THE DEVELOPMENT OF THE VASCULAR SYSTEM. 257 



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. 187 and Fig. 165). 



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 subclavin artery (see Fig. 182). 



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, in- 

 nominate, left common carotid, left vertebral, left subclavian. (b) Or 

 the order may be right common carotid, left common carotid, left subclavian, 

 right subclavian. In this case the proximal part of the right subclavian rep- 

 resents the portion of the right dorsal aortic root just cranial to the bifurca- 

 tion; the fourth arch on the right side disappears, (c) Or very rarely the 

 order may be right subclavian, right common carotid, left common carotid, 

 left subclavian. 



5. Five branches of the arch are rare. In order they are right sub- 

 clavian, right vertebral, right common carotid, left common carotid, left 

 subclavian. 



6. Very rarely there are six branches of the arch; right subclavian, right 

 vertebral, right .common carotid, left common carotid, left vertebral, left 

 subclavian. 



ANOMALIES OF THE VEINS. The two pulmonary veins on each side, more 

 frequently those on the left side, many unite into a common trunk before 

 opening into the atrium. This variation is probably due to the fact that the 

 absorption of the originally single pulmonary trunk into the wall of the 



