This is mainly due to the greater development of an unpaired, median portion, analogous to the mammalian corpus spongiosum, which is much less prominent in the Ratitae; the muscles of this type are derived solely from the anal sphincter.
The bulbs of the vestibule are two masses of erectile tissue situated one on each side of the vaginal orifice: above they are continued up to the clitoris; they represent the bulb and the corpus spongiosum of the male, split into two, and the fact that they are so divided accounts for the urethra failing to be enclosed in the clitoris as it is in the penis.
In a transverse section two of these cylinders (the corpora cavernosa) are placed above, side by side, while one, the corpus spongiosum, is below.
Posteriorly, at what is known as the root of the penis, the two corpora cavernosa diverge, become more and more fibrous in structure, and are attached on each side to the rami of the ischium, while the corpus spongiosum becomes more vascular and enlarges to form the bulb.
It has already been pointed out that the whole length of the corpus spongiosum is traversed by the urethra.
The anterior part of the penis is formed by the glans, a bell-shaped structure, apparently continuous with the corpus spongiosum, and having the conical ends of the corpora cavernosa fitted into depressions on its posterior surface.
The structure of the corpus spongiosum and glans resembles that of the corpora cavernosa, but the trabeculae are finer and the network closer.
At first it lies in the substance of the bulb and, later, of the corpus spongiosum, while finally it passes through the glans.