178 ZOOLOGY. 



pelvic fins : the remainder of the aorta behind this is 

 called the caudal artery. 



Now, at last, we can see the meaning of the great arterial 

 arches that come between heart and dorsal aorta in frog and 

 rabbit. Evidently they are the last traces of the arrange- 

 ment by which, in fishes, the blood is made to pass through 

 the gills to be oxygenated before it is distributed to the 

 capillaries of the tissues. This arrangement is not wanted 

 in animals which breathe air, but although there is a 

 tendency to get rid of it, even in the highest animals a 

 couple of arterial arches remain. 



10. Venous System. Instead of being narrow tubes, 

 many of the veins in the dogfish are irregular baggy sinuses, 

 enlarged to such an extent that some structures (particularly 

 nerves) appear to lie in them, though between the actual 

 nerve and the blood exactly the same layers intervene as 

 if the nerve ran alongside a vein. (Compare the case of 

 organs in the ccelom, chap., i. 6.) 



As in frog and rabbit, three main trunks enter the sinus 

 venosus. But the posterior one brings blood from the 

 liver only, and so is called the hepatic sinus. It may be 

 regarded as equivalent to the hepatic veins plus the most 

 anterior part of the postcaval, but its actual opening to the 

 sinus venosus is paired. At any rate, the main portion of the 

 postcaval is entirely absent in the dogfish. Owing to the 

 more anterior position of the heart (ventral to the throat) 

 the prexsav als come- to lie quite transversely instead of 

 obliquely forward, and are generally called the Cuvierian 

 sinuses. They receive near their confluence with the sinus 

 venosus the inferior ( = external) jugular vein* At their 

 dorsal origin they are formed by the meeting of the anterior 

 and posterior cardinal sinuses. The anterior cardinal sinus 

 (answering to the internal jugular vein of frog and rabbit) 

 lies dorsal to the gill-slits, and receives an orbital sinus from 

 around the eye. The posterior cardinal sinus receives a 

 subclavian and a lateral vein on either side and unites 

 posteriorly with its fellow in the middle line. This median 

 portion receives blood from the kidneys : it must not be 

 confused with the postcaval along with the right posterior 



