396 STRUCTURE OF VERTEBRATES. 



The important elements of the venous system are as follows : — 



(a) A sub-intestinal vein leading into the heart forms posteriorly the 

 caudal vein, and anteriorly the vitelline veins. The latter are at first 

 connected with the yolk, but persist in the hepatic-portal system, in 

 which veins from the intestine pass into the liver. 



(i5) Returning to the heart from the head region are two anterior 

 cardinals, returning from behind are two posterior cardinals. These are 

 really the forward and backward continuations of the duct of Cuvier, — 

 a short cross vein on each side by which the united cardinals enter 

 the heart or sinus venosus. In Fishes the anterior cardinals persist, 

 and the posterior cardinals unite with the above-mentioned caudal vein, 

 and also form in part the renal-portal system, in which venous blood 

 from the posterior parts passes through the kidneys on its way to the heart. 

 In other Vertebrates, the cardinals are for the most part replaced by 

 superior vense cavas (into which the anterior cardinals open as external 

 jugulars), and by an inferior vena cava, which receives at least the effer- 

 ent veins from the kidneys and the hepatic veins from the liver. 



(f ) In Amphibia and Reptiles some of the blood from the hind limbs 

 and from the allantoic outgrowth passes into the hepatic-portal system 

 via the epigastric vein or veins. In Birds and Mammals this arrange- 

 ment is found only in the embryos. ' 



The vascular system is developed in the mesoblast from the hollowing 

 out of strands of cells, the outer cells forming the walls of the vessels, 

 the inner cells forming blood-corpuscles. But it may be that some 

 elements of the blood are endodermic. 



Associated with the vascular system is the spleen, which in at least 

 some Vertebrates is a blood-making organ. It always lies in the mesen- 

 tery near the stomach. 



Developed in mesoblastic spaces, and continuous with the body-cavity 

 on the one hand and the blood-vessels on the other, is the system of 

 lymphatic spaces and vessels. The lymph fluid therein contained is 

 derived from the capillaries by exudation or filtration. Specially 

 important is the chyle-absorbing lacteal part of the lymphatic system Ijy 

 which food passes from the intestine into the blood. (See Chapter I. 

 pp. 17, 18. ) The force which propels the lymph is the same as that whiclj 

 drives the blood. 



Respiratory System. — In Balanoglossus, Tunicates, and 

 Amphioxus, the walls of the pharynx bear slits, between 

 whichthe blood is exposed in superficial blood-vessels to the 

 purifying and oxygenating influence of the water. 



In Cyclostomata, Fishes, all young and some adult Am- 

 phibians, there are not only clefts on the walls of the 

 pharynx, but gills associated with these. On the large 

 surface of the feathery or plaited outgrowths, the blood is 

 exposed and purified. 



In Reptiles, Birds, and Mammals, traces of gill-clefts per- 

 sist in the embryos ; but they do not serve any respiratory 



