prostatic urethra prostatic urethra the prostatic urethra, the widest and most dilatable part of the urethra canal, is about 3 cm long. prostatic urethrathe male urethra laid open on its anterior (upper) surface. (prostatic part labeled at upper right.)1: vas deferens2: seminal vesicle3: base of the prostate4: apex of the prostate5: prostatic urethradetailsidentifierslatinpars prostatica urethraeta98a09.4.02.004ta23445fma19673anatomical terminology it runs almost vertically through the prostate from its base to its apex, lying nearer its anterior than its posterior surface; the form of the canal is spindle-shaped, being wider in the middle than at either extremity, and narrowest below, where it joins the membranous portion. a transverse section of the canal as it lies in the prostate is horse-shoe-shaped, with the convexity directed forward. the keyhole sign, in ultrasound, is associated with a dilated bladder and prostatic urethra. additional images lobes of prostate zones of prostate structure of the penis vertical section of bladder, penis, and urethra. vesiculæ seminales and ampullæ of ductus deferentes, seen from the front.