THE MODIFICATIONS OP THE HEART. 97 



sentecl only during foetal life, by the ^imbilical vein or 

 veins. The eiierent veins of the kidneys open directly 

 into the trunk of the inferior vena cava, and the portal 

 vein is composed exclusively of radicles proceeding from 

 the chylopoietic viscera. 



Many of the veins of Amphioxus, the portal vein of 

 Mijxine, dilatations of the caudal vein in the Eel, the vena; 

 cavce and the iliac and axUlary veins of many Ampliihia, 

 the veins of the wing of Bats, possess a rhythmical con- 

 tractility, which, in combination with the disposition of theii- 

 valves, assists the circulation of the blood. 



In Vertebrata of all classes, and in very diverse parts of 

 the body, both veins and arteries occasionally break up into 

 numerous branches of nearly equal size, which may or may 

 not unite again into larger trunks. These are called retia 

 mirabilia. 



Modifications of the Heart. — Great changes go on in 

 the structure of the heart, pari passu with the modifica- 

 tions of the rest of the circvilatory system, in the develop- 

 ment of the highest Vertebrata. The primitively simple tube 

 becomes bent upon itself, and divided from before back- 

 wards into an aortic, or ventricular, and a venous, or auri- 

 cular, portion. A median septum then grows inward, divid- 

 insf the auricular and ventricular chambers into two. so that 

 a right am'icle and right ventricle become separated from 

 a left auricle and left ventricle. A similar longitudinal 

 division is effected in the cardiac aorta. The septa are so 

 disposed in the auriculo -ventricular chamber that the right 

 auiicle communicates with the venous sac and the trunks 

 of the visceral and body veins, while only the veins from 

 the lungs enter into the left auricle. And the cardiac 

 aorta is so divided that the left ventricle communicates 

 with the chief aortic trunk, the right with the pulmonary 

 artery. Valves are developed at the auriculo-ventricular 

 apertures and at the origins of the aortic and pulmonary 

 trunks, and thus the coiirse of the circulation is detennined. 

 The septum between the auricles remains incomplete for a 



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