PHYLUM CHORDATA 



93 



efferent vessels are not arteries, but thin-walled, non-elastic, 

 eolkpsihle tubes called veins. They receive the impure blood 

 from the capillaries, and unite into larger and larger trunks, 

 finally opening into one or other of the great veins, presently to be 

 described, by which the blood is returned to the heart. '. As a 

 general rule the vein of any part of the body runs parallel to its 

 artery, from which it is at once distinguished by its wider calibre, 

 by its dark colour— due to the contained bluish-purple blood seen 

 through its thin walls— by being gorged with blood after death, by 

 the conaplete collapse of its walls when empty, and by its usually 

 containing valves. In some cases the veins become dilated into 

 spacious cavities called sinuses ; but sinuses without proper walls 



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«Mr.a. 



tl.br a 



Fig. 773. — Diagram illustratiiigthecomseof thecirculationinarisll. Vessels containing aerated 

 bloodrcd, those containing non-a6rated blood blue, lymphatics black. B. capillaries of the body 

 generally ; E. of the enteric canal ; G. of the gills ; R. of the kidneys ; L. of the liver ; T. of 

 the tail. a. bv. a. afferent branchial arteries ; au. auricle ; e. a. conus arteriosus , d. ao. dursa 

 aorta ; c. hr. a. efferent branchial arteries ; h. j?. v. hepatic portal vein ; li. v. hepatic vein ; 

 Ic. lacteals ; ly. lymphatics ; pr. cv. v. precaval veins ; r. p. v. renal portal veins ; s. v. sinus 

 venosus ; v. ventricle ; v. ao. ventral aorta. The arrows show the direction of the current. 

 (From Parker's Elementary Biology.) 



such as occur in many Invertebrates, are never found in the 

 Craniata. 



The veins from the head join to form large, paired jugular veins 

 (Fig. 772, y. v.) which pass backwards, one on each side of the head, 

 and are joined by the cardinal veins (crd. v.) coming from the trunk, 

 each jugular uniting with the corresponding cardinal to form a large 

 precaval vein (pr. cv. v.) which passes directly downwards and enters 

 the sinus venosus. The blood from the tail returns by a caudal 

 vein (cd. v.), lying immediately below the caudal artery in the 

 haemal canal of the caudal vertebrae (Fig. 760, D). On reaching 

 the coelome the caudal vein forks horizontally, and the two 

 branches either become directly continuous with the cardinals 

 or pass one to each kidney under the name of the renal portal 

 veins (Fig. 772, r. p. v.). In the kidneys they break up into 

 capillaries (Fig. 773, K), their blood mingling with that brought 

 by the renal arteries and being finally discharged into the 

 cardinals by the renal veins (Fig. 772, r. v.). Thus the blood from 



