410 C. H. DANFORTH 



large passage which opens from the pericardium into the coelom, 

 the posterior mouth being in the usual position ventral to the 

 oesophagus and dorsal to the liver. The pericardial mouth opens 

 dorsally between the entrance of the right and left ducts of 

 Cuvier. In a specimen where the greatest lateral diameter of the 

 pericardial cavity was 36 mm., the smallest diameter of the 

 pericardio-peritoneal canal was 5 mm. The cana itself was 

 about 15 mm. long. 



The heart is attached to the pericardial wall anteriorly by 

 the truncus arteriosus and posteriorly by the union of the great 

 veins with the sinus venosus, by two cords consisting each of 

 a coronary vein and a posterior coronary artery, and by fine 

 strands running from the septum transversum to lymphoid masses 

 on the ventricle. The attachment of the sinus to the pericar- 

 dial wall is somewhat in the form of an upright H, of which 

 the great veins form the limbs. In the dorsal notch of the H 

 is the pericardial opening of the pericardio-peritoneal canal and 

 through the ventral notch pass the coronary vessels. These 

 are in two free strands each consisting of an artery and a vein. 

 Usually the left vein and right artery are large and the right 

 vein and left artery small. The veins were not studied in detail; 

 the arteries will be discussed further on. 



The sinus venosus is very asymmetrical. On the left its 

 anterio-posterior diameter is short so that the ve ns almost enter 

 the auricle, but on the right, lateral to the mouth of the veins, 

 there is a saccular dilation equal in capacity to about one-third 

 of the auricle. Posteriorly there are a few internal trabeculae 

 or reinforcing strands, but these are not much developed. The 

 arterial blood supply, is in part at least, by small twigs of cor- 

 onary orgin coming in from the septum transversum. 



Externally the auricle appears nearly symmetrical but slightly 

 inclined to the left. It has a smooth surface and is somewhat 

 crenulated around the margin. In sagittal section it is in the 

 form of a right triangle, high behind and low and thin in front. 

 Within there is a strong development of trabeculae, mostly 

 flattened and forked, all around the edge except for a short 

 distance on the left, lateral to the auriculo- ventricular opening. 



