notochord, while the posterior lie ventrolateral. The relation- 

 ship of the superior branchial sinuses is essentially like that 

 of the posterior cardinals. Each cardinal enters the duct of 

 Cuvier of its side. At metamorphosis, the left and right join 

 and enter the right duct of Cuvier; the left duct disappears. 



Each branchial arch has a large irregular sinus passing 

 through it externally. There is a velar (mandibular) sinus 

 joining the ventral cardinal channel below the otic capsule. 

 The large hyoidean sinus connects with both ventral and 

 lateral otic channels. The branchial sinuses develop directly 

 from the venous channels of the larva. 



There are paired inferior jugular veins in the early larva 

 which are later replaced by the median inferior jugular of 

 the adult, which lies mainly below but encloses the ventral 

 aorta and the jugular sinus. Anteriorly it connects with the 

 hyoidean sinus of either side. There are segmental connec- 

 tions along the course of the inferior jugular with the infe- 

 rior branchial sinus, which in turn communicates with the 

 ventral jugular and branchial sinuses. Posteriorly the in- 

 ferior jugular vein shifts to the right side to enter the sinus 

 venosus. 



In Petromyzon, the inferior jugular system is somewhat 

 different. The median inferior vessel proceeds forward from 

 the sinus venosus and at the level of the posterior (7th) gill 

 pouch gives off two large branches which parallel it. 



HAGFISH The veins of the Hagfish (Myxme or Eptatretus) are 

 included in two systems: the general system and the hepatic 

 portal system. From the head region, the deep cardinal veins 

 course posteriorly alongside the pharynx, internal to the 

 constrictor muscles. These veins pass under the first branchial 

 arch, just posterior to the skull, but over the second. About 

 2 to 3 cm behind the second arch, each deep vessel is joined 

 by a superficial one to form the anterior cardinal vein. This 

 passes back next to the pharynx, just external to the corres- 

 ponding lateral aorta and internal to the vagus nerve. 



Posterior to the vertical of the posterior end of the lingual 

 muscles, the courses of the two cardinals differ. The left 

 anterior cardinal continues back beside the vagus, above 

 the gills, between the left pronephros, from which it receives 

 a twig, and the alimentary canal, and empties into the an- 

 terolateral angle of the dilated posterior portion of the sinus 

 venosus. 



The right anterior cardinal, near the posterior end of the 

 jaw muscles mass, leaves the pharyngeal wall, passes down- 

 ward toward the posterior end of the "tongue muscle" and 

 the median line, and empties into the median inferior jugu- 

 lar vein. The posterior portion of the original right anterior 

 cardinal becomes the anterior portal vessel. This arises in 

 the right branchial region, a little in front of the posterior 

 end of the "tongue-muscle," just below and to the right of 

 the notochord. It continues backward into the fold (portal 

 septum) separating the inner and outer chambers of the 

 right pericardial cavity. It passes between the alimentary 

 canal and the right pronephros, opens into the roof of the 

 portal heart (the modified right duct of Cuvier) near the an- 



terior end of that structure. Just before entering the portal 

 heart, it receives a branch from the pronephros. Two or 

 three somatic veins lie opposite and posterior to the portal 

 heart. 



The median inferior jugular vein arises in the posterior 

 end of the "tongue-muscles," from which it emerges ven- 

 trally. It passes backward a little to the left of the median 

 line, immediately inside the body wall. After receiving the 

 right anterior cardinal vein, it continues back a little below 

 and to the left of the median ventral aorta. Finally it emp- 

 ties into the anterior end of the sinus venosus (Figure 9-28). 



There is some question as to the relationship between the 

 veins and the lymphatics in cyclostomes; it has been sug- 

 gested that the lymphatics are but venous sinuses. 



Genera\ observations Because the anterior cardinal system 

 is derived from a reticulum of channels, its comparison 

 from one group to another is rather difficult. In the head 

 this reticulum tends to be divided into an inner vena capitis 

 medialis and an outer vena capitis lateralis. 



The vena capitis lateralis is the primary channel in cyclo- 

 stomes, gnathostome fishes, amphibians, reptiles, mono- 

 tremes, and marsupials. It runs outside the auditory capsule 

 alongside the hyomandibular branch of the facial nerve, 

 through the cranioquadrate passage, and dorsal to the 

 spiracular gill pouch. In selachians, it passes dorsal to the 

 hyomandibular cartilage; in actinopterygians, below and 

 inside the hyomandibula; in tetrapods, dorsal to the colu- 

 mella auris or stapes. A new venous loop occurs in some 

 teleosts which passes outside of the hyomandibula. Occa- 

 sionally in cyclostomes, selachians, Polypterus, and anurans, 

 the vena capitis lateralis passes outside the trigeminal nerve, 

 but usually this vessel passes below and inside the nerve. 



In the neck region the cardinal system is aided by the 

 formation of an external jugular channel. A pair of inferior 

 jugulars is also present for draining the throat region (an- 

 terior jugulars). 



The venous systeiu of the head suggests that the various 

 groups have each developed a few modifications of a basic 

 plan. Just as in the head arteries there is evidence of a 

 common ancestor of the several groups of tetrapods, an an- 

 cestor much like the choanate fish and less like the ray- 

 finned fishes or sharks. Common ancestry of the gnathostomes 

 is suggested by agreement in many details. The cyclostome 

 pattern shows only the most general resemblance to that of 

 the gnathostome. 



CIRCULATION IN THE BODY 

 Arteries 



Mammah The dorsal aorta in the mammal sends branches 

 ventrally through the mesentery to the various parts of the 

 gut. The first branch is the coeliac. It has divisions going to 

 the liver, stomach, spleen, and to the underside of the duode- 

 num. These arteries form loops in some instances: a branch of 



366 



THE CIRCULATORY SYSTEM 



