38 TOPOGRAPHICAL ANATOMY OF THE 



arteriosum. The ligament of the adult represents the ductus 



arteriosus of the embryo, by which most of the blood of the pulmonary 



artery, not required by the non-functioning lungs, was transferred to the 



aorta. 



The aortic arch is enclosed within the pericardium to about the 



point of attachment of the ligamentum arteriosum and is crossed on the 



right by the trachea, the oesophagus and the left recurrent nerve. The 



left surface of the arch is related to the left vagus nerve, from which 



the left recurrent turns round the concavity of the arch to reach its 



right face. 



Dissection. — The common brachio-cephalic trunk and its branches 

 must be followed to the entrance to the thorax. This will be made 

 easier if the cranial vena cava is cut across just before it enters the 

 pericardium. In cleaning the blood vessels great care must be taken to 

 preserve intact the numerous nerves — some of them of small size — to 

 which they are related. 



Consideration of the arteries arising from the aortic arch in different 

 mammals leads to the conclusion that the rnost primitive arrangement 

 is one in which four vessels have independent origin from the arch. In 

 the order of their origin these are the right subclavian, the right 

 common carotid, the left common carotid and the left subclavian. The 

 right and left subclavian arteries supply blood to the right and left 

 thoracic limbs and the rioht and left sides of the wall of the chest 

 respectively ; while the right and left common carotid arteries furnish 

 branches to the two sides of the head and neck. 



The simple arrangement of four independent arteries, however, is 

 not common. In most mammals a greater or less amount of fusion 

 occurs whereby two or more arteries arise together from the aortic 

 arch. In man, for example, the right subclavian and the right common 

 carotid arteries spring from a common trunk, with the result that the 

 vessels leaving the aortic arch are reduced to three in number. In the 

 dog a further reduction is produced by the fusion of the left common 

 carotid with the common trunk ; so that, in this animal, the aortic arch 

 has only two arteries springing from it, namely, a brachio-cephalic and 

 a left subclavian : the brachio-cephalic giving off the left common 

 carotid and ending in the right common carotid and right subclavian 

 arteries. 



In the horse and the domestic ruminants the maximum of fusion 

 takes place, for in these animals the left subclavian has also joined the 

 common trunk. The single vessel leavins^ the aortic arch of these 

 animals is the common brachio-cephalic trunk, which divides into the 

 brachio-cephalic and left subclavian arteries. 



In the horse and ruminants, moreover, the right and left common 



