In the orbit it receives branches draining the palate, the 

 walls of the orbit and adjacent skin, and the brain (the mid- 

 dle cerebral). Necturus is similar to Cryptubranchus in most 

 details. 



The salamanders differ from the frog in that the maxillary 

 and mandibular veins join and enter the main stem rather 

 than the orbital vein. They also differ in the very superficial 

 position of the main stem. This lies in the position of an ex- 

 ternal jugular. 



The development of the venous channels of the frog 

 (anuran) has been described, and the general pattern follows 

 that of the higher forms. 



Choanafe fishes The main veins of Protopterus are similar 

 to the salamander; the mandibular, maxillary, and lateral 

 head veins join to form a jugular stem which passes down 

 through the body-wall musculature above the branchial 

 openings. The duct of Cuvier passes down behind the gill 

 cavity to reach the heart. The duct of Cuvier receives bra- 

 chial, subclavian, and inferior jugular branches. The lateral 

 head vein has orbital, palatine, and all three cerebral 

 branches, anterior, medial, posterior. There is no sagittal 

 sinus above the brain. These are paired inferior jugulars. 



Acfinopter/gion fishes In the antinopterygian fish, the ante- 

 rior cardinal (jugular) stem passes forward above the gills 

 where it receives a brachial vessel. Anteriorly the lateral 

 head vein passes above the spiracular pouch or medial to it. 

 It receives a posterior cervical vein through the jugular 

 foramen. There are vessels draining the operculum, the jaws, 

 and orbit; these are received by the lateral head vein an- 

 terior to its passage through the lateral canal — there is no 

 cranioquadrate fissure. The hypobranchial region is drained 

 by paired inferior jugulars. 



The development of the anterior channels is fairly direct 

 and has been described for Amia. The anterior cardinals 

 arise in a capillary network located ventrolateral to the 

 brain and ventromedial to the cranial ganglia and nerves. 

 In a 15-mm specimen, the channel passes below and inside 

 the vagus nerve; the posterior cervical vein exits in front of the 

 vagus root. The lateral head vein then passes above or be- 

 low the glossopharyngeal ganglion — above and below in 

 some cases. Above the geniculate ganglion of the facial 

 nerve, it receives external and pharyngeal channels. The 

 external channel extends back above the adductor hyo- 

 mandibularis muscle to the body wall above the branchial 

 chamber. From here it passes dovvm medial to the thymus to 

 enter the anterior cardinal at the duct of Cuvier along 

 with a thymus vessel. 



Between the geniculate and Gasserian ganglia, the lateral 

 head vein branches. One part follows out along the trigem- 

 inal nerve, while the main channel passes forward below the 

 Gasserian ganglion and within the lateral canal. In the 

 orbit this trunk divides into superior and inferior orbital 

 channels and into the hypophyseal cross connective. The 

 course of this vein is similar in the adult. 



Chondrichfh/es In the shark there are large anterior car- 

 dinal sinuses. Anteriorly each becomes a lateral head vein 

 which passes above the hyomandibula. Anteriorly this chan- 

 nel gives rise to the orbital sinus. The brain is drained by a 

 posterior cerebral vessel, passing through the jugular fora- 

 men, along with the vagus nerve, and an anterior cerebral 

 entering the orbit and orbital sinus. A dorsal myelonal vein 

 from the posterior cerebral root extends down through the 

 vertebral canal above the spinal cord. Ventral myelonal 

 (basilar) veins may also be present. The orbital sinus is con- 

 nected with a nasal sinus through the orbitonasal fissure. 



The anterior cardinal has a supraorbital division supply- 

 ing the top of the head. This also connects with the orbito- 

 nasal channel through the nasal capsule. There are bucco- 

 pharyngeal vessels an either side serving the roof of the 

 mouth. These connect anteriorly with the orbitonasal chan- 

 nel and posteriorly with the anterior cardinal. 



The hyoid vein connects the cardinal sinus with the in- 

 ferior jugular sinus. The nutritive veins of the branchial 

 arches are noteworthy. 



The elasmobranch fishes are somewhat peculiar in that 

 they frequently have greatly enlarged sinuses with tendon- 

 like strands passing from one wall to another. The anterior 

 cardinal is such a sinus, as is the postcardinal. The orbital 

 sinus, and cavernous sinuses of other organisms, are of 

 similar form. 



C/c(osfomes The head veins of the two groups of cyclo- 

 stomes deserve special attention. 



LAMPREY The venous system of the lamprey is not easily 

 described because of the relationship of veins to various 

 venous or lymphatic sinuses. Blood is returned to the heart 

 from the anterior part of the body through three channels; 

 a median inferior jugular and two lateral, anterior cardinals. 

 Both the anterior and the posterior cardinals utilize the 

 right duct of Cuvier in reaching the sinus venosus. The single 

 inferior jugular enters the sinus venosus independently. 



The branchial sinuses associated with these veins consist 

 of three roughly longitudinal channels. These are: a single 

 mid-ventral, posteriorly branching sinus, the ventral jugular 

 sinus (or ventral branchial sinus); bilateral inferior bran- 

 chial sinuses lying below the gill pouches; and bilateral 

 superior branchial sinuses overlying the gill pouches. 



These longitudinal sinuses are interconnected through 

 the branchial bars. The most anterior connective is the 

 hyoidean sinus. The branchial sinuses are irregular in shape 

 and tend to form an enclosing vascular sac for the gill 

 pouches. The ventral jugular sinus encloses the ventral 

 aorta. 



The anterior cardinals develop from channels passing 

 above, lateral, and below (next to the dorsal aorta) the otic 

 capsule. These unite in the region of the second branchial 

 arch. The relationship of the anterior cardinals to the dorsal 

 aorta is not the same as that of the posterior cardinals; the 

 anterior cardinals lie lateral or even dorsolateral to the 



CIRCULATION IN THE HEAD • 365 



