362 MANUAL OF ELEMENTARY ZOOLOGY 



snout. The internal carotid artery continues its course in 

 the carotid groove, towards the middle line, where with its 

 fellow it passes through the internal carotid foramen into 

 the cranium to supply the brain. Outside the carotid yet 

 another artery arises from the first efferent branchial vessel. 

 This is the hyoidean artery, which starts in a line with the 

 horizontal vessels which join the loops, runs forwards to 

 the spiracle, where it supplies the pseudobranch, crosses the 

 orbital floor, enters the cranium by a small foramen in the 

 inner wall of the orbit, and joins the crossed internal carotid 

 artery of the opposite side. The dorsal aorta ends in front 

 by breaking into two small posterior carotid arteries, which 

 curve outwards and join the common carotid trunks. Just 

 before it is joined by the last pair of epibranchial vessels it 

 gives off a pair of subclavian arteries, which pass backwards 

 and outwards to the fore-fins. Behind the pharynx it runs 

 backwards along the whole length of the body below the 

 backbone, lying, in the tail, in the haemal canal as the caudal 

 artery. Besides paired vessels to the body-wall, it gives off 

 to the viscera several median vessels, known successively as 

 the coeliac (of which the hepatic is a branch), anterior mes- 

 enteric (of which the genital is a branch), lienogastric, and 

 posterior mesenteric, and to the kidneys several paired 

 renal arteries. 



The sinus venosus receives the whole of the blood 

 returning to the heart, by a number of very large 

 veins which are called sinuses, though, unlike 

 the sinuses of the crayfish, they do not take the place of 

 capillaries as well as veins in the circulation, but are merely 

 enlarged parts of the veins. The blood from the liver 

 returns direct to the sinus venosus by two hepatic sinuses 

 which enter its hinder side. The rest of the blood is 

 returned by two large precavals or ductus Cuvieri which 

 join the sinus venosus, one on each side in the pericardium. 

 Into these the blood from the region of the body in front of 

 the fore-fins is conveyed by a pair of large dorsal anterior 

 cardinal sinuses and two smaller inferior jugular sinuses 

 below the throat. Each anterior cardinal sinus communi- 

 cates in front with an orbital sinus around the eye, and this 

 in turn with a nasal sinus around the olfactory organ. A 

 hyoidean sinus in the hyoid arch joins the anterior cardinal 



