148 Anatomy Applied to Medicine and Surge* y. 



limit of the left sterno-clavicular articulation. The left 

 subclavian, about two and a half inches in length, arises 

 on a plane posterior to, and a little to the left of the caro- 

 tid, and ascends almost vertically to the thoracic inlet, 

 where it curves outward over the apex of the left lung 

 and pleura to the inner border of the scalenus anticus 

 muscle. The variations that have been met with in the 

 aortic arch are numerous and are due to abnormalities in 

 the development of the vessel from the embryonic vessels, 

 i.e., from the ventral aortic stem, the dorsal stem and the 

 fourth and fifth branchial arches joining them. One of 

 these departures from the normal condition is occlusion 

 of the arch at its junction with the thoracic aorta just be- 

 low the ductus arteriosus, and, in this condition, the ana- 

 stomosis, whereby the circulation is still carried on, is as 

 follows : The internal mammary communicates with the 

 intercostals, with the deep epigastric, and with the phrenic 

 branch of the abdominal aorta, by means of the mus- 

 culo-phrenic and the comes nervi phrenici. The superior 

 intercostal from the subclavian communicates with the 

 first aortic intercostal; while the transversalis colli and 

 the suprascapular anastomose with the intercostals and 

 with the branches of the axillary artery. 



Landmarks for the Arteries. To mark out the 

 ascending aorta draw a line, with a slight convexity to the 

 right, from a point a little to the left of the centre of the 

 sternum on a level with the middle of the third costal car- 

 tilages, to the upper border of the second right costal car- 

 tilage. Since the transverse aorta passes almost directly 

 backwards behind the first piece of the sternum, it cannot 

 be readily mapped out, nor, can the descending aorta, be- 

 cause of its depth. The innominate is represented by a 

 line drawn from a point a little to the right of the centre 

 of the manubrium and about one inch below the upper 



