COMPARATIVE ANATOMY OF THE CIRCULATORY SYSTEM 209 



Now turn the animal dorsal side up and locate the lateral line. Make a 

 longitudinal incision above the gill slits on the left side along the lateral line in 

 the spiny dogfish and about one-third of the way between the lateral line and the 

 ends of the gill slits in the smooth dogfish. Deepen the incision until you break 

 into a large cavity with a smooth lining. This cavity is the anterior cardinal 

 sinus. Probe it anteriorly and follow your probe by an incision. The anterior 

 cardinal sinus can thus be traced forward above the spiracle to the eye, where 

 it connects with an orbital sinus surrounding the eyeball. At the level of the 

 posterior end of the eyeball is situated an opening in the ventral wall of the 

 anterior cardinal sinus. On probing into this it will be found to extend medially 

 into the skull. It is the opening of the interorbital sinus, which connects the 

 two orbital sinuses. Locate the hyoid arch. In the floor of the anterior cardinal 

 sinus, between the hyoid and third gill arches, is an opening. On probing this 

 it will be found to lead into a vessel which extends ventrally along the outer 

 surface of the hyoid arch. This vessel is the hyoidean sinus and it connects with 

 the inferior jugular vein. Next trace the anterior cardinal sinus posteriorly. 

 It turns abruptly ventrally and joins the posterior cardinal sinus. On probing 

 into the anterior cardinal sinus at the turn the probe will be found to emerge 

 into the posterior cardinal sinus. The union of the two sinuses forms the common 

 cardinal vein already described. 



Running along the lateral walls of the pleuroperitoneal cavity, immediately 

 external to the pleuroperitoneum, on each side is a conspicuous vein, the lateral 

 abdominal (often called simply lateral) vein. Note the parietal branches which 

 it receives segmentally from between the myotomes. Trace the right vein 

 anteriorly and find where it enters the common cardinal vein just in front of 

 the baglike expansion of the posterior cardinal sinus. Slit open the lateral vein 

 at its point of entrance into the common cardinal vein and find at the same 

 place immediately posterior to the pectoral girdle the opening of the subclavian 

 vein which drains the pectoral fin. The subclavian vein passes along the pos- 

 terior surface of the pectoral girdle in contact with the cartilage, and may be picked 

 up easily on the left side where the girdle has been cut across. The vein may also 

 be found by cutting across the base of the fin; it will then be seen in the section of 

 the fin as an opening dorsal to the fin rays in the posterior half of the fin. Next 

 trace the lateral vein posteriorly. It passes internal to the inner surface of the 

 pelvic girdle. In the spiny dogfish it then continues posteriorly along the lateral 

 margin of the cloacal aperture as the cloacal vein. At about the middle of the base 

 of the pelvic fin the lateral vein receives an iliac vein from the fin. The opening 

 of the iliac vein into the lateral vein will be found by slitting open the latter 

 vein. The iliac vein is a short vessel situated just under the skin of the dorsal 

 surface of the fin. In the smooth dogfish the two lateral veins connect with 

 each other by a cross vein which runs along the internal surface of the pelvic 

 girdle. Beyond this connection each vein continues as the cloacal vein along the 



