1052 THE VASCULAR SYSTEM. 



number of branches then, arising from the arch of the aorta is four, the additional vessel 

 being the left vertebral, which arises between the left common carotid and the left subclavian. 

 Occasionally the usual three branches from the arch are increased to four by the formation of a 

 new vessel, the " thyreoidea ima." This may be placed between the innominate and left carotid 

 trunks, in which case it represents a persistent ventral visceral branch from the ventral root of the 

 fourth left aortic arch ; in other cases the thyreoidea ima springs from the innominate artery and 

 represents a ventral visceral branch of the ventral root of the fourth right arch. Very rarely 

 the right vertebral artery arises separately, and forms a fourth branch of the arch of the aorta, 

 the rest of the branches being normal. This condition cannot be accounted for by any modifica- 

 tion of the ordinary developmental processes. It may possibly be due to the persistence of an 

 irregular or unimportant anastomosis between the ventral root of an aortic arch and the seventh 

 somatic segmental artery. 



Decrease in the number of branches from the arch of the aorta is most frequently due to 

 fusion of the ventral roots of the fourth aortic arches, the result being that a stem is formed 

 common to the right subclavian and the right and left common carotid arteries ; whilst the left 

 subclavian, arising separately, is the only other branch which springs from the arch of the aorta. 



If the fusion of the ventral roots proceeds further and includes those of the third arches, the 

 result, as regards the branches given off from the arch of the aorta, is the same, viz., there is a 

 common stem for the right subclavian and both carotids, and a separate left subclavian trunk ; but 

 the common stem now gives off the right subclavian artery, and then continues as a single vessel 

 for some distance before it divides into the two common carotids, of which the left crosses in 

 front of the trachea. This arrangement is common in many quadrumana and in some other 

 mammals. 



It is only in rare cases when the number of branches from the arch of the aorta is 

 reduced to two, that these consist of a right subclavian artery and of a single stem common to 

 the two carotids and the left subclavian artery. In such cases, however, the right common 

 carotid crosses in front of the trachea, and the variation is one of practical importance, but it does 

 not appear to exist as a normal condition in any mammal. Probably it is due to fusion of the 

 ventral roots of the fourth aortic arches, with absorption of the left fourth arch and the left sub-' 

 clavian into the stem so formed, whilst the right subclavian is relatively displaced. The two 

 common carotids may arise by a common stem, and the left subclavian arise separately from 

 the arch of the aorta, whilst the right subclavian springs from the descending aorta. This 

 arrangement probably results from the disappearance of the fourth right arch, the fusion of the 

 ventral roots of the fourth arches of opposite sides and the persistence of the dorsal roots of the 

 right fourth and sixth arches. 



Sometimes two innominate arteries, right and left, replace the usual three branches of the 

 arch of the aorta. This , is the normal arrangement in bats, moles and hedgehogs. It is 

 obviously the result of the disappearance of that portion of the arch which intervenes between 

 the left carotid and left subclavian arteries, and the consequent fusion of these two vessels. 



In a similar way may be explained the rarer condition in which the three ordinary 

 branches of the arch arise by one single stem, which divides into right and left innominate 

 arteries. In most ruminants, in the horse and in the tapir, this arrangement is constant. 



It will be evident that other combinations and modifications may be met with in the branches 

 of the arch of the aorta as the result of fusions and absorption. 



The right subclavian or the right vertebral may spring from the commencement of the 

 descending aorta. 



The bronchial arteries obviously correspond to splanchnic arteries and their continuations 

 to diverticula from the walls of the gut ; therefore the usual origin of the right bronchial artery 

 from the first right aortic intercostal artery must result from the persistence of an anastomosis 

 between a splanchnic artery and the first part of a somatic intersegmental artery ; the origin 

 of the right from the upper left bronchial artery, which sometimes occurs, is due to the fusion 

 of the roots of two splanchnic arteries. The occasional origin of a bronchial vessel from an 

 internal mammary artery can result only from the persistence and enlargement of an anastomosis 

 between a splanchnic artery and the ventral branch of a somatic segmental artery. The origin 

 of a bronchial branch from a subclavian artery may have the same or a different significance 

 on opposite sides of the body. A bronchial artery arising from the left subclavian artery 

 corresponds with the origin of the right bronchial artery from the first aortic intercostal artery ; 

 it is due to the persistence of an anastomosis between a splanchnic artery and the root of a 

 somatic intersegmental artery ; and the origin of a bronchial artery from a right subclavian artery 

 may be due to a similar cause. It may, on the other hand, be due to the enlargement of an 

 anastomosis between a splanchnic branch of the descending aorta and a splanchnic branch of the 

 fourth right aortic arch. When, as occasionally happens, the bronchial artery arises from the 

 inferior thyreoid, it is due to the persistence and enlargement of an anastomosis between splanchnic 

 arteries. 



Intercostal Arteries. Variations of the intercostal arteries are not very common, but 

 they are significant and interesting. Corresponding vessels of opposite sides may arise from a 

 common stem which has been formed by the fusion of the roots of two somatic intersegmental 

 arteries after or simultaneously with the fusion of the primitive dorsal aortae. The number of 

 intercostal arteries may be reduced, one artery supplying two or more intercostal spaces ; in these 

 cases the roots of origin of some of the somatic intersegmental arteries in the thoracic region 

 have disappeared, and the precostal anastomoses between their ventral branches have persisted. 



Occasionally the number of the aortic intercostal arteries is increased, an additional artery 



