THE DEVELOPMENT OF THE VASCULAR SYSTEM. 287 



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

 companied 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. 230). 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 condition 

 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.) 



The dorsal aorta, particularly the abdominal part, is occasionally found to 

 consist of two parallel, imperfectly separated vessels a condition known as 

 double aorta. This anomaly is due to an imperfect fusion of the two primitive 

 aortae (p. 239 and Fig. 194). 



Numerous variations are met with in the larger branches of the aorta, many 

 of which are explained by referring them to embryonic conditions. Especially 

 noteworthy are the branches from the arch of the aorta, since their development 

 is so closely associated with the changes in the aortic arches. The normal 

 arrangement, passing from the heart, is innominate artery, left common 

 carotid artery, left subclavian artery (see Fig. 220). 



1. All these branches may be collected into a single trunk, a condition 

 characteristic of the horse. 



2. Two branches may arise from the arch, (a) The left common carotid 

 unites with the innominate, and the left subclavian arises separately. This is 

 the normal arrangement among the apes, and is probably the most common 

 variation in man. (b) Very rarely there are two innominate arteries, each 

 formed by the union of a common carotid and subclavian a condition char- 

 acteristic of Birds. 



3. Three branches may arise from the arch but in a manner differing from 

 the normal. Each subclavian arises separately and the two common carotids 

 are united into a single vessel. This arrangement is found in some of the 

 Cetacea. 



4. Four vessels may arise from the arch, (a) These are, in order, innomi- 

 nate, left common carotid, left vertebral, left subclavian. (b) Or the order may 



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