The male organ of copulation, or phallus. In mammals the penis consists basically of three elongated masses of erectile tissue. The central corpus spongiosum (corpus urethrae) lies ventral to the paired corpora cavernosa. The urethra runs along the underside of the spongiosum and then normally rises to open at the expanded, cone-shaped tip, the glans penis, which fits like a cap over the end of the penis. Loose skin encloses the penis and also forms the retractable foreskin, or prepuce. The organ is held firmly in place by fibrous tissue and ligaments that bind it to the posterior surface of the pubic arch. In some mammals, although not in man, there may be spines on the glans penis, or there may be a bone (the baculum) within the penis; both of these characters are useful in classifying certain types of mammals.