1.

1. Despite the many papers that have been published on the lymphatics of the heart, much obscurity still prevails concerning their existence in the endocardium and their extent in the myocardium.

2.

2. Because neither the development of the channels in the human heart, nor that and the distribution of their valves have been studied before, the present communication aims to offer information in this matter.

3.

3. At the end of the second lunar month of intrauterine life, two plexiform extensions from the mediastinal lymphatics have entered the base of the heart. One, arising from the upper reaches of the thoracic duct and passing down in front and to the left of the aortic arch, comes to lie between pulmonary artery and ascending aorta and then grows ventral to the latter and along the right coronary artery. The other, a derivative of the pretracheal lymphatic plexus runs behind and to the left of the pulmonary artery to be prolonged along the left coronary artery. Both of these primary channels are permanently retained to constitute the efferent lymph conduits of the heart. During the third month a net of vessels growing out from them expands rapidly over the periphery of the heart. While that of the ventricular territory eventually becomes intricate and close-meshed, that over the auricular remains less well-developed. Extensions from this superficial or pericardial plexus invade the myocardium to form its plexus.

4.

4. At the end of the third or the beginning of the fourth month of prenatal life, the first valves have their inception in the developing pericardial lymphatic plexus, especially in the main pathways accompanying the coronary vessels; subsequently, they spring up also in the more distant stretches of the network, but in smaller numbers. Their mode of genesis—similar to that in other peripheral lymph channels of the body—varies according to whether they originate at confluences already completed or at confluences just being formed. In one case the valve arises from an endothelial proliferation that encircles the entrance of one vessel into another; in the other, it is produced from the tip or bud of a small vessel that grows at a tangent into the lumen of a bigger vessel.

5.

5. Regardless of the early demands put on the heart in the life of the individual, it acquires its perfect lymphatic organization no earlier than any other organ or region, with the exception of the jugular and mediastinal districts in which the earliest valves make their appearance at two months. In this respect, the heart does not even surpass an organ like the lung which is not called upon to serve until birth.

6.

6. As to distribution, more lymphatic valves are found on the anterior side than on the diaphragmatic side of the heart, at any rate in the fetus. With few exceptions all of these valves occurred on the ventricular portion; for example, of the nearly 300 valves counted in the heart of a 3.8 mo. fetus, not more than four or five were atrial in location. In the depth of the myocardium these structures are absent.

7.

7. Finally, the physiology of the lymph circulation of the heart is discussed: the direction of its flow, the mechanical factors influencing it, and its function.