508 THE ARTERIES 



running in this is the siipraseai)ular artery. The sul^clavian is crossed by this 

 artery unless the arm is drawn -well downwards. Hence one of the reasons for 

 depressing the shoulder, and thus avoiding the suprascapular artery, in the opera- 

 tion of ligaturing the subclavian. Close to the outer margin of the sterno-mastoid, 

 the external jugular vein pierces the fascia, and crosses the subclavian artery to 

 open into the subclavian vein. As this vein lies between the two layers of fascia, it 

 receives on its external side the suprascapular, transverse cervical, and other veins 

 of the neck, which together form a plexus of large veins in front of the artery. The 

 nerve to the subclavius, and when present the accessory l)ranch from this nerve to 

 the phrenic, also here cross in front of the artery. In very muscular subjects the 

 sterno-mastoid may extend further outwards along the clavicle than usual, and in 

 such a case will form one of the coverings of the artery. 



Behind, the artery is in contact with the scalenus medius, and with the cord of 

 the brachial plexus formed by the union of the eighth cervical and first dorsal 

 nerve. 



Below, the artery rests in the posterior of the two grooves on the upper surface 

 of the first rib. 



Above is the ]:»rachial plexus of nerves and the posterior ])elly of the omo-hyoid 

 nuiscle. The cord formed l)y the fifth and sixth cervical nerves is also alwve the 

 artery, but on a somewhat anterior plane. It is close to the vessel, and has been 

 mistaken for the artery in the application of a ligature. 



As a rule there is no branch given off from the third portion of the subclavian. 

 At times, however, the suprascapular (fig. 329) or the posterior scapular artery may 

 arise from the third portion of the sul^clavian, instead of from the thyroid axis and 

 from the transverse cervical respectively as here descril:)ed. 



Chief Variations in the Subclavian Artery 



(A) The right subclavian artery may arise directly from the arch of the aorta, and then come 

 off as the first, second, tliird or fourth branch of that vessel. When it arises as the first branch, 

 it takes the place usually occupied by the innominate ; wlien it arises as the last branch, it 

 courses behind the trachea and oesophagus to gain the groove on the first rib. As the second or 

 third branch of the aortic arch it is very rare ; in both instances it then runs behind the right 

 common carotid. The explanation of the right subclavian arising as the last branch of the arch 

 of the aorta, is that the right aortic arch has remained pervious, whilst the normal root of the 

 subclavian artery has become obliterated. An arteria aberrans, given off from the right subclavian 

 or from the superior intercostal, can generally be traced to the third thoracic vertebra behind the 

 oesophagus, and in a number of such cases can be followed across the s]iine to anastomose with a 

 branch of the thoracic aorta given off below the ductus arteriosus. It is the enlargement of this 

 anastomosis — which is itself the remains of what was the primitive right dorsal aorta in the 

 embryo — that gives rise to the abnormality in question. The inferior laryngeal nerve in such 

 cases, in consequence of the right fourth arch which firms the first portion of the subclavian being 

 obliterated, follows a direct course to the larynx instead of winding recurrently round the sub- 

 clavian artery. 



(B) The righ{ subclavian may arise higher or lower in the neck than usual, according as the 

 innominate divides above or below the normal situation. 



(C) It may perforate the scalenus anticus or pass in front of that muscle. 



(D) It may ascend as high as an inch and a half above the clavicle, or remain below the level 

 of that bone. 



(E) The third^ iiart of the artery may be covered by the trapezius or sterno-mastoid, or by a 

 clavicular origin of the omo-hyoid. 



(F) The subclavian vein may accompany the artery behind the scalenus anticus. 



Branches of the Si'bcl avian Artery 



From the first portion of the subclavian artery are given off: from the upper 

 and back ])art and al)out three-quarters of an inch (2 cm. ) from its origin, the ver- 

 tebral ; a little further outwards, from the front })art, the thyroid axis ; and from 

 the lower part — usually o)>])()sit(' the thyroid axis, or else ])etween the thyroid axis 

 and the verteljral — the internal mammary. 



From the second portion arises, from the back of the vessel, the superior 

 intercostal. 



The third portion as a rule, gives off no branch. 



