CIRCULATORY SYSTEM 805 



side and bicuspid (mitral) on the left. In the monotremes, however, 

 both valves have three cusps. 



The pulmonary artery and aorta all have three-lobed semilunar 

 valves. 



In the monotremes the renal portal system is better developed than 

 in other mammals although in all mammals it functions but for a short 

 time and disappears with the degeneration of the mesonephroi (Wolffian 

 bodies). 



A part of the capillary system of the mesonephroi enlarge during 

 the degenerative process to form a main trunk which connects the post- 

 cava with the caudal portions of the postcardinal veins. It is the post- 

 cardinals that drain the tail, iliacs, and metanephroi. 



The left postcardinal largely disappears later with the exception of 

 that portion which connects with the suprarenal and gonad of the left 

 side. All the blood from the posterior part of the body is therefore re- 

 turned through the right postcardinal and the postcava, whose origin 

 appears to be at the union of the iliac veins. 



In the turtle the postcaval vein unites with that part of the renal 

 portal system which lies caudal to the kidneys and the renal portal sys- 

 tem then passes out of existence. 



This can be understood the better if it be remembered that the renal 

 portal veins are the caudal portions of the posterior cardinal veins, and 

 that the subcardinal veins (particularly the right subcardinal) form the 

 postcaval vein which lies between the kidneys. 



In mammals the postcaval vein is formed principally of the distal 

 ends of the posterior cardinal veins, and of the right subcardinal, of the 

 (vitelline) hepatic veins close to, and cephalad to, the liver, as well as 

 of the hepatic veins which lie between the liver and the hind limbs. As 

 the postcaval vein is made up of so many different sources, there are 

 bound to be many variations in the adult state due to more or less per- 

 sistent embryonic conditions. 



The more anterior portion of the postcardinal veins loses its con- 

 nection with the portion connecting with the excretory organs, and with 

 the thoracic portion of the supracardinals, to become the azygous vein 

 on the right side and the hemiazygous on the left. Either of these may 

 disappear or, as in man, there may be a cross connection between these 

 two veins. In such a case the anterior part of the hemiazygous is known 

 as the superior intercostal vein. 



The abdominal veins are quite important in foetal life as they bring 

 blood from the placenta to the embryo. 



In the higher vertebrates, including man, an innominate vein ex- 

 tends across from the carotid-subclavian trunk from one side to the other. 

 All the blood is thus returned to the heart by means of the base of the 

 right trunk, which is now called the precava or vena cava anterior. 



The Cuvierian duct remains only as the coronary sinus. 



