VERTEBRAL ARTERY. 383 



Varieties. — The variations in origin of the suhclaA-ian 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 arterj' crosses the neck a little higher than the 

 clavicle, but it is sometimes, especially on the right side. j)laced as high as an 

 inch or even an inch and a half alcove the level of that bone. 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 sub- 

 clavian vein. That vein has been also seen to pass with the ai-tery behind the 

 scalenus muscle. 



Bmnehcs. — Besides the variation in the position of the branches already refen-ed 

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

 have been found moved inwards from their usual position, or outwards to another 

 division of the subclavian. Sometimes two, and much more rarely tliree branches 

 arise from the third part of the vessel. 



BRANCHES OF THE SUBCLAVIAN ARTERY. 



1. Vertebral Artery. — The vertebral artery, which is usually the 

 first and largest branch of the subclavian, arises from the upper and 

 back part of that vessel, and ])assing upwards and a little backwards, 

 enters the transverse foramen of the sixth cervical vertebra — not unfre- 

 quently that of one of the liigher vertebrae. The vessel then ascends in a 

 vertical direction through the series of foramina of the transverse j^ro- 

 cesses, as far as to the ujjper border of the axis ; thence it inclines out- 

 wards 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 side to the fi'ont of the 

 medulla oblongata on the basilar process of the occipital bone, unites 

 with the vessel of the opposite side, at the lower border of the pons 

 Yarolii, to form the hasikrr artery. 



At its commencement, the vertebral artery lies behind the internal 

 jugular vein, and on approaching the vertebra 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 tlie foramina of the cervical vertebra3, the artery is 

 accompanied by a plexus of the sympathetic nerves and by the verte- 

 bral vein, which, as the vessels issue i'rom the foramen of the sixth 

 vertebra, is in front of the artery : the cervical nerves as they emerge 

 from the intervertebral foramina lie behind it. The suboccipital 

 nerve passes out beneath it, where it lies on the groove of the 

 atlas, and at that point the artery is covered by the superior oblique 

 muscle. 



Within the skull it turns round the side of the medulla oblongata, 

 between the origin of the hypoglossal nerve and the anterior root of the 

 sub-occipital, and then lies between the anterior surface of the medulla 

 and the basilar process of the occipital bone. 



Branches. — A. Cervical Branches : 



(«) In the neck, the vertebral artery sends ofi at different points of its coui'se 

 several small branches named lateral xjj'nial arteries. Each of these entering the 

 spinal canal through an intervertebral foramen divides into two branches ; one 



