It is a peculiarity of the cardiac circulation that there are channels that pass from coronary arterioles, from the capillary bed, and from the cardiac veins directly into the lumen of the heart. Irregular thin-walled channels of larger than capillary size, which are called “myocardial sinusoids,” also receive blood from the coronary arterioles or the capillary bed and communicate with the smallest cardiac veins that open directly into the chambers of the heart, particularly into the atria. It has been assumed that the stream in these veins can be reversed and thus help in nourishing the ischemic myocardium in case of coronary obstruction. Some of these openings in the cardiac cavity can be seen with naked eye by inspection of the endocardial lining. They vary from pinpoint size to almost 1 mm in diameter.