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This investigation was begun with the object of determining the relative number of capillaries in normal human hearts as contrasted with the number in abnormal hearts fr'om patients who had shown clinical signs of cardiac failure. In order to approach the problem it was first necessary to establish the size of the capillary bed and the distribution of the capillaries in normal hearts. The present paper, therefore, deals with the capillaries of the normal myocardium as determined b y injections through the coronary arteries in man, cats and rabbits, and with the methods used in making these injections.The coronary vessels have held the attention of numerous investigators for the past 3 centuries. Earlier workers studied their distribution by careful tedious dissections; and during the last few decades the vessels have been injected and outlined with various dyes and masses which have made possible a detailed study of the finer branches. Unfortunately, however, the injection masses used in most instances have failed to penetrate the capillaries and, as a result, the existing knowledge of this important group of vessels is very meagre. The same could be said, moreover, of the capillaries elsewhere in the body until the recent studies of Dale and Laidlaw (1) and Dale and Richards (2) on the effect of histamine upon the circulation, demonstrated the enormous capacity of the capillary bed. Krogh (3) subsequently directed attention to the number and behavior of capillaries in skeletal muscle, and, by his brillant investigations, opened a new pathway to this extensive but obscure branch of the vascular tree. In 1919, Richards (4, 5) made direct observations upon the kidney of a living frog and was able to study the behavior of the capillaries in that organ.The heart does not permit of direct observation of the circulation in its walls, although Drury and Smith (6) have observed the blood flow in the branch of the coronary artery which supplies the first part of the pulmonary artery in the 273

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