X CIRCULATORY ORGANS 457 



veins (scl. v, sc) from the pectoral fins. In addition to the 

 dorsally situated jugular sinuses, there are paired inferior 

 jugulars (Fig. 112, inf. jug} y bringing back the blood 

 from the ventral parts of the head and each opening into 

 the corresponding precaval together with the subclavian. 

 As we have seen, several of the veins, e.g., the pre- 

 cavals, jugulars, cardinals, and the genital veins, are 

 dilated into spacious cavities called sinuses (Fig. 122). 

 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 (pp. 374 and 377). In the 

 dogfish, as in the frog and Vertebrates 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 frog as well as in the cray- 

 fish and mussel, i.e., 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 (Fig. 123). 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 outgoing 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 ; a vein is 

 a thin-walled vessel bringing back the blood from the tissues 

 to the heart. 



Moreover, the circulation in the dogfish is, as in the frog, 

 complicated by the presence of the two portal systems, renal 

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

 hepatic portal, which, instead of joining with larger and 

 larger veins and so returning its blood directly to the heart, 

 breaks up, after the manner of an artery, in the kidney or 

 liver, the blood finding its way into the ordinary venous 

 channels after having traversed the capillaries of the gland 

 in question. 



Thus an ordinary artery arises from the heart or from an 

 artery of higher order and ends in capillaries ; an ordinary 

 vein arises from a capillary network, and ends in a vein of 



F F *** 



