COMPARATIVE ANATOMY OF THE CIRCULATORY SYSTEM 221 



the cloaca. They course along the body wall, resembling the parietal arteries, 

 and after giving off a network of branches into the walls of the cloaca and anas- 

 tomising anteriorly with the posterior end of the ventral abdominal artery, they 

 enter the pelvic fins. The dorsal aorta continues into the tail as the caudal 

 artery which is situated in the haemal canal immediately ventral to the centra 

 of the vertebrae. 



Draw the dorsal aorta and its branches. 



8. The structure of the heart. — The heart of elasmobranchs is a tube bent 

 into an S-shape and differentiated into four chambers. These chambers have 

 already been named. They are originally arranged in a straight line, but the 

 bending of the heart tube brings the ventricle in contact with the sinus venosus 

 and the auricle in contact with the conus arteriosus. The sinus venosus has 

 already been examined. It is a thin-walled chamber, triangular in form in the 

 dogfishes, tubular in the skate. Cut across the connections of the sinus venosus 

 with the transverse septum and also across the base of the ventral aorta, and 

 remove the heart from the body. Look into the previously opened sinus 

 venosus and find the large sin- auricular aperture which leads into the auricle. 

 It is guarded by a pair of valves formed of the smooth free edges of the sinus 

 wall. Note the shape of the auricle. It is a broad thin-walled chamber with 

 large lateral expansions on each side of the ventricle. Slit open the auricle and 

 wash out the contained blood clots. Note the folds in its wall. Find the 

 auricido-ventricular opening into the ventricle. It is guarded by two valves. 

 Each of these is a pocket of thin tissue, the opening of the pocket being directed 

 into the ventricle. Cut off the ventral half of the ventricle and also slit open 

 the conus arteriosus by a longitudinal ventral incision. Note the small U-shaped 

 cavity of the ventricle and its very thick, spongy walls forming numerous cavi- 

 ties and crevices in which the blood is held. Examine the auriculo-ventricular 

 valve from the ventricular side, and note the two pockets of which it is composed 

 and the attachment of the pockets to the ventricular wall. On the wall of the 

 conus arteriosus note the pocket-shaped semilunar valves, the pockets opening 

 anteriorly. In the spiny dogfish there are nine valves in three circles of three each ; 

 two circles are near the posterior end of the conus, while the third circle composed 

 of larger and stronger valves is near the junction of the conus with the ventral 

 aorta. In the smooth dogfish there are two circles of valves of three each, situ- 

 ated at the posterior end of the conus. The conus arteriosus of the skate bears 

 three longitudinal rows of valves with five valves in each row. To distinguish 

 the valves, run the point of a probe along the conus wall from the anterior end 

 backward, thus opening the pockets. 



Make a drawing to show the structure of the heart. 



The heart of elasmobranchs contains only venous blood. This enters the sinus venosus 

 from the systemic veins and passes in turn through the auricle, ventricle, and conus arteriosus, 

 and out into the ventral aorta which distributes the blood to the gills by way of the afferent 



