Ejaculatory Role of Pelvic Urethra

Above: Transverse section through the prostatic urethra of a dog (color added). The urethral lumen (asterisk) is surrounded by lamina propria-submucosa (blue). The colliculus seminalis produces a bulge in the dorsal submucosa. Smooth muscle has an insignificant sphincter role, none surrounds the urethral lumen. Small amounts of smooth muscle are present on the surface of the prostate and within trabeculae partitioning prostatic lobules (arrows). Thus the prostatic urethra has a passive, elastic role during ejaculation. The wall of the urethral lumen is passively stretched as sperm and prostatic fluid are added under pressure and the ejaculate is temporarily trapped by contraction of sphincters cranial and caudal to the prostatic urethra.

Below: Transverse section through the canine postprostatic urethra (color added). The urethralis muscle (brown) forms a thick striated muscle coat encircling the lamina propria-submucosa (blue) and urethral lumen (asterisk). Venous sinus profiles of the stratum spongiosum within the submucosa (red) become engorged during erection, which decreases the volume and stiffens the wall of the urethral lumen.

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