280 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 
venze cave 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. ARTERIZA. 
1. 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 foetal 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 
