Most mammals have a bone called the baculum or os penis in the penis, but it is absent in humans and horses. Although there are no bones in a human penis, a man can still suffer from penile fracture, which is when the thick membrane surrounding the erectile tissue tears. There is usually a "cracking" sound, an immediate loss of erection, and a great deal of pain. In extreme cases, the urethra can be damaged, which affects the ability to urinate.