BY DR. BURDON-SANDERSON. 261 



right and left aorta. Of these aortic, which part from each 

 other at the middle line, the left is the larger. The posterior 

 wall of the right auricle extends backwards into a club-shaped 

 appendage, the sinus venosus. This body may be described 

 as the dilated end of the large vena cava inferior. It first 

 extends vertically upwards in the middle line, in continuity 

 with that vein, applying itself against the oesophagus behind, 

 and opening towards the front into the right auricle, from 

 which it is separated by a slight furrow. At the top it re- 

 ceives on either side the two venae cavas superiores, which, 

 however, are relatively small. The two auricles are separated 

 from each other by a septum, which stretches as a curtain from 

 before backwards, between them. This curtain ends below in 

 a crescentic margin, beneath which the two cavities communi- 

 cate freely. The orifice leading from the sinus venosus into 

 the right auricle is guarded by a well-marked Eustachian valve, 

 which hangs downwards and towards the right. The auriculo- 

 ventricular valve consists of an anterior and a posterior cur- 

 tain, both of which are continuous at their edges with the 

 auricular septum. 



The mode of exposing the heart has already been described. 

 The facts to be observed when the pericardium is opened are 

 the following: The series of muscular movements which are 

 performed by the heart each time it contracts is seen to begin 

 at the upper end of the vena cava inferior and sinus venosus. 

 From the sinus the peristaltic wave extends to the auricles; 

 but it is not until the auricular contraction is complete that 

 the ventricle suddenly draws itself together. Before this last 

 act is accomplished, it is usually seen that the sinus venosus 

 is full, and the auricles are already filling. In a moment they 

 become distended and contract, transferring the blood they 

 contain to the now empty and flaccid ventricle, which in its 

 turn forwards it onwards to the bulbus aortas and arterial 

 system. In consequence of the fact that during the contrac- 

 tion of the ventricle the auricles are already filling with blood, 

 and that the ventricle does not fill until the auricle contracts, 

 the successive appearances presented by the heart during each 

 cardiac period are very much as if there were a constant ex- 

 change of blood between the two great chambers into which 

 the organ is divided, and at once suggest the notion that the 

 auricles and ventricle dilate and contract alternately, the one 

 seeming to contract while the other dilates, and vice versa. 

 It is easy, however, for any one who possesses the faculty of 

 observation to satisfy himself that this is not the case, and 

 that, while the ventricular contraction is determined by the 

 auricular, and the auricular by that of the sinus, the last 

 originates of itself i. e., independently of any previous 

 movement. 



