286 TEXT-BOOK OF EMBRYOLOGY. 



septum is the persistence of the foramen ovale. The entire foramen may 

 remain patent, or, as is more frequently the case, a smaller opening may per- 

 sist between the ventral (anterior) border of the foramen and the valve of the 

 latter (p. 229). 



The atrial septum may be wholly lacking, but this always occurs in conjunc- 

 tion with other defects. It sometimes happens that the primary atrial septum 

 (septum superius), which grows from the cephalic side of the common chamber, 

 fails to fuse with the septum of the atrio-ventricular aperture (p. 229 and 

 Fig. 200). 



Defects in the ventricular septum occur less frequently than in the atrial 

 septum. It may happen that the cephalic (upper) border of the ventricular 

 septum fails to fuse with the septum which divides the aortic trunk and bulb 

 into the aorta and pulmonary artery. This affects the cephalic (upper) part of 

 the septum sometimes called the pars membranacea (p. 230 and Fig. 203) ; and 

 since the defect is situated near the opening of the aorta it brings about the so- 

 called "origin of the aorta from both ventricles." Stenosis of the pulmonary 

 artery usually accompanies this condition. Rarely is there a deficiency in the 

 caudal (lower) part of the ventricular septum. Complete absence of the 

 ventricular septum may occur, and along with it also an absence of the atrial 

 septum, so that the heart is simply two-chambered; or 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 posi- 

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

 (p. 232). 



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. 232). Variations 

 in the valves may or may not be accompanied by functional disturbances. 

 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. 230). 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. 



