822 



SUBCLAVIAN ARTERIES. 



Varieties occasionally observable in the verte- 

 bral arteries. ]. Of origin. n. It has already 

 been mentioned that the vertebral artery 

 may arise from different portions of the first 

 stage of the subclavian artery being sometimes 

 nearer, and sometimes further removed, from 

 the innominata; but, independently of these 

 varieties, the vertebral artery on the right 

 side is 



b. Sometimes furnished by the common 

 carotid artery. In all the cases where this 

 anomaly lias been observed, the right subcla- 

 vian artery was given off directly as a branch 

 of the aorta. Again, 



c. The vertebral artery sometimes comes 

 off from the arch of the aorta. This irre- 

 gularity is as [infrequent on the right, as it is 

 common on the left side. When the left 

 vertebral artery springs from the arch of the 

 aorta, it usually arises between the left carotid 

 and the left subclavian arteries, though some- 

 times its origin has been found to the left of 

 all the other branches of the arch. 



d. In a few instances, more than one vessel 

 has been found to constitute the origin of the 

 vertebral artery; thus, it may be formed by 

 the union of two roots, both arising from the 

 subclavian artery, or one from the subclavian 

 and the other from the aorta. In one exam- 

 ple, where it was formed by three roots, two of 

 these were derived from the subclavian, and 

 the third from the inferior thyroid artery. 

 These roots of the vertebral artery in some 

 instances united before the artery had become 

 engaged in the vertebral foramina, whilst in 

 others the union took place subsequently. 



2. Of size. There is often a considerable 

 difference in the size of the two vertebral arte- 

 ries, which is stated to be most frequently in 

 favour of that of the left side ; thus, in 98 ob- 

 servations made by Mr. Davy, the left verte- 

 bral artery was the larger in twenty-six, and 

 the right in eight instances only. 



3. Of course and relations. The vertebral 

 artery may enter the transverse process of the 

 last cervical vertebra (though the contrary has 

 been asserted), or it may enter one of the fo- 

 ramina higher than that in the transverse pro- 

 cess of the sixth, which latter it usually selects. 

 When the artery enters any vertebra higher 

 than the sixth cervical, it always occupies an 

 unusually superficial position in the neck, 

 lying external and parallel to the common 

 carotid artery, for which, consequently, it is 

 liable to be mistaken. (Vide CAROTID AR- 

 TERY.) 



The vertebral vein corresponds to the cer- 

 vical stage only of the artery. Its origin is 

 found in some branches from the deep muscles 

 at the back of the neck, joined by one from 

 the occipital vein, and by another which 

 passes through the posterior condyloid fo- 

 ramen. The vertebral vein traverses the 

 canal in the transverse process of the atlas, 

 and descends through the same foramina by 

 which the artery ascends ; whilst here it lies in 

 front of the artery, and has the same relations 

 as that vessel. Emerging from the foramen in 



the sixth vertebra, the vertebral vein (liable 

 to the varieties already specified) opens into 

 the vena innominata close to its junction with 

 the internal jugular. 



II. Internal mammary artery. This ar- 

 tery arises (more externally than the ver- 

 tebral) from the anterior surface of the sub- 

 clavian, and near to the inner margin of the 

 anterior scaknus muscle. From this origin it 

 runs, first forwards, then downwards and 

 inwards, and enters the thorax, lying between 

 the pleura and the internal layer of intercostal 

 muscles. 



Previous to entering the thorax, the in- 

 ternal mammary artery is crossed anteriorly 

 by the vena innominata and by the phrenic 

 nerve : the latter intersects the artery ob- 

 liquely from above and without, downwards 

 and inwards. In the thorax, however, the 

 nerve attains a position much posterior to the 

 artery. 



The mammary artery descends on the back 

 of the anterior parieties of the chest a little 

 external to the junction of the costal cartilages 

 with the sternum, and is covered posteriorly 

 by the pleura at the line of reflexion of that 

 membrane to form the side of the anterior 

 mediastinum. Arrived at the cartilage of 

 the third rib, the artery in its farther 

 descent inclines a little outwards, and be- 

 comes separated from the pleura by the 

 fibres of the triangularis sterni; the vessel is 

 now placed between that muscle, which lies 

 behind it, and the internal layer of intercostal 

 muscles and the cartilages of the lower true 

 ribs, which constitute its anterior relations. 

 Opposite the cartilage of the seventh rib the 

 mammary artery terminates by dividing into 

 two branches, an external and an internal. 



Branches of the mammary artery. 1. 

 Mediaslinnl branches, which are distributed 

 to the thymus gland (thymic arteries), and 

 the cellular issue of the anterior mediasti- 



num. 



2. A descending muscular branch to the 

 diaphragm (superior phrenic), also termed, from 

 its so constantly accompanying the phrenic 

 nerve, the comes nervi phrenici. This branch 

 accompanies the nerve in a tortuous manner, 

 between the pleura and the pericardium, to 

 reach the upper surface of the diaphragm, 

 where it anastomoses with the phrenic arte- 

 ries from the aorta. 



3. The anterior intercostal arteries. These 

 are distributed to the six upper intercostal 

 spaces ; but their number is greater than that 

 of the spaces for which they are destined, as 

 two branches are frequently found between 

 adjacent ribs, and this arrangement may even 

 prevail in all of these intercostal spaces. 



The anterior intercostal arteries pass out- 

 wards in the intervals between the two planes 

 of intercostals, in which muscles some of their 

 branches terminate ; others are lost in anasto- 

 mosing with the intercostal arteries from the 

 aorta, whilst many perforate the muscular 

 fibres, and, arriving on the external surface 

 of the thorax, dip into the pectoral muscles 



