THE AORTIC ARCH. 723 



of the left lung, where it divides into two branches to be distributed to the lobes of the lung. 

 From the upper border of the artery a short cj-lindrical cord passes to the under surface of the 

 transverse portion of the aortic arch, a little beyond the point at which the left subclavian artery 

 arises from its upper convex surface. This cord is the remains of ^ communication between 

 the pulmonary and systemic aortae which exists in fcetal life, when the lungs are not functional, 

 and is termed the ductus arteriosus. It represents the outer portion of the vessel of the sixth 

 branchial arch of the left side, and its lumen usually becomes occluded during the first few 

 months after birth, so that, as a rule, the cord is solid in the adult. 



Variations. — The majority of the variations that have been observed in the pulmonary 

 aorta are associated with serious malformations of the heart which usually result in early death, 

 and are consequently to be classed as pathological rather than as merely anomalous conditions. 

 A precocious division of the main stem of the pulmonary aorta occasionally occurs, absence of 

 the right pulmonary artery has been observed, and an accessory coronary artery has been noted 

 arising from the pulmonary aorta. 



Failure of the ductus arteriosus to undergo complete occlusion is a not infrequent occur- 

 rence, and is often associated with a persistence of the foramen ovale. The ductus has also 

 been observed to arise directly from the right ventricle. 



THE SYSTEMIC AORTA. 



The systemic aorta, or, as it is more commonly and more simply termed, the 

 aorta, is the main arterial stem for the supply of the tissues of the body. It arises 

 from the base of the left ventricle and curves in an arch-like manner to the left side of 

 the vertebral column, along which it runs to the level of the fourth lumbar vertebra. 

 There it gives of? a pair of large common iliac arteries, and is continued onward, much 

 reduced in size, along the ventral surface of the sacrum and coccyx, being termed in 

 this portion of its course the middle sacral artery. 



It may be regarded, for the purpose of description, as being composed of three 

 portions : (i) the aortic arch, which extends from the heart to the left side of the 

 body of the fourth thoracic vertebra ; (2) the thoracic aorta, extending from the 

 lower end of the aortic arch to the diaphragm ; and (3) the abdo])ii)ial aorta, extend- 

 ing from the diaphragm to the fourth lumbar vertebra. The middle sacral artery 

 may most conveniently be treated as a branch of the abdominal aorta. 



THE AORTIC ARCH. 



The aortic arch arises from the base of the left ventricle (Figs. 679, 690), and 

 in the first or ascending portion (aorta ascendcns) of its course is directed upward and 

 somewhat forward and to the right. It then curves to the left and backward as the 

 transverse portion (arcus aortae), and finally bends downward as the descending 

 portion along the left side of the body of the fourth thoracic vertebra, to become 

 continuous wdth the thoracic aorta. 



At its origin the aortic arch presents three rounded swellings, one anterior and the 

 other two postero-lateral, marking the position of the sinuses of Valsah'a (sinus aortae). 

 The diameter of the ascending portion is about 2.7 cm. and that of the descending 

 portion about 2 cm. , the diminution appearing rather suddenly below the origin of 

 the left subclavian artery and forming what has been termed the aortic isthmus. 

 Where the ascending portion passes over into the transverse an enlargement of the 

 diameter occurs which is especially well marked in older individuals, and is presuma- 

 bly due to the impact of the blood forced out of the ventricle by its contractions. 



At about the junction of its transverse and descending portions the arch has 

 attached to its under surface the fibrous cord which represents the foetal ductus 

 arteriosus. 



Relations. — The ascending portion of the arch is enclosed throughout 

 almost its entire length (about 5 cm., or 2 in.) in the sheath, formed by the visceral 

 layer of the pericardium, which it shares with the pulmonary aorta. At its origin 

 it lies behind and somewhat to the left of that vessel, but higher up crosses it 

 obliquely, so that it comes to lie upon its right side ; to the right and left it is in 

 relation with the corresponding auricles, and anteriorly its upper portion is separ- 

 ated from contact with the sternum by a more or less abundant fattv tissue in which 

 are the remains of the thymus gland. Posteriorly it is in relation with the anterior 

 surface of the auricles. 



