This vessel may be easily injected from the superior or inferior vena cava. It arises from the infundibulum of the right ventricle: its direction is upwards, backwards, and to the left side ; and after a course of about an inch and a quarter, it terminates by dividing into a right and left branch. In the angle between these branches, but more connected with the left than with the right, the ductus arteriosus arises : this vessel in the foetus equals in size, and seems like a continuation of, the pulmonary artery; it terminates in the concave side of the arch of the aorta, a little beyond the origin of the left subclavian artery. Superiorly, and to the right side of the bifurcation of the pulmonary artery, we see the bifurcation of the trachea into the right and left bronchial tubes. Between the division of the artery below, and that of the trachea above, we find a space somewhat of a lozenge shape, which is filled with a considerable quantity of areolar tissue, a number of black bronchial glands, together with numerous branches of the pulmonary plexuses of nerves, chiefly those derived from the posterior. The pulmonary artery, after its origin, forms a curvature, the convexity of which looks forwards and to the left side, and is covered by the serous layer of the pericardium, with the interposition of some adipose tissue: its concavity looks backwards and to the right side, and corresponds to the commencement of the aorta: on either side it is related to the appendix of the corresponding auricle. The pulmonary artery, unlike the aorta, does not in the undisturbed state retain its cylindrical form; this is owing to the comparative thinness of its proper or middle elastic coat. We have already mentioned that this vessel and the commencement of the aorta have a common sheath formed by the reflexion of the serous layer of the pericardium : within this sheath, and behind and between the vessels, filaments of the sympathetic nerve descend to form the coronary plexuses. If we now cut into the artery, and examine its interior, we observe that there are three semilunar valves at its orifice, and that an incision through its anterior part will divide one of them; whereas an incision into the anterior part of the aorta would nearly separate two of them, viz., the right from the left.

The middle or proper coat of the pulmonary artery will be found to take its origin from the arterial zona tendinosa situated at the termination of the infundibulum of the right ventricle, by a festooned margin presenting three convexities or inverted arches, separated from each other by a small triangular interval, in which we find no proper arterial tunic. The connection between the three inverted arches and the zona tendinosa will be best seen by dissecting the parts from the interior of the ventricle. The muscular fibres of this portion of the ventricle will be seen attached to the lower margin of the tendinous zone, whilst the three inverted arches of the middle coat of the artery will be found connected with its upper margin by condensed areolar tissue. Corresponding to each of the three small triangular intervals between the inverted arches of the middle coat, we will find a fibrous prolongation sent up from the upper margin of the zona tendinosa; this becomes ultimately incorporated with the condensed areolar tunic external to the middle coat. The endocardium within, and the serous layer of the pericardium without, though but partially, complete the connection between the artery and the ventricle. Between each of these convexities and the area of the vessel, we find a corresponding semilunar valve formed by the lining membrane of the artery, the concavity of which looks upwards and is strengthened by a small body called the corpus sesa-moideum or corpus Arantii.

The Right Pulmonary Artery

The Right Pulmonary Artery crosses transversely behind the aorta and superior cava, to which consequently its anterior surface corresponds with the interposition of the serous sheath of the aorta. Posteriorly and superiorly it corresponds to the right bronchus, and infe-riorly to the right auricle.