THE DEVELOPMENT OF THE VASCULAR SYSTEM 287 



the single ventricle may open into two atria. The causes of these defects 

 are obscure. 



ANOMALIES OF THE VALVES, There may be congenital variations in the 

 size and number of the atrio-ventricular valves, depending upon abnormal 

 position, fusion, or division of the pad-like masses from which the valves 

 develop (p. 237). 



There may be also a greater or lesser number of semilunar valves in the 

 aorta and pulmonary artery. This irregularity can probably be referred 

 back to an atypical division of the aortic trunk and bulb, and a corresponding 

 atypical division of the protuberances which give rise to the valves (p. 237). 

 Variations in the valves may or may not be accompanied by functional dis- 

 turbances. The congenital diminution in the number of valves should be 

 distinguished from the acquired, where chronic endocarditis may cause a 

 fusion. 



ANOMALIES OF THE LARGE VASCULAR TRUNKS. 



ANOMALIES OF THE ARTERIES. There may be a transposition of the aorta 

 and pulmonary artery. This results from an anomalous division of the aortic 

 trunk and bulb. The partition develops in such a way as to put the aorta in 

 communication with the right ventricle, and the pulmonary artery with the 

 left ventricle (p. 235). Or the aorta and pulmonary artery may remain in 

 direct communication on account of an imperfect development of the 

 partition. Rarely the two vessels remain as a common stem. 



Congenital stenosis (constriction) of the pulmonary artery may occur, 

 accompanied by an increase in the size of the aorta, possibly due to an unequal 

 division of the aortic trunk and bulb. After birth little or no blood can pass 

 to the lungs, and the result is a general damming (stasis) of the venous blood 

 with marked cyanosis. This is at least one explanation of the so-called " blue 

 babies." Less frequently there is a stenosis of the proximal end of the aorta, 

 with excessive size of the pulmonary artery, also due to an unequal division 

 of the aortic trunk and bulb (p. 235). These stenoses are usually, though not 

 always, accompanied by defects in the ventricular septum. 



Persistence of the ductus arteriosus may occur without any other defect; 

 but usually the persistence is associated with anomalous conditions of the 

 aorta and pulmonary artery. 



Occasionally the arch of the aorta is found on the right side. This condi- 

 tion is due to the persistence of the fourth aortic arch on the right side instead 

 of the corresponding arch on the left side; this is the normal condition in 

 Birds. Rarely both fourth aortic arches persist, which results in a double 

 arch of the aorta the normal condition in Reptiles. (Compare Figs. 219 

 and 220.) 



