BRANCHES OF THE SUBCLAVIAN. VERTEBRAL. 367 



Another branch, in the great majority of instances, arises from the third 

 part of the artery. This is most frequently the posterior scapular artery, a 

 branch which otherwise is derived from the transverse cervical, one of the 

 divisions of the thyroid axis. This circumstance is of surgical interest, as 

 the third part of the subclavian artery is the portion of the vessel usually 

 tied for axillary aneurism. As the right subclavian artery is likewise 

 accessible to the surgeon in its first part, it is proper to mention that the 

 distance between the origin of the vessel and its first branch is usually 

 between half an inch and an inch, and that it very rarely is less than half 

 an inch or more than an inch and a half. 



PECULIARITIES. The variations in origin of the subclavian arteries have been con- 

 sidered along with the peculiarities of the arch of the aorta. 



Course. The height to which these vessels reach in the neck is liable to some 

 variation. Most commonly the artery crosses the neck a little higher than the 

 clavicle, but it is sometimes placed as high as an inch or even an inch and a half 

 above the level of that bone. The greater extent of elevation above the clavicle, how- 

 ever, is especially seen in the artery of the right side. Occasionally the subclavian 

 artery perforates the anterior scalenus muscle, and in a few rare cases it has been seen 

 altogether in front of the muscle, and close to the subclavian vein. That vein has 

 been also seen to pass with the artery behind the scalenus muscle. 



Branches. Besides the variation in amount of the branches already referred to, 

 it may be noticed that, in a few cases, one or more of the three first branches have 

 been found moved inwards from tlieir usual position, or outwards to another division 

 of the subclavian. Sometimes two, and much more rarely three branches arise from 

 the third part of the vessel. 



BRANCHES OF THE SUBCLAVIAN ARTERY. 



I. VERTEBRAL ARTERY. 



The vertebral artery, which is usually the first and largest branch of the 

 subclavian, arising from the upper and back part of that vessel, passes 

 upwards and a little backwards, and enters the transverse foramen of the 

 sixth cervical vertebra not unfrequently that of some higher vertebra. 

 The vessel then ascends in a vertical direction through the series of foramina 

 of the transverse processes, as far as to the upper border of the axis ; there 

 it inclines outwards to reach the corresponding foramen of the atlas, and 

 after passing through that aperture winds backwards and inwards in the 

 groove on the neural arch of that vertebra, and, piercing the dura mater, 

 enters the skull through the foramen magnum. Finally, it proceeds 

 upwards and forwards, and turning round from the sile to the front of the 

 medulla oblongata on the ba^ilar process of the occipital bone, unites with 

 the vessel of the opposite side, at the lower border of the pons Varolii, to 

 form the basilar artery. 



At its commencement, the vertebral artery lies behind the internal jugular 

 vein, and on approaching the vertebrse passes between the longus colli and 

 the scalenus anticus muscle. On the left side, the thoracic duct in ascending 

 crosses in front of the vessel from within outwards. 



While within the foramina of the cervical vertebrae, the artery is accom- 

 panied by a plexus of the sympathetic nerves and by the vertebral vein, 

 which, as the vessels issue from the foramen of the sixth vertebra, is in 

 front of the artery : the cervical nerves as they emerge from the inter- 

 vertebral foramina lie behind it. The suboccipital nerve passes out beneath 



