256 TEXT-BOOK OF EMBRYOLOGY. 



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 j 

 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. 206). 



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.. 206). 

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

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

 distinguished from the acquired, where chronic endocarditis may cause a 

 fusion. 



ANOMALIES or THE LARGE VASCULAR TRUNKS. 



ANOMALIES or 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 1 the j 

 left ventricle (p. 204). 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, j 

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

 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. 204) . 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; ;i 

 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. 181 

 and 182.) 



