Besides the inlet from the bladder by which the urine escapes, there are a number of openings into the urethral canal. In the prostatic portion, on either side of a small ridge of mucous membrane, is the orifice of the "ejaculatory duct", through which the seminal fluid is discharged in the act of copulation. These openings in aged stallions are of considerable size, in some instances sufficiently large to admit the point of the index-finger. This fact must be borne in mind when passing the catheter, lest the instrument becomes lodged in one of them. To guard against it the point should be well pressed down on to the floor of the canal by the fingers acting through the perineum.
A little behind the ejaculatory ducts are two rows of small openings from the prostate gland, and beyond these occur others similarly arranged proceeding from Cowper's glands.