The Left Pulmonary Artery, shorter than the right, and less horizontal, ascends in front of the left bronchus, being covered anteriorly by the serous layer of the pericardium, except in the immediate vicinity of the lung, where it is covered by its corresponding veins. Above and behind it, is the arch of the aorta ; beneath it, 10 the superior wall of the left auricle, and in front of it is the left auricular appendix.

It may not be considered out of place to quote the following observations of Dr. Hope, as to the relative positions of the heart and its great vessels with regard to the parietes of the chest.—"A line drawn from the inferior margin of the third ribs, across the sternum, passes over the pulmonic valves a little to the left of the mesial line, and those of the aorta are behind them, but about half an inch lower down. From this point the aorta and pulmonary artery ascend; the former inclining slightly to the right, coming in contact with the sternum, when it emerges from beneath the pulmonary artery, and following, or perhaps rather exceeding, the mesial line till it forms its arch; the pulmonary artery, which is, from the first, in contact with the sternum, inclining more considerably to the left, until it arrives at the interspace between the second and third ribs above described. A vertical line coinciding with the left margin of the sternum has about one-third of the heart, consisting of the upper portion of the right ventricle, on its right; and two-thirds, composed of the lower portion of the right ventricle, and the whole of the left, on its left. The apex beats between the cartilages of the fifth and sixth left ribs, at a point about two inches below the nipple, and one inch on its sternal side.

" The lungs descend along the margins of the sternum, about two inches apart, and overlap the base of the heart, slightly on the right side and more extensively on the left: then, receding from each other, they leave a considerable portion of the right ventricle, and a less extent of the lower portion of the left, in immediate contact with the thoracic walls.

"The right auricle is in front of the heart, at its right side and upper part. One portion of it is overlapped by the right lung, and another, principally the appendix, is in contact with the sternum. The left auricle is situated deeply behind and to the left of the heart at its upper part, opposite to the interval between the cartilages of the third and fourth ribs. The extremity of the appendix is visible in front, but, when the volume of the heart is natural, it is not in contact with the sternum, being considerably overlapped by the left lung. The auricular orifices are situated opposite to the interspace between the third and fourth ribs, and the right is rather lower down than the left."*

* Hope on Diseases of the Heart, etc., pp. 3-4.