Topographically the penis is composed of three principal divisions: the root (radixpenis), the body (corpuspenis), and the glans (glanspenis). The root of the penis is composed of the two crura and the bulb of the penis. The body is primarily comprised of the two adjacent corpora cavernosa. The glans is subdivided into a bulbus glandis and a pars longa glandis. In the nonerect state the glans penis is entirely withdrawn into the prepuce. The prepuce is attached to the ventral abdominal wall except for its distal open end, which is free. The penis has two primary surfaces, a dorsal (dorsumpenis) and a ventral or urethral surface (fades urethralis). Measurements of the non-erect penis in more than 150 mature dogs of assorted breeds show that its length ranges from 6.5 to 24 cm, with an average of 17.9 cm.
The crus of the penis (cms penis) is the proximal end of the corpus cavernosum penis that is attached to the lateral aspect of the ischial arch and is covered by the ischiocavernosus muscle (). The thick tunica albuginea that covers the corpus cavernosum penis provides for this attachment. Within each crus is the blood-filled corpus cavernosum penis that extends distally into the body of the penis. The caudal external surface of each root is covered by an ischiocavernosus muscle (), which arises from the medial end of the ischial tuberosity.
The bulb of the penis (bulbus penis) (), formerly urethral bulb, is a partially bilobed, spongy, blood-filled sac that lies between the crura close to the ischial arch (). The bulb is continuous with the corpus spongiosum () surrounding the caudal part of the pelvic urethra and all of the penile urethra. The external surface of the bulb is covered by the median bulbospongiosus muscle ().
The corpus penis, or body, begins where the two crura join distal to the bulb. The cavernous bodies of each crus remain distinct as they join to form the body, because each is enveloped by a thick covering of coUagenous and elastic fibers, the tunica albuginea. On transection () the paired corpora cavernosa appear somewhat triangular and are separated by a median fibrous septum. Ventrally they form a groove for the ventrally lying penile urethra with its surrounding corpus spongiosum. The latter is not wrapped by the tunica albuginea. The body of the penis at midlevel is compressed in such a manner that its cross-sectional diameter is greatest dor-soventrally. This allows for lateral bending without twisting it upside down as the male dismounts to face in the opposite direction. This was described by Grandage (1972) as “flexible rigidity.” At the level of the glans the paired corpora have very thick fibrous wrappings and are fused with each other on the midline. They end by attaching to the base of the os penis. (In reality the os penis develops within the terminations of the corpora cavernosa and thus they are attached by virtue of their manner of formation , as shown by Evans ).
The corpus spongiosum, a continuation of the bulbus penis (), has its origin surrounding the urethra at the level of the prostate gland and extends caudally around the pelvic urethra and entire penile urethra. At the ischial arch it expands dorsocaudally into the bulb of the penis. It continues through the penile body and glans to the termination of the penile urethra. Thus the corpus spongiosum lies within the urethral groove of the os penis. Within the proximal glans there are numerous shunts from the corpus spongiosum that pass ventral to the margins of the urethral groove to supply the surrounding bulbus glandis with blood. The bulbus glandis is the most distensible portion of the penis in the dog. From one to four valves are located in each of the venous connections between the bulbus glandis and the corpus spongiosum, preventing blood from leaving the bulbus glandis by this route. At the distal quarter of the glans the urethra and its corpus spongiosum lie beneath the cartilaginous termination of the os penis before turning ventrally to terminate at the urethral orifice.