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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 

Collateral Circulation.—After ligature of the third part of the subclavian artery, the collateral circulation is established mainly by three sets of vessels, thus described in a dissection:
  1. A posterior set, consisting of the transverse scapular and the descending ramus of the transverse cervical branches of the subclavian, anastomosing with the subscapular from the axillary.
  2. A medial set, produced by the connection of the internal mammary on the one hand, with the highest intercostal and lateral thoracic arteries, and the branches from the subscapular on the other.
  3. A middle or axillary set, consisting of a number of small vessels derived from branches of the subclavian, above, and, passing through the axilla, terminating either in the main trunk, or some of the branches of the axillary below. This last set presented most conspicuously the peculiar character of newly formed or, rather, dilated arteries, being excessively tortuous, and forming a complete plexus.
  The chief agent in the restoration of the axillary artery below the tumor was the subscapular artery, which communicated most freely with the internal mammary, transverse scapular and descending ramus of the transverse cervical branches of the subclavian, from all of which it received so great an influx of blood as to dilate it to three times its natural size. 1
  When a ligature is applied to the first part of the subclavian artery, the collateral circulation is carried on by: (1) the anastomosis between the superior and inferior thyroids; (2) the anastomosis of the two vertebrals; (3) the anastomosis of the internal mammary with the inferior epigastric and the aortic intercostals; (4) the costocervical anastomosing with the aortic intercostals; (5) the profunda cervicis anastomosing with the descending branch of the occipital; (6) the scapular branches of the thyrocervical trunk anastomosing with the branches of the axillary, and (7) the thoracic branches of the axillary anastomosing with the aortic intercostals.

Branches.—The branches of the subclavian artery are:
Vertebral.
Internal mammary.
Thyrocervical.
Costocervical.
  On the left side all four branches generally arise from the first portion of the vessel; but on the right side (Fig. 520) the costocervical trunk usually springs from the second portion of the vessel. On both sides of the neck, the first three branches arise close together at the medial border of the Scalenus anterior; in the majority of cases, a free interval of from 1.25 to 2.5 cm. exists between the commencement of the artery and the origin of the nearest branch.
  1. The vertebral artery (a. vertebralis) (Fig. 514), is the first branch of the subclavian, and arises from the upper and back part of the first portion of the vessel. It is surrounded by a plexus of nerve fibers derived from the inferior cervical ganglion of the sympathetic trunk, and ascends through the foramina in the transverse processes of the upper six cervical vertebræ 2 it then winds behind the superior articular process of the atlas and, entering the skull through the foramen magnum, unites, at the lower border of the pons, with the vessel of the opposite side to form the basilar artery.

Relations.—The vertebral artery may be divided into four parts: The first part runs upward and backward between the Longus colli and the Scalenus anterior. In front of it are the internal jugular and vertebral veins, and it is crossed by the inferior thyroid artery; the left vertebral is crossed by the thoracic duct also. Behind it are the transverse process of the seventh cervical vertebra, the sympathetic trunk and its inferior cervical ganglion. The second part runs upward through the foramina in the transverse processes of the upper six cervical vertebræ, and is surrounded by branches from the inferior cervical sympathetic ganglion and by a plexus of veins which unite to form the vertebral vein at the lower part of the neck. It is situated in front of the trunks of the cervical nerves, and pursues an almost vertical course as far as the transverse process of the atlas, above which it runs upward and lateralward to the foramen in the transverse process of the atlas. The third part issues from the latter foramen on the medial side of the Rectus capitis lateralis, and curves backward behind the superior articular process of the atlas, the anterior ramus of the first cervical nerve being on its medial side; it then lies in the groove on the upper surface of the posterior arch of the atlas, and enters the vertebral canal by passing beneath the posterior atlantoöccipital membrane. This part of the artery is covered by the Semispinalis capitis and is contained in the suboccipital triangle—a triangular space
Note 1.  Guy’s Hospital Reports, vol. i, 1836. Case of axillary aneurism, in which Aston Key had tied the subclavian artery on the lateral edge of the Scalenus anterior, twelve years previously. [back]
Note 2.  The vertebral artery sometimes enters the foramen in the transverse process of the fifth vertebra, and has been seen entering that of the seventh vertebra. [back]

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