This section is from the book "Anatomy Of The Arteries Of The Human Body", by John Hatch Power. Also available from Amazon: Anatomy of the Arteries of the Human Body, with the Descriptive Anatomy of the Heart.
The Vertebral Artery is usually the first branch of the subclavian, and comes off from the superior and posterior portion of that vessel: it may be divided into four stages. In the first it ascends almost vertically in the neck as high as the foramen in the transverse process of the sixth cervical vertebra: in the second it passes through the foramina of the transverse processes; in the third it passes horizontally inwards, behind the occipito-atlantoid articulation; and in the fourth it passes obliquely upwards, forwards, and inwards, on the side of the medulla oblongata.
In its first stage, at its origin from the subclavian artery, it lies a little to the outside of the carotid, and passes upwards and backwards, situated in an angular space formed between the scalenus anticus muscle externally, and the longus colli internally. In this course it lies on the inferior cervical ganglion of the sympathetic nerve, and is covered in front by the vertebral vein, and by the inferior thyroid artery, which crosses its course and separates it from the common carotid.
Vertebral. Internal Mammary.
Thyroid Axis. Cervicalis Profunda.
In its second stage it enters the foramen in the transverse process of the sixth cervical vertebra, and passes through the corresponding foramina of the vertebrae above it. In this course it is accompanied by the vertebral vein and by a plexus of branches given off from the inferior cervical ganglion : it ascends between the anterior and posterior intertransverse muscles, and in front of the anterior branches of the cervical nerves, along each of which it sends a small artery to the spinal marrow; these small branches are called the lateral spinal arteries. It also gives off some muscular branches in its course which anastomose with the cervicalis superficialis and ascendens colli arteries. After the vertebral artery has passed through the foramen in the transverse process of the second vertebra, it inclines upwards and outwards to reach that of the atlas, which extends farther outwards than the transverse process of the dentata; in its course from the one process to the other it describes a curve, the convexity of which looks downwards, backwards, and outwards.
In its third, stage it is horizontal. After the artery has passed through the transverse process of the atlas, it is placed at the inner side of the rectus capitis lateralis muscle, which here separates it from the occipital artery which lies at the outer side of the muscle: from this point the vessel is directed at first backwards and inwards, and then winds forwards and inwards to pierce the posterior occipito-atlantoid ligament. In this course, its concavity, turned forwards, embraces the articulation between the atlas and the condyle of the occipital bone: its convexity, turned backwards, may be seen in a triangular space, bounded internally or towards the middle line by the rectus capitis posticus major muscle, above by the obliquus superior muscle, and below by the obliquus inferior. Inferiorly it lies in a groove on the upper surface of the posterior arch of the atlas, but is here separated from the bone by the interposition of the ganglionic dilatation of the tenth or sub-occipital nerve: whilst resting on this portion of the atlas, the horizontal curve of the artery is situated on a plane superior and posterior to the first cervical nerve as it escapes from the spinal canal behind the inferior oblique process of the atlas. Superiorly the vertebral artery is covered by a production of the posterior occipito-atlantoid ligament, which converts the groove upon the atlas for the artery, into a canal. In this stage the artery gives off minute branches which anastomose with others from the occipital and cervicalis profunda arteries.
In its fourth stage the vertebral artery pierces the dura mater beneath the insertion of the first tooth of the ligamentum dentatum, passes upwards and inwards upon the front of that structure, which consequently separates the artery from the spinal accessory nerve as it is passing upwards and outwards behind the ligament. The artery then runs either before or through the midst of the fibrils composing the ninth nerve, applies itself to the side of the medulla oblongata, and afterwards, getting in front of this body, it joins the vertebral of the opposite side at the posterior inferior margin of the pons, and forms the basilar trunk.
The branches given off by the vertebral arteries before their junction to form the basilar artery, are the following:
The Lateral Spinal Arteries are given off from the artery as it is passing through the foramina in the transverse processes; they pass in along the spinal nerves to the interior of the spinal canal, and are distributed to these nerves, to the medulla spinalis and its membranes, and to the back part of the bodies of the cervical vertebrae: they anastomose with the other spinal arteries in the interior of the canal.
The Muscular Arteries are given off from the vertebral in its second and third stages: these supply the deep muscles of the neck and anastomose with the cervicalis superficialis and ascendens colli arteries.
The Anastomotic Branches are comparatively large: they come off from the vertebral in its third stage, pass backwards and outwards and anastomose with branches from the occipital in its second stage.
The Posterior Meningeal Artery, described by Haller and Soemmering, arises from the vertebral artery, generally speaking, in the third stage, passes through the occipital foramen, and is distributed to the dura mater lining the inferior occipital fossae, and to the falx cere-belli: there may be two of these arteries present. The branch described by Soemmering enters the cranium along with the sub-occipital nerve.