Circulatory System 73 



necting more or less directly with the heart include a pair of anterior 

 cardinal (precardinal or jugular) veins collecting blood from the 

 head and neck, and a pair of posterior cardinals (postcardinals) 

 situated on the dorsal wall of the coelom and extending to the pos- 

 terior end of the abdominal cavity. Just lateral to the heart, the an- 

 terior and the posterior cardinal of each side meet and join to form a 

 short transverse precava (anterior vena cava, or duct or sinus of 

 Cuvier) which opens into the venous sinus. Each precava receives a 

 subclavian vein from the adjacent pectoral appendage and, at least 

 in the fishlike vertebrates, a lateral vein collecting from the lateral 

 abdominal wall. The blood from the liver is collected into one or 

 two hepatic veins (or sinuses) which pass forward directly into the 

 venous sinus (Fig. 74, h). 



In most adult fishes the two postcardinals persist as chief veins 

 of the trunk. In the lungfishes (Dipnoi) and all other vertebrates, the 

 veins of the embryonic trunk undergo a complex metamorphosis result- 

 ing in formation of a single median vein. Certain veins, a pair of sub- 

 cardinals and a pair of supracardinals, which may be regarded as 

 derivatives or collaterals of the postcardinals, arise in connection with 

 the development of the kidneys (Figs. 74, 75). The several veins of 

 this postcardinal system acquire complex and transitory connections 

 with one another. Gradually, from this early embryonic maze 

 of small veins, a central main channel begins to emerge (Fig. 75F-K) 

 and eventually becomes one large median vein extending from the re- 

 gion of the kidneys forward along the roof of the abdominal cavity, 

 whence it descends to enter and pass through the liver directly into the 

 venous sinus or right auricle (Figs. 71B-E, 76). Thus is formed the 

 postcava (posterior vena cava) which functionally more or less com- 

 pletely replaces the postcardinals, although in part derived from them 

 or from their associated subcardinals and supracardinals. The an- 

 terior segment of this built-up postcava consists of an embryonic he- 

 patic vein (Fig. 151, K). The embryonic history of the postcaval veins 

 of vertebrates of the several classes is not uniform. The definitive 

 veins differ as to the fractions of embryonic veins which constitute 

 them, and therefore do not exactly correspond — or are not completely 

 homologous (see p. 362). 



With the shifting of the main venous flow from the paired postcar- 

 dinals to the median postcava, the anterior regions of the postcardinals 

 become reduced or may be represented by mere vestiges. A pair of small 

 azygos veins just behind the heart in mammals have been regarded as 

 remnants of postcardinals, but recent study of their origin indicates 

 that only their anterior extremities are postcardinal, their greater part 

 being probably of supracardinal origin (Fig. 15G-K). 



