Figure 1. Comparison of penile structure in various species; cross sections of the corpora cavernosal and longitudinal aspect of the glans penis are from A to C and D to F, respectively. (A) In rats, the corpus cavernosum, devoid of the medial septum and intracavernosal pillars (ICPs), is positioned between the deep dorsal vein (arrow) and the urethra (arrowhead) (hematoxylin-eosin, reduced from 7x). (B) In dogs, a complete septum (arrow) and abundant ICPs are obvious (1x). (C) In human beings, an ICP is not uncommonly encountered (data not shown). A septum (arrow) is significant but is incomplete and dorsally fenestrated. Note the clear delineation of the inner circular and outer longitudinal layers of the tunica albuginea (1x). (D) In a rat, a short os penis is positioned between the glans penis and the corpus cavernosum (left panel, 3x). The amount of glanular tissue is scanty. The junction between the glans penis and the corpus cavernosum is similar to a knee joint and provides a flipping action during mating. The short os penis (right panel, 1x) can be better demonstrated after clearing and alizarin red S staining because only bony tissue is observed. (E) In a dog, the os penis (double-headed arrow) is enveloped with a unique glans penis of 2 compartments (arrowhead and arrow). Similar to the rat penis, the corpora cavernosa are not intromitted. However, they are reinforced with abundant ICPs and a complete septum (1x). (F) In a human being, the distal ligament (arrow) within the glans penis is obvious and should be regarded as a ligamentous structure rather than sinusoidous only. The distal ligament is an aggregation of the outer longitudinal layer of the tunica albuginea and acts as a buttress for the glans penis (1x).