Third-class levers are plentiful in human anatomy, with the elbow and biceps brachii combining to provide the most commonly used example.
Just as there are many remote causes of plague—heat, insects, rats—but only one common cause, the plague bacillus, there are a great many remote causes of cancer—tar, rays, arsenic, pressure, urethane—but there is only one common cause into which all other causes of cancer merge: the irreversible injuring of respiration.