcygeus M
3- Ischiococcygeus M
4- Vertebral part of retractor penis M
5- External sphincter ani M
6- Perineal M
7- Bulbospongiosus M
8- Left penile crus
A photograph showing the perineal region (left lateral view).
1- External sphinter ani M
2- Perineal M
3- Perineal body
4- Ischiococcygeus M
5- Pubococcygeus M
The anal part (Suspensory apparatus of the anus), includes the rectococcygeus and the retractor penis muscle. The former, (Figures 10 and 11a) is the extrarectal continuation of the rectal musculature to the coccygeal vertebrae. The median dorsal longitudinal rectal muscle fibers coalles at the level of the pelvic out let to form a caudo dorsal projection inserted in the ventral aspect of the 5th caudal vertebra.
A photograph showing the perineal region (deep dissection).
1- Retractor penis M
2- Rectum (cut)
3- Pelvic urethra
5- Urinary bladder
6- Ischiococcygeus M
7- Pubococcygeus M
8- Iliococcygeus M
9- Ventral sacrococcygeus M
10- Left penile crus the yellow arrow indicates the obturator nerve
11- Rectococcygeus M
Figures 11(a,b): A photograph showing suspensory apparatus of the anus.
1- Medial coccygeus group (reflected)
2- Rectococcygeus M
3- Vertebral part of retractor penis M
4- External sphincter ani M
5- Perineal M
6- Perineal body
7- Bulbospongiosus M
The black arrow indicates the retractor penis M
The blue arrow indicates the rectal part
The green arrow indicates the penile part
The red arrow indicates the anal part of retractor penis M
The Retractor penis muscle (Figures 5,6,10,11a,11b,12), it consists of two parts; large and vertebral, the former is represented in right and left thin flat tape like in shape bundles. Each arises from the ventral aspect of the 1st and 2nd caudal vertebrae. It is directed in a caudoventral direction to run on the dorsolateral aspect of the anorectal junction. At the level of the dorsal insertion of the medial coccygeal muscle group, it inclines laterally on the anal canal. It passes on the deep face of the ischiococcygeus muscle, where the muscle divides into strong anal, penile and rectal parts. The former, is the caudal one and extends along the dorsolateral side of the anal passage, it proceeds deeply to the para-anal sinus, where the bundles terminate between the anal sphincter muscles. The penile part is the longest and crosses the lateral side of the anal canal in a vertical direction. It reaches the ventral border of the latter, where both fibers of the opposite sides are collected commonly in a single bundle and pass caudally between both perineal muscles. The muscle extends along the ventral border of penis to the glans and enrolled in the deep penile fascia. The rectal division is the cranial smaller part and its fibers are encircled with that of the terminal part of the rectum.
A photograph showing the penile region, deep dissection (lateral view).
1- Deep penile fascia
2- superficial penile fascia
3- Retractor penis M
4- Ischiocavernosus M
The vertebral part (Figures 8,11a,11b), is a thin flat fascicles derive from the ventral aspect of the 3rdcaudal vertebra. The muscle passes ventrally and vertically on the caudal aspect of the external sphincter ani muscle and deeply to the ischiococcygeus muscle. The muscle fibers of both sides terminate in the bulb of penis.
The intermediate part
The perineal body is a fibrous mass ventrally located to the anal opening where the perineal fasciae attach (Figures 9 and 11a).
The perineal muscle (Figures 8,9,11a,7) is the right and left ventral median longitudinal continuation of the external sphincter ani muscle. Each short bundle ascends from the perineal body on the lateral aspects of the penile part of retractor penis muscle. The muscle bundles encircle both sides of the anal canal respectively and joined together forming the external anal sphincter.
The ventral part
The perineal fasciae: The superficial perineal fascia comprises the superficial and membranous parts; the former figure 2 is a fibro-fatty layer composed of a thin fibrous sheath firmly adhered to the perineal skin and continues laterally to the superficial gluteal fascia. This layer is wrapped in fatty coat separating it from the skin externally and the membranous layer internally.
The perineal fat coat covers the ischiorectal fossae and the area of urogenital triangle, where over the former, the fat bad supports the muscles of the dorsal part of the pelvic diaphragm caudally and laterally. While on the urogenital triangle, the coat divides it from the skin externally figure 2.
The membranous layer (Colle’s fascia), (Figures 5,6,13,14) is a fibrous sheath extending from the perineal body dorsally and bilaterally stretched to the ischial tuberisities. It covers the caudoventral aspect of the ischial arch and extends ventrally to envelope the root of penis. At the arch, it coats the bulbospongiosus muscle and wraps around the penile part of retractor penis muscle. A fibrous division arises to separate between the bulbospongiosus and ischiocavernosus muscle. The fascia continues around the intrascrotal part of penis, where it receives the scrotal septum at the median plane. It proceeds cranially to join the tunica dartus of penis (superficial penile fascia) and the dorsal aspect of the latter, attached to the suspensory ligament of penis. Cranially, the dorsal extension of the superficial penile fascia, continued with the scrapa fascia of the abdominal wall. Colle’s fascia encloses a space or pouch (superficial perineal pouch) divides the deep penile fascia (internally) and perineal fat (externally). It contains the blubospongiosus and ischiocavernosus muscle.
Figures 13: A photograph showing the penile region (lateral view).
1- Suspensory ligament of penis
2- Deep penile fascia
3- Superficial penile fascia
The dotted line indicates the junction between the superficial penile fascia and scrapa
A diagram showing the line of extension of colle’s fascia.
The yellow lines indicates the colle’s fascia (superficial perineal fascia)
The blue line indicates the superficial penile fascia
The red line indicates the scrapa fascia of the abdominal wall
The green line indicates the perineal membrane
The brown line indicates the dorsal layer of the urogenital diaphragm
The Deep perineal fascia (Urogenital diaphragm) figures (7,12,14) is a double membranous layers that guard the ventral aspect of the pelvic outlet (Ischial arch). It is related to the prostate gland and pelvic urethra dorsally and consists of superficial (ventral) and deep (dorsal) parts. The former, is the perineal membrane that emerges from the ventral aspect of the pubic symphysis, where it sends the suspensory ligament of penis. It passes along the dorsal aspect of the root of the latter and at the ischial arch, the fascia is wrapped laterally to the ischial tuberosities. It reflects on the penile crurae and body to form the deep penile fascia. On the superficial aspect of bulbospongiosus and ischiocavernosus muscle it extends to form the colle’s fascia. The perineal membrane covers the ischiurethralis muscle laterally and proceeds cranially to the symphysis on the floor of the pelvic outlet, ventrally to the pelvic urethra to form the dorsal layer of the dorsal layer of the urogenital diaphragm. A narrow deep perineal pouch is formed between the two layers of the deep fascia and encloses the ischiocavernosus muscle figure 7.