The ability of certain parts of the body to function in an essentially normal manner despite interruption of the major source of arterial inflow was recognized prior to the understanding of the true nature of the circulation. While studying the nature and treatment of arterial aneurysms in 200 A.D., Antyllus1 realized that in some instances the major vessel to a limb might be ligated without loss of the extremity. The first investigator to study collateral circulation experimentally was John Hunter,7 who ligated the main artery of an antler in a young deer, noted that the antler continued to grow, and observed the development of new vessels around the site of arterial occlusion. His succinct explanation of the development of this phenomenon was, "The blood goes where it is needed." This concept of collateral circulation has been improved upon only in detail in the past two centuries. The present