CIECULATORY SYSTEM 223 



2. The heart is a single tube twisted on itself in an 

 S-shape, and dilated to form a series of chambers 

 separated by transverse constrictions. 



Determine the position and general characters of the cham- 

 iers in order from behind forwards, pressing them aside when 

 ■necessa/ry, hut making no further dissection. 



a. The sinus venosus is a thin-walled transversely 



placed tube, attached along its whole length to 

 the posterior and dorsal waU of the pericardial 

 cavity. It receives laterally the veins returning 

 blood from the body, and opens in front into the 

 auricle by a median aperture. 



To expose the sinus venosus, press the ventricle slightly 

 forwards. 



b. The auricle is a large thin- walled sac, occupying the 



dorsal half of the pericardial cavity. It is tri- 

 angular in shape, with its posterior angles pro- 

 duced into processes which, in the natural position 

 of the parts, project laterally at the sides of the 

 ventricle. 



To expose the auricle, push the globular ventricle to one 



c. The ventricle is a thick-walled globular sac lying 

 ventral to the auricle, and forming the most con- 

 spicuous part of the heart. 



d. The conus arteriosus is a cylindrical tube, running 

 forwards from the ventricle to the anterior end of 

 the pericardial cavity. 



The pericardio-peritoneal canal is a median passage about 

 an inch long, opening in front into the hinder end of 

 the pericardial cavity dorsal to the sinus venosus, and 

 running back along the ventral surface of the ceso- 

 phagus, to open behind into the abdominal cavity. Its 

 anterior opening is large and conspicuous ; the open- 

 ing into the abdominal cavity, which may be double,, 

 is smaller and slit-like. 



