THE CIRCULATORY SYSTEM 25 



connects the cavities. The septum is formed as a bridge, 

 enabling the dorsal blood vessels to pass to the heart, which 

 is ventral in position. 



2. External Aspects of the Heart. 



The heart is attached at its posterior and anterior ends 

 by deflections of the parietal pericardium, which may be 

 located by lifting up the heart. The visceral pericardium 

 is an extension of this, which covers the heart and is indis- 

 tinguishably fused with its wall. 



The wings of the sinus venosus may be located on either 

 lower corner of the main mass of the heart as thin-walled 

 wide tubes. Make a very small incision in the right wing 

 and insert the probe and run it towards and into the main 

 mass of the heart. The wings of the sinus venosus are in 

 the transverse septum. Non-aerated blood from the various 

 parts of the body is returned to the heart through the sinus 

 venosus. 



During development the heart is at first a straight tube, 

 but later it becomes bent upon itself dorso-ventrally in 

 the form of a letter S. The walls of the tube are differen- 

 tiated into chambers which should be located on the ex- 

 terior. The first chamber is the sinus venosus already 

 located. The thin-walled second chamber just anterior to 

 the sinus venosus and on the dorsal side of the heart is 

 the auricle or atrium. It expands on each side of the an- 

 terior portion of the heart so that a portion of it may be 

 seen from a ventral viewpoint. The third chamber is the 

 thick-walled, heart-shaped, ventricle. The posterior lateral 

 end of the ventricle is the apex, the anterior side is the base. 

 The fourth chamber is the conus arteriosus, a heavy-walled 

 tube which originates at the base of the ventricle and pene- 

 trates the anterior pericardial wall. The blood, all of 

 which is non-aerated, circulates through the chambers of 



