NOTE ON THE CONFIGURATION OF THE HEART. 77 



extensively to the diaphragm. The ventricular surface of the 

 heart, which rests on the diaphragmatic pericardium, is flattened 

 and marked off from the back of the auricles by a rounded 

 border. The mesial plane cuts through the left third of the 

 right ventricle (Plate VI. figs. A. B. C). 



Order Marsupialia. 



Heart of Macro2)us rohustus. — Position in the thorax slightly 

 oblique. Lies behind the second, third, fourth, and fifth costal 

 cartilage of the left side, and the second, third, and fourth 

 of the right side. The apex is rounded and situated a little 

 to the left of the middle line. The mesial plane passes 

 through the anterior inter-ventricular septum. The heart is 

 somewhat conical in shape, and is not much longer than it is 

 broad. The inferior cava is long, and very little of the peri- 

 cardium touches the diaphragm. This is chiefly due to the 

 large, much curved ' azygos ' lobe of the right lujig intervening 

 between the diaphragm on the one hand, and the lower surfaces 

 of the ventricles of the heart and pericardium on the other. 



The pericardium is loosely attached to the back of the 

 sternum. 



Order Eodentia. 



Hearts of (a) Lagostotnus trichodactylus. 



(b) Sciurus vulgaris. 



(c) Mus decumamcs. 



More mesially situated than that of the marsupial, the apex 

 in each is blunt and rounded. The heart lies behind the second 

 and third costal cartilages of both sides. The mesial plane 

 passes through the anterior inter-ventricular furrow. The 

 heart in each animal is short and thick, and the under surface 

 of the pericardium is attached to the diaphragm by a thin 

 membranous fold of connective tissue. This spreads out as it 

 approaches the diaphragm, being narrow above, where it is 

 attached to the under surface of the pericardium. The attach- 

 ment of the pericardium to the diaphragm is very slight. 



