12 8o THE CIRCULATORY SYSTEM. 



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 

 venaj cavcX 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. ARTERIiE. 



I. A. pulmonalis, the Pulmonary Artery (Fig. 115,/). 



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. 116,7). 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 fcetal 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 



