PECULIARITIES OF THE AORTIC ARCH. 337 



the left common carotid, the second the right common carotid, the third the right 

 subclavian, and the fourth the left subclavian, passing behind the oesophagus to 

 reach its destination. 



II. Peculiarities affecting the Primary Branches. 



Tti.e situation of the brandies. Instead of springing from the highest part of the 

 arch, the branches are frequently moved altogether to the right, and take origin from 

 the commencement of the transverse portion, or even from the end of the ascending 

 portion of the arch. In the ordinary arrangement the origin of the left carotid is 

 nearer to the innominate than to the left subclavian ; but the branches sometimes 

 arise at equal distances from each other, or are unusually widely apart. A very 

 frequent change consists in the approximation of the left carotid towards the in- 

 nominate artery. 



TJie number and arrangement of the branches. These are extremely various. The 

 most frequent change met with is their reduction to two, from the left carotid being 

 united in a common trunk with the innominate artery. In cases of rare occurrence, 

 the carotid and subclavian arteries of the left side, as well as those of the right, 

 are conjoined in an innominate artery. 



On the other hand, the number of the primary branches has been found to be 

 augmented to four, by the separation, as it were, of the innominate artery into the 

 right carotid and subclavian arteries, both arising directly from the aorta. In those 

 cases, the right subclavian artery is most frequently the last branch given off, pro- 

 ceeding from the back part of the arch, and passing behind the oesophagus to reach 

 its destination ; but sometimes it is given off in its proper order, as the first branch, 

 and in rare instances, it has been the second or third branch given off, in which 

 case it has passed behind those which preceded it, to reach the limb. 



In those instances in which the right subclavian is the last vessel given off, and in 

 some other abnormal arrangements, a small pouch-like dilatation is sometimes found 

 on the arch, which is a vestige of the right aortic root, and is accounted for by the 

 changes in development which have led to the unusual disposition of the branches. 



A remarkable variety is on record, in which the aorta divided at once into two 

 vessels, as is the usual arrangement in some quadrupeds, all the arteries for the head, 

 neck, and upper limbs, taking origin by a single trunk. In those cases the single 

 trunk referred to passed vertically upwards and divided into three branches, in the 

 form of a cross. 



An abnormal arrangement of three branches springing from a normally arranged 

 arch has been observed, in which the two carotids have arisen by a common trunk, 

 and the two subclavians separately the right subclavian, in most instances, being 

 transferred beyond the other branches to the left end of the arch. 



III. Peculiarities in which one or more Secondary Branches, usually given from 

 the Subclavian, are derived directly from the Aorta. 



In nearly all instances belonging to this section, there is only one secondary branch 

 taking origin from the aorta ; and the occurrence may either accompany the ordinary 

 arrangement of the primary branches, or co-exist with a diminution or with an 

 increase in their number. The additional branch, when it is a normal artery trans- 

 posed, is almost invariably the left vertebral, which in nearly all those cases arises 

 between the left carotid and left subclavian arteries ; but it has also been observed 

 to proceed from the aorta beyond the last-named trunk. Very rarely the additional 

 branch is the right vertebral. 



The thyroidea ima, a small supernumerary artery occasionally found ascending to 

 the thyroid body, sometimes arises from the arch of the aorta. 



Development of variations in the aortic arch and its branches. Many of the most 

 frequent variations in the arrangement of the arch of the aorta and its branches may 

 be explained on referring to the development 'of those vessels, by supposing that one 

 of the usual branches has become obstructed in early foetal life, and that the circula- 

 tion has been carried on by the persistence of one of the original vessels which 

 otherwise would have been obliterated. 



Thus in the case of double aortic arch, both the fourth branchial vascular arches 



