VEINS 



317 



asymmetrical andforces the hepatic veins to empty into the right side of the 

 heart (fig. 336)y\^he heart in all cycostomes has a rudimentary conus. 

 The hepatic portal receives a vein from the head, and then passes back to a 

 contractile portal heart, just before it enters the liver. /\ 



FISHES. — In the fishes, the dipnoi excepted, the circulation corresponds 

 rather closely in its main features with the primitive condition or embryonic 

 described above. The purely venous heart is situated in the floor of the pharyn- 



FiG. 335. — Oblique ventral view of venous system of Petromyzon, drawn from a 

 corrosion preparation (Princeton, 669); ac, precardinal; c, caudal; gs, genital sinus; hv, 

 hepatic vein; ij, inferior jugular; pc, postcardinal ; sv, sinus venosus; va, ventral aorta. 



geal region, between and behind the last pair of branchial arches. The atrium 

 is dorsal to the ventricle. In the elasmobranchs and ganoids the valves of the 

 conu are arranged in several transverse rows (3-8), but in the teleosts (J5M/yrJnM5 

 and Osteoglosstifti excepted) they are reduced to a single row, apparently corre- 

 sponding to the first of the lower forms. The aortic arches correspond in number 

 to the functional gill clefts — six or seven in the notidanid sharks, five in other 

 elasmobranchs, and at most four in ganoids and teleosts. The carotids of the 

 two sides are usually connected by a transverse 

 vessel. Paired inferior jugulars are usually pres- 

 ent, but they are lacking in Polypierus, while in 

 Lepidosteus and in many teleosts they are united 

 at the base into a single trunk which empties 

 directly into the sinus venosus. Epigastric veins 

 are usually present, but are lacking in many 

 bony fishes.'^he elasmobranchs are charac ter ized 

 by the presence of very large sinuses in the post- 

 cardinal and hepatic veins. >^ Valves in the veins 

 are common in the elasmobranchs. ^ 



DIPNOI. — In this group the atrium, in cor- 

 relation with the development of lungs, becomes 

 partially divided, as described above (p. 297). 

 No true atrioventricular valves occur, their place 

 being taken by a strong ridge on the inner side 



of the atrial cavities. This, at the time of systole, closes the canal and at 

 the same time partially divides the ventricle into arterial and venous halves. 

 The spirally twisted conus has eight rows of valves and in Ceratodus the short 

 truncus shows the beginning of a division (completed in Protopterus) separating 

 the arterial from the venous arches. This separation is effected by a longitudi- 

 nal fold which may be the result of the fusion of a row of conal valves. In all 

 there are four aortic arches, the anterior two in Protopterus receiving arterial 

 blood. The principal veins are shown in fig. 334, the most important features 



Fig. 336. — Heart of lamp- 

 rey, after Rose, a, atrium; cd, 

 cs, left and right postcardinals; 

 h, hepatic vein; jd, right jug- 

 ular; ji, js, inferior and su- 

 perior left jugulars; /, truncus 

 arteriosus; v, ventricle. 



