NECTURUS 67 



and contains the larger part of the viscera. The anterior division 

 is the pericardial cavity ; it is lined by the pericardium and is very 

 small, containing only the heart. These two divisions are separated 

 by the false diaphragm, which is composed of the posterior wall 

 of the pericardium and the anterior wall of the peritoneum. 



Note that the organs in the abdominal cavity are attached to its 

 walls by delicate membranes. These are the mesenteries, which are 

 folds of the peritoneum. This membrane forms a closed sac lining 

 the abdominal cavity; the various organs of this cavity project 

 into the sac and are suspended in peritoneal folds which form the 

 mesenteries and extend into its interior from its walls. 



The most prominent organ in the abdominal cavity is the liver, 

 an elongated, dark-colored body which is attached to the ventral 

 wall of the cavity by a median, vertical mesentery. At the right 

 of it (the animal's left) is the stomach, which is of about the same 

 length as the liver, and at the left of the stomach is the elongated, 

 dark-colored spleen. Between the hinder end of the stomach and 

 the liver will be seen the cream-colored pancreas. Behind the 

 stomach are the coils of the intestine, attached to the dorsal body 

 wall by an extensive mesentery. Between the stomach and spleen 

 and the dorsal body wall may be seen the narrow, cylindrical left 

 lung. At the posterior end of the abdominal cavity is the urinary 

 bladder, a large sac which may appear large and distended or very 

 much contracted. 



The Heart and the Pericardial Cavity. Carefully cut away the 

 body wall over the heart and expose the pericardial cavity. The 

 heart is composed of five divisions,— a single ventricle, two auri- 

 cles, the sinus venosus, and the truncus arteriosus. 



The ventricle is the conical posterior portion of the heart. 

 The auricles are a pair of thin-walled and dark-colored sacs in 

 front of the ventricle; they are not completely separated from 

 each other. The sinus venosus is a large, thin-walled sac which 

 lies dorsal to the ventricle and is in communication with the 

 right auricle; it will be seen by turning the ventricle to one 

 side. The truncus arteriosus is the forward continuation of the 

 ventricle; it is composed of two portions,— the conus arteriosus, 

 which is immediately in front of the ventricle, and the bulbus 



