THE THORAX 9^3 



2. Branches ol the Arch and their Positions. — Varieties in these respects are 

 very numerous. The normal number of branches arising from the arch is three. 

 There may, however, be four. The most common additional branch is the left 

 vertebral artery, its place of origin being between the left common carotid and 

 left subclavian arteries. The right vertebral artery sometimes arises from . 

 the arch, but this is somewhat rare. An arteria thyroidea ima may arise 

 from the arch betw-een the innominate and left comm.on carotid arteries. 

 In rare cases the internal mammary artery, or the inferior thyroid, may spring 

 from the arch. The innominate artery may be absent, in which cases the 

 right subclavian and right common carotid have independent origins. Under 

 these circumstances the right subclavian artery may be the last of the branches 

 from the upper part of the arch, and, when this is so, in order to reach the 

 right side of the neck, it crosses in front of the vertebral column, lying behind 

 the oesophagus, or between the trachea and the oesophagus. 



Development of the Arch of the Aorta and its Branches. — As already 

 stated, the ascending aorta is developed, along with the pulmonary artery, 

 from the aortic bulb. The arch of the aorta is developed chiefly from the 

 fourth left arterial arch, but slightly also from the root of the left primitive 

 ventral aorta, that is, the part as high as the fourth left arterial arch. 



The innominate artery is developed from the root of the right primitive 

 ventral aorta — that is, the part as high as the fourth right arterial arch. 



The left common carotid artery is developed from that portion of the left 

 primitive ventral aorta which lies between the fourth and third left arterial arches. 



The left subclavian artery is developed from a segmental artery. 



For the description of the descending aorta, see p. 1023. 



Pulmonary Artery. — ^This is the great vessel which, by means 

 of its right and left divisions, carries the venous blood from 

 the right ventricle of the heart to the lungs. It is therefore an 

 example of an artery which conveys venous blood, and in this 

 respect resembles the hypogastric and umbilical arteries of the 

 foetus. It arises from the infundibulum, or conus arteriosus, of 

 the right ventricle of the heart, on a level with the upper 

 margin of the third left costal cartilage at its junction with 

 the sternum. It is directed upwards and backwards, and, after a 

 course of about 2 inches, breaks up into two divisions, right and 

 left, within the concavity of the arch of the aorta. The vessel lies 

 in the middle mediastinum, and, along with the ascending aorta, it 

 is contained within the pericardium, the serous portion of which 

 forms one common sheath for the two arteries over about the first 

 i^ inches of their course. 



Relations — Anterior. — ^The sternal extremity of the second left 

 intercostal space and second left costal cartilage ; and the left 

 pleura and left lung. Posterior. — ^The root of the ascending aorta ; 

 the commencement of the left coronary artery ; and the left auricle 

 of the heart. Right. — ^The right coronary artery ; the right auricular 

 appendix ; and the ascending aorta. Left. — ^The left coronary artery ; 

 and the left auricular appendix. 



The only branches of the artery are the two terminal divisions. 



The right pulmonary artery passes outwards to the right, behind 

 the ascending aorta and superior vena cava, to the root of the right 

 lung, where it divides into two branches, upper and lower. The 

 upper branch is distributed to the upper lobe, and the lower branch, 

 which is the larger of the two, is distributed to the middle and 

 lower lobes. 



