72 DR CHARLES J. PATTEN. 



intervening). This surface is not distinctly tiattened, and looks 

 backwards as well as downwards.^ 



5. The inferior vena cava within the thorax consists of a very 

 short intra-pericardial stage (as in man), and a longer extra- 

 pericardial stage. 



The formalin heart in man contrasts with the above as 

 follows : — 



1. It is rather trihedral than conical in shape. 



2. It lies very obliquely in the thoracic cavity. 



3. The margo acutus is, according to position, much more of 

 an inferior than a right border. 



4. The whole of the facies diaphragmatica (so-called posterior 

 surface of the ventricles) rests on the diaphragm (the pericardium 

 intervening), is flattened, and looks almost directly downwards. 

 The extra- as well as the intra-pericardial stages of the thoracic 

 inferior vena cava are very short. 



The difference in position of the margo acutus of the human 

 heart as compared with the same in lower animals seems to 

 depend on — 



Firstly, the oblique position which the human heart has 

 assumed in the thoracic cavity. 



Secondly, the shortening of the cxtra-pericardial paYt of the 

 inferior vena cava. This took place as the heart more intimately 

 approached the diaphragm, the organ subsiding, as in the case 

 of many other viscera, when man assumed the erect posture. 

 The result is that in man the right auricle in its lower part 

 approaches and even touches the diaphragm (the pericardium 

 intervening), and extends lower down than in the case of 

 quadrupeds. 



In tracings taken of the human heart in situ (not necessarily 

 hardened in formalin), it may be seen that the lower end of 

 the right auricle extends nearly as far down on the right side 

 of the thorax as the ventricular apex extends on the left side. 

 Hence the border joining the two runs almost transversely 

 across the chest (Plate V., m.a.), from right to left. This border 

 is the margo acutus. 



The position of the riyht border or margin of the heart //; 



^ The above animals are supposed to be standing erect like man. 



