+Treatment.+ As in other malignant tumors, the indication is for absolute and radical removal whenever possible. Unfortunately, this very seldom can be done, because at the time the bone has become affected by extension to any great degree, radical operation is impossible. Many times, however, extensive operations must be undertaken for the removal of bone. +Cysts of Bone+ are rare lesions which practically always occur secondary to other lesions. They may occur as the result of the degeneration and softening of bone sarcomata. Some of the cases of bone cysts undoubtedly represent the entire destruction of sarcomatous processes. Occasionally echinococcus cysts of bone occur. +Treatment.+ Cysts of bone due to softening of the centre of sarcomatous tumors, like sarcomata themselves, are to be treated by complete removal, best usually by amputation. Cysts of bone not due to the presence of sarcomatous tissue, should be opened and drained in some cases. Cysts due to the presence of echinococcus, should be opened and drained, with the removal of every vestige of the echinococcus. CHAPTER XI +DISEASES AND INJURIES OF THE ARTERIES AND GANGRENE+ +Gangrene+ is a term employed to denote the death of a part of the body, in mass. _Necrosis_ and _mortification_ are terms used in a similar sense though necrosis is reserved in surgery to mean death of bone. Gangrene may result from the gradual or sudden cessation of the arterial supply, or from a stoppage of the venous outflow. In general the etiology of gangrene comprises: 1. Traumatic causes. 2. Constitutional causes. 3. Thrombosis and embolism. 4. Cold. 5. The effect of certain chemicals. Before entering into a consideration of these subjects, it is wise to first consider the varieties of gangrene. There are two forms in which gangrene is observed: _dry_ and _moist_. +Dry gangrene+, or mummification, is a condition which occurs in consequence of a gradual diminution and final cessation of the blood supply, with the venous outflow intact. In this way, aided by evaporation and the venous return, there is a gradual drying of the parts. Diseases of the arteries and increasing pressure upon them from growing tumors, causes this variety. +Moist gangrene+ is due to the sudden arrest of the arterial supply, or a similar obstruction to the venous return. This is the variety commonly met with from crushing or cutting accidents; from the effects of carbolic and other acids; from cold; and from thrombosis and embolism. A _thrombus_ is a blood clot occluding the lumen of a vessel. An _embolus_ is a loosened part of a thrombus or any other foreign substance, free in the blood stream, such as a drop of fat, an air globule, or a detached particle of tissue from growths in the heart or vessels. Any one of these may find lodgment in a terminal vessel, and plug it.