The tunica albuginea  of the corpora cavernosa is a bi-layered structure with which the collagen bundles of the inner layer are arranged circumferentially and those of the outer layer are arrayed longitudinally (Figure1). The inner layer completely contains and, together with the intracavernosal pillars, supports the sinusoids. There is a paucity of outer layer bundles, resulting in a weak sector which may be susceptible to surgical trauma during penile implant , at the region between the clockwise 5 and 7 o’clock positions, where there are two triangular ligament structures. These structures, termed the ventral thickening, are a continuation of the left and right bulbospongiosus muscles. There is close contact between the corpora cavernosa and the corpus spongio-sum. On the dorsal aspect, between the 11 and 1 o’clock positions, there is again a dorsal thickening of the outer longitudinal tunica, a radiating aspect of the bilateral ischiocavernosus muscles. Distally they are grouped into the glans penis, forming the distal ligament, an indispensable continuation of the outer longitudinal layer of the tunica located at the 12 o’clock position of the distal urethra. It is arranged centrally, and acts as a trunk of the glans penis. The median septum is incomplete with dorsal fenestration at the pendulous portion of the penis, which is diverged so widely from that depicted in the traditional textbook. Its amount of fenestration is commensurate with the quantity of the intracavernosal pillars. Thus, at a distal penis, the pillars are numerous where the septum is most incomplete, in order to allow for the tensile capability.
The tunica albuginea had been consistently described as a single layer with uniform circumferential thickness. It is, however, unequivocally a bi-layered structure in which the outer longitudinal layer can be regarded as a tendon-like tissue of the finger in the extremities. Thus, it is indeed a continuing structure of the skeletal muscles positioned proximally. The two-layered design is functionally similar to that of a bicycle’s tire, where the inner tire is responsible for confining the inflated air (blood in a sinusoid of the penis), and the outer tire is responsible for supplying strength. It is, therefore, a bi-layered tunica which can meet the requirement for rigid erection. This anatomical knowledge is deemed to be a prerequisite for clinical surgeons who attempt to perform surgeries on the delicate tunica albuginea.
The distal ligament of the glans penis has been overlooked in our published reports. This unique anatomical arrangement may explain why the glans penis is strong enoughto bear the buckling pressure of coitus, as well as how an erect penis is sufficiently rigid but never compresses the corpus spongiosum, which, otherwise, would present an obstacle to ejaculation. Without this strong ligament, it would be like a Christmas tree without a trunk and an umbrella without a stick. A detailed comprehension of the penile anatomy is, therefore, certainly meaningful, and could provide a further foundation for penile surgical techniques and methods.
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