Perinaeum, the Ligamentous Seam betwixt the Cod and the Fundament. Blancard’s Phys. Dict. (ed. 2), 1693
Development of the perineum
By the end of the third week of gestation, the mesoderm has raised swellings around the cloacal membrane as the initial development of the primary perineum (see Fig. 16-1 ). The swellings, in turn, become divided transversely by the perineal spur into anterior genital and posterior anal swellings. A primitive cloacal sphincter develops around the cloacal membrane.
The secondary perineum is a late development that accompanies regression of the tail. It arises as the distal part of the urorectal septum divides the cloaca transversely into urogenital and rectal portions (see Fig. 13-8 ). The point of contact of the septum with the membrane forms the central tendon of the perineum.
Anterior perineal differentiation is dependent on the development of the external genitalia, as described in Figures 16-3 to 16-14 . In brief, the genital tubercle develops into a phallus from its origin at the cranial end of the cloacal membrane. The urethral plate of endoderm, covered on its lower surface with ectoderm from the primary urethral groove, extends onto the phallus. The edges of the groove are elevated to form genital folds that pass around the urogenital membrane reaching almost to the anus. When the membrane breaks down, both the urinary and genital passages open at the base of the phallus. The urethral groove deepens and the stage is set for male and female differentiation. In the male, the genital folds join to create a urethra.
Development of the perineal musculature
The cloacal sphincter muscle , rather than the internal muscles that form the pelvic floor, is the source of the perineal musculature proper. This muscle develops around the cloacal membrane at 8 weeks and is innervated by the pudendal nerve, as are its subsequent divisions ( Fig. 11-1 A). It becomes stratified into two planes. The deep plane forms the external anal sphincter, the striated urethral musculature, and the deep transverse perineal muscles. From the superficial plane, the bulbospongiosus muscle, the superficial transverse perineal muscles, and a superficial portion of the external anal sphincter are derived.
By the 12th week, the sphincter is divided into an external anal sphincter and the sphincter of the urogenital sinus at the same time that the common cloacal opening becomes divided ( Fig. 11-1 B).
In the male, by the 20th week, the sphincter of the urogenital sinus has formed the ischiocavernosus and bulbospongiosus muscles and also the striated urethral sphincter ( Fig. 11-1 C). Finally, the transverse perineal muscles are formed in association with the bulbospongiosus muscle.
By term, the ischiocavernosus and bulbospongiosus , as well as the transverse perineal muscles , are independent of the striated urethral sphincter ( Fig. 11-1 D).
The levator ani , a muscle of the pelvis, arises from a higher mesodermal source in conjunction with the coccygeus, and later, descends to the level of the bladder and prostate to become secondarily related to the sphincters and perineal musculature.
Boundaries of the perineum
The diamond-shaped perineum lies below the pelvic outlet, which is separated from the pelvis by the pelvic diaphragm (see Fig. 10-11 ). It is limited anteriorly by the pubic arch and the arcuate pubic ligament that joins the lower borders of the symphysis. Posteriorly, it is restricted by the tip of the coccyx and on either side by the inferior margins of the pubic and ischial rami, the ischial tuberosity, and the sacrotuberous ligament. It includes all of the soft parts that connect the lower portions of the digestive, genital, and urinary tracts to the walls of the true pelvis. A line drawn between the ischial tuberosities divides the perineum into an anterior urogenital triangle that is different in the male and female, and a posterior anal triangle common to both sexes.
Descriptions of the perineum are still handicapped by the tendency of anatomists to tailor the muscular and aponeurotic layers to conform to concepts they have derived from their own dissections and to ascribe original names to selected layers. The whole concept of a urogenital diaphragm has yet to be satisfactorily resolved, mainly because the urethral sphincters penetrate its proposed layers. The present illustrations are based on classic descriptions that incorporate recent findings but do not include all the modifications made by every anatomist.
It should be stressed that the classic concept of a two-layered urogenital diaphragm as described by Henle 138 years ago has not been uniformly identified by subsequent anatomists; the most conspicuous difference has been the inability of more recent observers to locate a superior fascial layer. However, by considering this layer to be composed of the deep perineal muscles and their associated fascia rather than a special anatomic sandwich, its perineal relations can be described and understood.
Compartments and fascias of the perineum
Two compartments can be dissected, a superficial perineal space and a deep perineal space. The superficial perineal space contains the superficial urogenital muscles: the bulbospongiosus, both of the ischiocavernosus muscles, and the superficial transverse perineal muscles. The deep perineal space encloses the deep urogenital muscles: the striated urethral sphincters and the deep transverse perineal muscles. What has been called the urogenital diaphragm is best defined as the layer of deep urogenital muscles and the fascias that accompany them.
Three fascial layers define the two spaces. The first is the membranous layer of the superficial fascia (Colles’ fascia) that forms the roof of the superficial perineal space. The intermediate layer is the perineal membrane or inferior fascia of the urogenital diaphragm that provides both the floor of the superficial perineal space and the roof of the deep perineal space. The deepest layer is the inferior fascia of the urogenital diaphragm that serves as the floor of the deep perineal space. These compartments and layers are defined for anatomic convenience and as such provide convenient points for description and reference. During dissection or surgery, many structures are found not to be strictly bounded by such distinct layers because of the tenuous nature of the tissues of the so-called diaphragm and because of anatomical variations.
Table 11-1 outlines the layers of the perineum.
|Superficial perineal fascia (areolar and membranous layers)|
|Inferior fascia of the urogenital diaphragm|
|Superior fascia of the urogenital diaphragm|
Superficial perineal fascia
The membranous layer of the superficial fascia or Colles’ fascia has a structure resembling an aponeurosis. It forms a roof over the superficial perineal space that extends anteriorly from the dorsal margin of the perineal membrane ( inferior fascia of the urogenital diaphragm ) and the perineal body . One portion, as the major leaf of Colles’ fascia, forms a partition between the superficial perineal space and the scrotum ( Fig. 11-2 A). More anteriorly, Colles’ fascia joins the dartos muscle of the scrotum as the scrotal dartos and forms part of the scrotal septum. It is continuous over the penis as the superficial penile fascia or penile dartos that covers the penile or Buck’s fascia over the corpora cavernosa and corpus spongiosum. A thin connective tissue layer, the tela subfascialis of Eberth, intervenes between the superficial penile fascia and Buck’s fascia. The superficial penile fascia also encloses the ischiocavernosus and bulbospongiosus muscles within individual fascial compartments. As a continuation of Colles’ fascia, it joins Scarpa’s fascia, the membranous layer of the superficial fascia over the lower abdomen. Laterally, it is attached to the pubic rami and ischia . These connections limit the spread of extravasated urine.
Colles’ fascia covers the superficial perineal space containing the ejaculatory muscles, the ischiocavernosus and bulbospongiosus . These muscles, in turn, surround Buck’s fascia , which encloses the corpus spongiosum and the crus of the corpus cavernosum on each side ( Fig. 11-2 B). The penile dartos, scrotal dartos , and scrotal septum are continuations of Colles’ fascia.
The perineal membrane (inferior fascia of the urogenital diaphragm), an extension of Colles’ fascia, covers the deep transverse perineal muscles and forms the roof of the deep perineal space, through which the membranous urethra passes and in which the pudendal arteries and veins lie. Deep to this space are the superior fascia of the urogenital diaphragm and the levator ani group of muscles.
Superficial perineal space
After the covering membranous layer of the superficial fascia (Colles’ fascia) is removed, the superficial perineal space is exposed. The floor of the space is the perineal membrane (inferior fascia of the urogenital diaphragm) that extends from the obturator fascia to reach the fascias surrounding the urethral and anal sphincters, effectively lining the medial wall of the ischiorectal fossa. The perineal membrane is continuous with the fascia of the obturator internus, fuses with the superior fascia anteriorly, and joins the fascia of the external sphincter ani and the anococcygeal raphe posteriorly, thus forming the upper limit of the ischiorectal fossa.
The posterior edge of the perineal membrane reaches the perineal body and fuses there with the superior fascia of the urogenital diaphragm beneath the superficial transverse perineal muscles. The perineal membrane is also continuous with Colles’ fascia. Anteriorly, the perineal membrane becomes more dense, forming the transverse perineal ligament and terminates to allow passage of the deep dorsal vein and the dorsal nerve of the penis. The rectourethralis muscle is formed from a few strands of the anterior longitudinal layer of the rectum at the rectal ampulla that pass forward to the perineal body. The prostatic levators are those portions of the levator ani that pass alongside the prostate to attach to the perineal body.
Within the superficial perineal space are the superficial urogenital muscles, the bulbospongiosus and both ischiocavernosus muscles ( Fig. 11-3 ). This space also contains the paired superficial transverse perineal muscles that run across from the anterior and medial parts of the ischial tuberosities . They join the perineal body in the midline in conjunction with muscle fibers from the superficial part of the external anal sphincter and the bulbospongiosus muscle.