388 THE DOGFISH less. 



is taken by a pair of hepatic veins {Ii. v) to the sinus 

 venosus. 



The iliac veins {i/. v) from the pelvic fins pour their blood 

 into the lateral veins {lat. v), paired trunks running forwards 

 in the side walls of the body to the sinus venosus, and 

 receiving at their anterior ends the subclavian veins {scl. v) 

 from the pectoral fins. 



Some of the veins, especially the cardinals and spermatics, 

 are dilated into spacious cavities called sinuses. These are, 

 however, of a totally different nature from the sinuses of the 

 crayfish, which are mere spaces among the tissues devoid of 

 proper walls. In the dogfish, as in Vertebrata generally, the 

 blood is confined, throughout its course, to definite vessels, 

 the heart, arteries, capillaries, and veins invariably forming 

 a closed system of communicating tubes. 



The general course of the circulation will be seen to agree 

 with that already described in the crayfish and mussel : />., 

 the blood is driven by the contractions of the heart through 

 the arteries to the various tissues of the body, whence it is 

 returned to the heart by the veins or sinuses (compare 

 Figs. 88, 96, and 104A). But whereas in both crayfish 

 and mussel the respiratory organs are interposed in the 

 returning current, both their afferent and efferent vessels 

 being veins, in the dogfish they are interposed in the out- 

 going current, their afferent and efferent vessels being 

 arteries. An artery, it must be remembered, is a vessel 

 taking blood from the heart to the tissues of the body, and 

 having thick walls td resist the strain of the heart's pulsa- 

 tion ; a vein is a thin-walled vessel bringing back the blood 

 from the tissues to the heart. 



Moreover, the circulation in the dogfish is complicated by 

 the presence of the two portal systems, renal and hepatic. 

 In both of these we have a vein, renal portal or hepatic 



