Original question.
Enumerate the different muscles, membranes, and vessels which you would meet with in dissecting from the external integument down to the level of Cowper's glands.
Cowper's glands are the paired bulbourethral glands, deep to the perineal membrane on either side of the membranous urethra. The relevant dissection is of the male anterior perineum, or urogenital triangle. A broad exposure reveals structures at different lateral positions; they are not all piled one above another along a single central line.
From superficial to deep, the principal layers and contents are:
- Skin and superficial fascia. The superficial fascia includes a fatty layer and a deeper membranous layer, Colles' fascia. The latter attaches laterally to the ischiopubic rami and posteriorly towards the posterior border of the perineal membrane and perineal body; anteriorly it is continuous with fascia of the scrotum and lower abdominal wall. Superficial vessels and nerves accompany these layers. Cutaneous arterial supply includes branches of the perineal artery and, more anteriorly, contributions associated with the external pudendal circulation.
- The superficial perineal compartment. Removing the membranous fascia exposes the superficial pouch and the investing fascia over its muscles. Centrally, bulbospongiosus covers the bulb of the penis and proximal corpus spongiosum. On either side, ischiocavernosus covers a crus of the penis. Posteriorly, the superficial transverse perineal muscle runs towards the perineal body. Older descriptions call these the accelerator urinae, erector penis and transversus perinei respectively. The names should be matched to the structures rather than counted as additional muscles.
- Superficial perineal vessels. The perineal artery, a branch of the internal pudendal artery, supplies this region and gives posterior scrotal and muscular branches; accompanying veins drain it. These vessels lie in relation to the pouch and its muscles, rather than constituting a complete vascular sheet. The bulb and crura are the associated erectile tissues, not additional membranes.
- The perineal membrane. This dense fibrous sheet spans the anterior pelvic outlet between the ischiopubic rami. It is the structure traditionally described as the triangular ligament's inferior layer. It is pierced by the urethra and by the ducts of the bulbourethral glands, as well as related vascular passages. The artery of the bulb of the penis passes to the bulb in close relation to the membrane. The deep and dorsal penile arterial routes lie more anteriorly; they should be identified where the exposure reaches them rather than imagined to cross every central dissection plane.
- The deep perineal structures. Beyond the perineal membrane are the external urethral sphincter complex, traditionally described deep transverse perineal fibres, the membranous urethra and the bulbourethral glands. The glands lie in close relation to sphincteric skeletal muscle. Their ducts run forwards through the perineal membrane and open into the bulbar urethra, so the position of a duct's opening is not the position of its gland.
The superficial muscles, erectile bodies and pudendal branches identify the shallow compartment; the perineal membrane separates it from the gland-bearing deeper region.[ref]
Anatomy, Abdomen and Pelvis: Superficial Perineal Space (StatPearls). Structure and Function; Blood Supply and Lymphatics; Muscles
The deep space contains the sphincter complex and male bulbourethral glands, with their ducts traversing the membrane.[ref]Anatomy, Abdomen and Pelvis: Deep Perineal Space (StatPearls). Introduction; Structure and Function; Blood Supply and Lymphatics
The main internal pudendal vessels themselves run laterally in the pudendal canal, in relation to the fascia over obturator internus, before their terminal distribution. A median approach to a gland does not necessarily expose their whole trunks. The deep dorsal vein of the penis has a distinct anterior route beneath the pubic arch and should not be confused with the small accompanying perineal veins. These positional qualifications are necessary for an accurate enumeration of the vessels one may meet.
Nineteenth-century descriptions often arranged the deep structures as a urogenital diaphragm enclosed between two neat fascial layers, the superior and inferior layers of the triangular ligament. Modern dissection finds a more complex sphincteric and connective-tissue arrangement. The historically named layers explain the older vocabulary, but the perineal membrane and surrounding muscles should not be made into a fictitious uniform sandwich. Nor does reaching the bulbourethral glands require dissecting through the whole pelvic diaphragm above them.
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