Intravascular complications of surgical operations are common and, to a great extent, mysterious. Too often phlebitis is regarded simply as a nuisance and embolism as an act of providence. Yet the courses of these complications, and their relationships as well, are uniform enough in many cases to indicate the value of certain procedures designed either to prevent the disease or to treat it after it has developed. Many problems present themselves immediately. Some of them are highly theoretical, others are entirely academic, but many are extremely practical. Unfortunately, even the most practical aspects must be based to a large extent on theory, because direct investigations of intravascular disorders and physiology are rare. This study has avoided as much theory as possible and has been elaborated about one central problem—the development of therapeutic guides. To establish a comprehensive background, a survey was first made of 250 cases of thrombophlebitis of the