Soft tissue, on the other hand, isn't as dense, and contains mostly lower atomic number elements, like carbon, hydrogen, and oxygen. So more of the X-rays penetrate tissues like lungs and muscles, darkening the film.

　　相反的，软组织不是那么致密，大部分含有较低原子序数的元素，如碳、氢及氧。所以比较多的X-射线能穿透如肺及肌肉组织，使底片变得较黑。