The male urethra (urethra masculina) carries urine, semen, and seminal secretions to the distal end of the penis. In a 25-pound mature dog, the urethra averages 25 cm in length. It is divisible into a pelvic part (parspelvind) and a penile part (parspenind). The pars pelvina includes a preprostatic portion and a prostatic portion. In the dog the preprostatic part is essentially absent. The prostatic portion (parsprostaticd) of the urethra passes through the prostate gland (). The walls of the prostatic urethra are made up of a variable number of longitudinal mucosal folds. When distended, all the folds, except the dorsally located urethral crest (crista urethralis), are obliterated. The seminal hillock (colliculus seminalis) is an oval enlargement, located at the center of the urethral crest, which protrudes into the lumen of the urethra. The ampullae of the deferent ducts open on each side of the colliculus seminalis. The opening of each duct is usually not visible macroscopically unless fluid is forced from the ducts and ampullae into the urethra. Numerous prostatic ducts also open into the urethra adjacent to and surrounding the urethral crest. Instead of being round or oval, a cross-section through the middle of the prostatic urethra appears U-shaped. The center of the colliculus seminalis contains a minute opening into a tiny tube, the uterus masculinus, which runs craniodorsally into the prostate. The uterus masculinus, also known as the prostatic utricle and utriculus prostaticus, is a homologue of the caudal portion of the paramesonephric ducts in the female. Caudal to the prostate, the pelvic urethra contains a thin layer of vascular tissue, the stratum spongiosum. This cavernous tissue is continued into the penile part of the urethra as the corpus spongiosum, which includes its expansion at the ischial arch as the bulb of the penis.
The penile part of the urethra is a continuation of the pelvic part located in the pelvis. The penile part begins at the ischial arch. It is surrounded by corpus spongiosum for its entire length. At the ischial arch, the bulb of the penis is an expansion of this spongy tissue.
The stratum spongiosum of the urethra is composed of vascular erectile tissue that is continuous in the penis with the corpus spongiosum. Peripheral to the vascular layer of the pelvic urethra is the urethral gland layer (glds. urethrales) consisting of isolated small branched glands. The muscular layer (tunica muscularis) consists of smooth muscle, primarily longitudinal, which extends from the prostate, which it overlaps slightly, to the point where the stratum spongiosum ends at the bulb of the penis. Within the pelvis, peripheral to the smooth muscle layer, is the thick striated m. urethralis, which consists of transversely running fibers that are separated dor-sally by a thin, longitudinal fibrous raphe.
Male Urethra: Vessels and Nerves
The prostatic part of the pelvic urethra is supplied with blood through the prostatic artery. The stratum spongiosum is supplied by small urethral arteries that branch off the internal pudendal, urethral, or prostatic arteries. The penile part is supplied through the artery of the bulb of the penis. The urethral veins are satellites of the arteries, draining into the internal pudendal vein. The smooth muscles of the urethra are innervated by the autonomic nerves derived from the pelvic plexus. The striated urethralis muscle is innervated by somatic efferent axons in the pudendal nerve.
Male Urethra: Anomalies and Variations
The length and diameter of the urethra vary within wide limits. When the penis is flaccid, the urethral mucosa is folded longitudinally and the lumen is obliterated. During urination or ejaculation, the urethral walls are distended. Only that part of the cavernous urethra that passes in the ventral groove of the os penis is limited in its expansion. Fracture of the os () can obstruct the urethra. The urethra may infrequently open on the ventral surface of the penis (hypospadias) or on the dorsal surface (epispadias). Hypospadias () is considered to result from failure of the urethral groove to close normally, whereas epispadias is caused by an embryonic displacement of cells forming the cloacal membrane, resulting in a reversal of the cloacal membrane location.