520 THE BLOOD-VASCULAR SYSTEM. 



Branches. The branches given off from the subclavian artery are : 



Vertebral. Thyroid axis. 



Internal mammary. Superior intercostal. 



On the left side all four branches generally arise from the first portion of the 

 vessel ; but on the right side, the superior intercostal usually arises from the 



second portion of the vessel. On both 

 sides of the body the first three branches 

 arise close together at the inner margin 

 of the Scalenus anticus ; in the majority 

 of cases, a free interval of from half an 

 inch to an inch exists between the com- 

 mencement of the artery and the origin of 

 the nearest branch ; in a samller number of 

 cases, an interval of more than an inch ex- 

 ists, but it never exceeds an inch and three- 

 quarters. In a very few instances, the 

 interval has been found to be less than half 

 an inch. The vertebral artery arises from 

 the upper and posterior part of the artery, 

 the internal mammary from the lower part 

 f thc branches f tbe right of the artery ; the thyroid axis from in front, 



and the superior intercostal from behind. 

 The Vertebral Artery (Fig. 289) is generally the first arid largest branch of 

 the subclavian ; it arises from the upper and back part of the first portion of the 

 vessel, and, passing upward, enters the foramen in the transverse process of the 

 sixth cervical vertebra, 1 and ascends through the foramina in the transverse 

 processes of all the vertebrae above this. Above the upper border of the axis it in- 

 clines outward and upward to the foramen in the transverse process of the atlas, 

 through which it passes ; it then winds backward behind its articular process, runs 

 in a deep groove on the upper surface of the posterior arch of this bone, and, pass- 

 ing beneath the posterior occipito-atlantal ligament, pierces the dura mater and 

 arachnoid, and enters the skull through the foramen magnum. It then passes for- 

 ward and upward, inclining from the lateral aspect to the front of the medulla 

 oblongata. It unites in the middle line with the vessel of the opposite side at the 

 lower border of the pons Varolii to form the basilar artery. 



Relations. At its origin, it is situated behind the internal jugular and vertebral 

 veins, and is crossed by the inferior thyroid artery : it lies between the Longus 

 colli and Scalenus anticus muscles, having the thoracic duct in front of it on the 

 left side. It rests on the transverse process of the seventh cervical vertebra and 

 the sympathetic nerve. Within the foramina formed by the transverse processes of 

 the vertebrae it is accompanied by a plexus of nerves from the inferior cervical 

 ganglion of the sympathetic, and is surrounded by a dense plexus of veins which 

 unite to form the vertebral vein at the lower part of the neck. It is situated in 

 front of the cervical nerves, as they issue from the intervertebral foramina. While 

 winding round the articular process of the atlas, it is contained in a triangular 

 space (suboccipital triangle) formed by the Rectus capitis posticus major, the 

 Superior and Inferior oblique muscles ; and at this point is covered by the Corn- 

 plexus muscle. The suboccipital nerve here lies between the artery and the bone. 

 Within the skull, as it winds round the medulla oblongata, it is placed between the 

 hypoglossal nerve and the anterior root of the suboccipital nerve, beneath the first 

 digitation of the ligamentum denticulatum, and finally ascends between the basilar 

 process of the occipital bone and the anterior surface of the medulla oblongata. 



1 The vertebral artery sometimes enters the foramen in the transverse process of the fifth ver- 

 tebra. Dr. Smyth, who tied this artery in the living subject, found it, in one of his dissections, pass- 

 ing into the foramen in the seventh vertebra. 



