The urethra (d and K, fig. 220) is a long tube extending from the bladder to the end of the penis. It conducts the urine out of the body, and serves also to convey the spermatic fluid and some other secretions accessory to reproduction into the female organs of generation in the act of copulation.

In proceeding from the neck of the bladder, the urethra passes backwards for a short distance along the floor of the pelvis; then curving downward over the ischiatic arch, it takes a direction forward between the two branches or crurae of the corpora cavernosa, from which it continues onward, enclosed in the corpus spongiosum, to the glans penis. There it ends in a small cylindrical projection termed the "urethral tube". On leaving the neck of the bladder it is surrounded by the prostate gland, and then for a farther short distance by a thin muscle, the " compressor urethrae", after which it is enclosed in the corpus spongiosum for the remainder of its course, excepting the small terminal prolongation from the glans penis.

These three divisions are distinguished as (l) the "prostatic", (2) the "muscular" or "membranous", and (3) the "spongy" portion. An inspection of the interior of the canal shows it to vary in diameter in different parts of its length. The smallest part is the external opening, which will admit a catheter about half an inch in diameter. It may be concluded, therefore, that any instrument that will enter this opening may be safely pressed on into the bladder. The larger parts are the prostatic, and those situated at either extremity of the spongy portion.