Frenulum.

Frenulum.
The frenulum is a band of tissue that extends from the ventral tip of the glans penis to the prepuce. Separation of the penis from the sheath normally occurs spontaneously at 3-4 months of age in the boar. When the frenulum persists there is frequently a small artery present in the center of the tissue band comprising the frenulum. Persistence of the frenulum prevents the penis from fully extending.

Evans LE (2009)