ABNOKMALITIES OF AETEEIES. 1055 



The right vertebral may arise from the common carotid or from the arch of the aorta. 

 Occasionally it springs from the descending aorta, an arrangement associated with the persistence 

 of the dorsal roots of the fourth and fifth right arches. 



The left vertebral artery not infrequently springs from the arch of the aorta, arising between 

 the left common carotid and left subclavian arteries ; this is evidently due to the, absorption of 

 the stem of the seventh segmental artery into the aortic arch. Very exceptionally the left 

 vertebral is a branch of an intercostal artery. 



In its course upwards either vertebral artery may enter the vertebrarterial foramen of any of 

 the lower six cervical vertebra. 



The cases in which it does not enter one of the lowest of these are apparently associated with 

 its formation, in part, from the precostal instead of from the postcostal anastomosing channels. 



The artery may enter the vertebral canal with the second instead of with the first cervical 

 nerve, or, after leaving the foramen in the transverse process of the third vertebra, it may divide 

 into two branches, one of which accompanies the second and the other the first cervical nerve ; the 

 two branches unite together again in the vertebral canal to form a single trunk. 



Sometimes, though rarely, it gives off superior intercostal and inferior thyreoid branches. The 

 upper end of one of the vertebrals is sometimes very small, or it may be entirely wanting ; in 

 the latter case the basilar artery is formed by the direct continuation of the opposite vertebral. 



The basilar artery may be double in part or the whole of its extent, or its cavity may be 

 divided by a more or less complete septum. It may terminate in one instead of two posterior 

 cerebral arteries, the missing vessel being supplied by the enlargement of the posterior com- 

 municating branch of the internal carotid. 



THE ARTERIES OF THE UPPER LIMB. 



Subclavian Arteries. The variations, so far as regards the origins of the subclavian 



sries, have already been mentioned (p. 1051). Other interesting modifications are met with in 

 respect of its position and branches. 



The subclavian artery may reach as high as 25 or 37 mm. (1-H in.) above the clavicle, though 

 as a rule it does not reach higher than 19 mm. above that bone. On the other hand, it may 

 not rise even to the level of the upper border of the clavicle. These differences appear to be 

 associated with the descent of the clavicle and sternum, which occurs as age increases. 



The artery may pass in front of or through the scalenus anterior instead of behind it, or 

 the vein may accompany it behind the muscle. 



The branches of the subclavian artery may be modified with reference to their points of origin ; 

 thus, those of the first part may be further medial or lateral than usual, the transverse scapular 

 or some other branch of the thyreo-cervical trunk may arise separately from the third part 

 of the subclavian, and not uncommonly the descending branch of the transverse cervical artery 

 is a branch of that part. The abnormalities of the vertebral branch have already been described ; 

 those of the thyreo-cervical trunk and its branches are numerous but not important. 



The internal mammary artery, usually a branch of the first part of the subclavian, is very 

 variable as regards its origin. It may arise from the second or third parts, or from the thyreo- 

 cervical, or it may spring from the aorta, or from the innominate or axillary arteries. All these 

 variations are due to obliteration of the normal origin and the opening up of anastomoses. 

 The internal mammary artery sometimes descends in front of the cartilages of one or more of 

 the lower true ribs ; and occasionally it gives off a large lateral branch (a. mammaria lateralis) 

 which descends on the inner side of the chest wall, close to the mid -axillary line, a point of 

 importance in paracentesis. 



A few cases have also been noticed in which a bronchial artery has arisen from the internal 

 mammary. 



The superior intercostal branch of the costo-cervical trunk may be absent, and the pro- 

 funda cervicis branch may arise directly from the subclavian trunk. The superior intercostal is 

 sometimes formed from a postcostal instead of a precostal primitive channel. In such cases it 

 passes between the necks of the ribs and the transverse processes of the vertebrae instead of, as 

 usual, in front of the necks of the ribs. 



The axillary artery does not vary much as regards its origin or course. Its relations may be 

 modified by the existence of a muscular or tendinous " axillary arch," which, passing from the 

 latissimus dorsi to the pectoralis major, crosses the distal part of the artery superficially ; and 

 a further interesting modification is associated with an anomalous arrangement of its branches. 

 Occasionally the sub-scapular, circumflex, and profunda and superior ulnar collateral arteries 

 arise from the axillary by a common stem. In those cases the chief branches of the brachial 

 plexus are grouped round the common stem instead of round the main trunk. The arrangement 

 is due to the persistence of a different part of the original vascular plexus. 



Sometimes the axillary artery divides into the radial and ulnar arteries, and more rarely the 

 interosseous artery may spring from it. 



Obviously there is no brachial artery when the radial and ulnar arteries are formed by the 



vision of the axillary ; its place is taken by the two abnormal vessels which, as a rule, are 

 separated by the median nerve as they run through the arm ; the radial is usually more super- 

 ficial than the ulnar, and crosses laterally in front of it at the bend of the elbow. 



The brachial artery is rarely prolonged beyond its usual point of bifurcation ; not uncommonly, 

 however, it bifurcates at a more proximal level. Of the two terminal branches of the brachial, 



"~ may divide into radial and interosseous, the other forming the ulnar ; or one may divide into 



one 



