XXIX CIRCULATORY ORGANS 387 



From the systemic as from the respiratory capillaries the 

 blood is collected into vessels which join into larger and 

 larger efferent trunks. But these trunks are not thick-walled 

 elastic arteries, but thin-walled, non-elastic, collapsible tubes, 

 having valves at intervals, called veins. As a general rule 

 every part of the body has a vein running alongside its 

 artery, the blood in the artery flowing to the part in 

 question, that of the vein away from it. 



The blood from the head is brought back by a pair of 

 ^ugular veins {J. v) : each of these enters a large precaval 

 vein {pr. cv. v), which passes vertically downwards and 

 enters the sinus venosus. The blood from the tail is re- 

 turned by a caudal vein {cd. v) lying immediately beneath 

 the caudal artery in the haemal canal : this vessel enters the 

 coelome and then divides into right and left branches, the 

 renal portal veins (r, /. v), which pass to the kidneys and 

 join with the capillaries of these organs, the impure blood 

 brought from the tail mingling with the pure blood of the 

 renal arteries {r. a). From the kidneys the blood is returned 

 into a pair of immense cardinal veins (crd. v\ which pass 

 forwards, receiving veins from the reproductive organs (sp. v\ 

 muscles, &c., and finally join each with the corresponding 

 jugular to form the precaval vein. 



From the stomach, intestine, spleen, and pancreas the 

 blood is collected by numerous veins, which all join to form 

 a large hepatic portal vein (h. p. v). This behaves in the 

 same way as the renal portal : instead of joining a larger 

 vein on its way to the heart, it passes to the liver and breaks 

 up to connect with the capillaries of that organ, its blood, 

 deprived of oxygen but loaded with nutrient matters from 

 the enteric canal, mingling with the oxygenated blood 

 brought to the liver by a branch of the cceliac artery. After 

 circulating through the capillaries of the liver the blood 



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