The pericardium is a sac enclosing the heart. It lies in the middle mediastinum, and the two halves of the mediastinal septum with their fat may be dissected away from it. Its wall is composed of two layers, an external firm fibrous layer and an internal layer of flattened epithelial cells similar to the peritoneal epithelium (=serous= layer). The fibrous layer forms a sac which repeats roughly the form of the heart. This sac is attached to the aorta at the point of origin of the subclavian artery, to the pulmonary artery at its bifurcation, and to the venæ cavæ and pulmonary veins near their entrance into the heart. At these points it is continuous with the fibrous coats of the vessels named, and from them it is reflected over the heart, forming a complete sac enclosing it but not attached to it anywhere. The heart lies within this sac. The serous layer lines the fibrous sac and gives to the surface of the heart and fibrous layer a smooth glistening appearance. It is reflected over the heart. The relation of the heart to it is much the same as the relation of the intestine to the peritoneal sac. The serous layer consists therefore of two portions, =parietal=, lining the sac, and =visceral=, covering the heart. The parietal and visceral portions are continuous along a line which runs approximately parallel to the auriculoventricular groove and encloses all the great blood-vessels. Within this line the heart lies against the fibrous layer of pericardium and is not covered by the serous layer. The serous layer is easily dissected free from the heart-wall, but its parietal portion is closely adherent to the fibrous layer. II. THE ARTERIES. ARTERIÆ. 1. =A. pulmonalis, the Pulmonary Artery= (Fig. 115, _f_). The pulmonary artery passes craniodorsad and slightly to the left from the cranial end of the conus arteriosus. One to one and a half centimeters from the conus it divides into right and left branches (Fig. 116, _j_). Just before the division the dorsal surface of the pulmonary artery is connected by the short =ligamentum arteriosum=, or ligamentum Botalli, with the aorta. This is the remnant of a canal which in fœtal life forms a free communication between the pulmonary artery and the aorta; this canal is known as the =ductus Botalli=. The ligament is almost or quite obliterated in the adult cat. The left branch of the pulmonary artery passes to the left lung, crossing ventrad of the thoracic aorta. It then divides into branches which pass to the lobes of the left lung. The point of division of the left branch of the pulmonary artery lies craniad of all the lobes of the lung, so that the lobes of the left lung are said to be all =hyparterial=, i.e., below (or caudad of) the artery.