IX CIRCULATORY ORGANS 441 



bringing back the blood from the ventral parts of the head, 

 and each opening into the corresponding precaval. 



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

 jugulars, cardinals, and the genital veins, are dilated into 

 spacious cavities called sinuses (Fig. 113). 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 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 crayfish 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. 114). But whereas in both cray- 

 fish and mussel the respiratory organs are interposed in the returning 

 current — both their afiferent and efferent vessels being veins, in the dog- 

 fish they are interposed in sthe 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, compli- 

 cated 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 higher order or in the heart. 

 But the hepatic and renal portal veins end in capillaries after the man- 

 ner of arteries, and the efferent branchial arteries begin in capillaries 

 after the manner of veins. 



