At metamorphosis the left and right posterior cardinals 

 join anteriorly and drain through the right duct of Cuvier 

 into the sinus venosus. 



HAGFiSH In the tail, small twigs enter lateral vessels arising 

 from two caudal venous hearts. The hearts are elongated 

 tubes which in section are pyramidal, the apex pointing up- 

 ward. Usually both are filled with red corpuscles, but ordi- 

 narily one is distended more than the other; from this it is as- 

 sumed that they contract alternately. 



The lateral veins accompany arteries on either side of the 

 cartilaginous median ventral plate in the caudal region. They 

 unite to form the median caudal vein, which runs forward im- 

 mediately beneath the caudal artery. In the cloacal region, 

 the caudal vein divides into the right and left posterior car- 

 dinal veins. These vessels run just below and on each side of 

 the dorsal aorta, internal to the niesonephros (i.e. subcar- 

 dinal channels). The right posterior cardinal is much smaller 

 than the left. The posterior cardinals are joined by a large 

 number of commissural vessels. 



The posterior cardinals receive no veins from the intes- 

 tine, but they do receive renal and somatic branches. An- 

 teriorly, a short distance behind the heart, the right and 

 left posterior cardinals unite, and the left becomes the main 

 channel to the sinus venosus. 



The "subintestinal" vein arises from the ventral wall of 

 the intestine toward the anterior end. It passes forward 

 along the median ventral line of the intestinal wall and, on 

 reaching the hepatic ligament, passes down along its poste- 

 rior margin to the posterior lobe of the liver. In some speci- 

 mens it passes through the tissue of the liver for a consider- 

 able distance, but in others it runs forward on the ventral 

 and external aspect. It receives branches from the posterior 

 lobe of the liver, and becomes the "posterior hepatic vein." 

 As such it passes upward, parallel to and near the bile duct 

 of the posterior lobe, and empties into the posterior end of 

 the sinus venosus. 



The veins of the anterior lobe of the liver converge to 

 form the anterior hepatic vein, which lies on the dorsal sur- 

 face of the lobe. This vein runs forward and upward and 

 empties into the left side of the sinus venosus, a little be- 

 hind the sinoatrial opening. 



The portal system of the myxinid presents several inter- 

 esting features. The anterior cardinal division of it has 

 already been described (Figure 9-27). The intestinal vein 

 receives the blood from the entire intestinal wall, except 

 the ventroanterior region drained by the subintestinal; it 

 runs forward just above the intestine a little to the right of 

 the median line, within the mesentery. It lies to the right of 

 the vagus nerve and the mesenteric arteries. In the region 

 of the reproductive organs, this vein receives several genital 

 veins which descend through the mesentery. These veins are 

 formed by the plexus of small venous twigs in the special 

 genital fold of the mesentery. 



On reaching the pericardial region, the "intestinal" vein 

 turns to the right side of the intestine, where it receives the 



cystic vein from the gall bladder. It then passes through the 

 pericardio-peritoneal foramen, beside the intestine and be- 

 low the right mesonephros, crosses the roof of the outer 

 chamber of the right pericardial cavity, just below the 

 right vagus nerve, and enters the roof of the portal heart 

 posteriorly. 



The portal heart lies in the pericardial fold which forms 

 the septum in the right pericardial cavity. It is an elongated 

 sac, somewhat irregular in shape and variable in size. It 

 stretches diagonally across the pericardial cavity, and lies 

 nearly opposite the ventricle. The points of entrance of the 

 anterior portal vein and the intestinal vein are dorsal and are 

 guarded by semilunar valves. At its posterior (ventral) ex- 

 tremity, the portal heart empties into its efferent vessel, the 

 common portal vein. The opening into the common portal 

 vein is guarded by a pair of semilunar valves. 



The common portal vein extends backward and inward 

 toward the median line passing above the anterior lobe of 

 the liver, to which it gives off ventrally a large branch. This 

 branch is the anterior hepatic portal vein, and it descends 

 almost vertically alongside the hepatic duct of the anterior 

 lobe. The common portal vein then crosses the median line 

 to the left side and continues backward and downward 

 alongside the hepatic duct of the posterior lobe as the pos- 

 terior hepatic portal vein. It enters the posterior lobe about 

 the center of the dorsal surface. Blood is pumped to the liver 

 by the portal heart and drains into the sinus venosus from the 

 liver lobes by way of the anterior and posterior hepatic 

 veins. The latter also drains the subintestinal channel. 



GENERAL OBSERVATIONS 



In examining the circulatory system and its development, 

 one comes to the conclusion that it is derived from an indif- 

 ferent "capillary" or intercellular space network which 

 permeates the body. The final channels are in most cases 

 the result more of functional circulatory needs (hydrody- 

 namics) than they are of an inherently fixed pattern. The fixa- 

 tion of pattern that does exist is explainable on the basis of 

 the similarity in general body form and of functional 

 problems of the early stages of all vertebrates. In cases 

 where vessels follow circuitous routes, these routes have been 

 produced by modification at stages after the establishment 

 of the basic pattern which becomes fixed with the develop- 

 ment of the vessel walls. Once the connective tissue walls of 

 the blood vessels begin to develop, blood no longer has the 

 choice of seeking the most expedient channel. 



The development of the arterial system presents several 

 interesting points. The first of these is that a premandibular 

 arch is never present, although it has been assumed that the 

 external carotid and other structures such as the choroid 

 gland of the eye are remnants of such an arch. The lack ot 

 such an aortic arch has been one of the strong points used in 

 arguing against the existence of such a somite. Even in the 

 lamprey, the mandibular aortic arch is the most anterior; it 

 is associated with the velum which marks the extreme an- 



376 • THE CIRCULATORY SYSTEM 



