Tunica albuginea (penis)
The tunica albuginea is the fibrous envelope that extends the length of the corpora cavernosa penis (containing erectile tissue) and corpus spongiosum penis (containing the male urethra). It is a bi-layered structure that includes an outer longitudinal layer and an inner circular layer. It consists of approximately 5% elastin, with the remainder mostly consisting of collagen. The tunica albuginea is directly involved in maintaining an erection; that is due to Buck's fascia constricting the erection veins of the penis, preventing blood from leaving and thus sustaining the erect state. The erection veins include the deep  dorsal vein, two cavernosal veins, and four para-arterial veins. The trabeculae of the tunica albuginea are more delicate, nearly uniform in size, and the meshes between them smaller than in the corpora cavernosa penis: their long diameters, for the most part, corresponding with that of the penis. The external envelope or outer coat of the corpus spongiosum is formed partly of unstriped muscular fibers, and a layer of the same tissue immediately surrounds the canal of the urethra.