From its origin, the vertebral artery assumes a direction upwards, backwards, and slightly outwards to the foramen, in the transverse process of the sixth cervical vertebra ; having passed through this, it is transmitted from vertebra to vertebra by the foramina which their transverse processes present, until finally it traverses the foramen magnum, where its cerebral, or cranial, stage commences.
its origin until it enters the transverse process of the sixth cervical vertebra, the vertebral artery is placed in a muscular interspace, between the longus colli and the scalenus anticus muscles, where it is related in front to the trunk of the subclavian artery, to its own vein, and to the inferior thyroid artery ; the last of these, as it passes transversely inwards, is interposed between the vertebral vessels and the sheath of the carotid artery (in the sheath, the internal jugular vein exactly corresponds to the anterior sur face of the vertebral artery). By its posterior surface, the vertebral artery is in relation with the inferior cervical ganglion of the sympa thetic nerve.
From the sixth to the second cervical ver tebra inclusive, the vertebral artery is lodged in the bony foramina of the transverse pro cesses; in the spaces between these, it is enclosed by the pairs of intertransverse muscles, being in close contact with the anterior set, whilst from the posterior it is separated the spinal nerves in their trajet outwards. In this part of its course the artery is slightly tortuous, a provision, no doubt, to guard it against injury in the free and varied motions of this portion of the spine. For a similar reason its tortuosity increases remarkably in the subsequent stage.
Having passed through the transverse pro cess of the axis, the vertebral artery inclines outwards and slightly upwards, to reach the foramen in the transverse process of the atlas, a deviation from the former vertical course of the artery which is rendered necessary by the superior breadth of the first vertebra.
Whilst engaged in the curved canal (not foramen) of the transverse process of the atlas, the artery bends abruptly backwards and inwards, so that on emerging from this bone it becomes horizontally placed on the upper surface of the posterior arch of the atlas. Between the occipital bone and the atlas the artery describes a curve, of which the concavity, looking forwards and inwards, embraces the occipito-atlantal articulation, whilst the convexity, directed backwards and outwards, is contained in a triangular space, circumscribed in the following manner by the small rotator muscles of the head. The sides
of this triangle are constituted by the superior and inferior oblique muscles respectively, the apex is at the transverse process of the atlas, where both these muscles are attached ; whilst the rectus•capitis posticus major (placed in ternally) represents the base. Deeply situated in this triangular space, the vertebral artery is covered by the splenius and complexus muscles, and rests on the posterior arch of the atlas ; the bone presents a groove for the reception of the artery ; but the sub-occipital nerve, which frequently forms its ganglion in this situation, is interposed. The space which has just been described is occupied by yellow, granular-looking fatty matter, and the occi pital artery winds along its upper boundary, freely anastomosing with the vertebral.
The vertebral artery next passes beneath the lower edge of the posterior occipito-atlan tal ligament, then perforates the dura mater, and, taking a direction upwards, forwards, and inwards, enters the cranium through the foramen magnum. The posterior occipito atlantal ligament, by arching over the groove on the upper surface of the atlas, forms a foramen for the transmission of the artery.
As the vertebral artery advances through the foramen magnum, it passes between the first and second tooth-like insertions of the ligamentum dentatum, and then ascends, lying on the anterior surface of the first of these processes, by which it is separated from the spinal accessory nerve, which latter passes upwards on the posterior surface of the liga ment ; here the lingual nerve passes outwards to the anterior condyloid foramen, above the level of the artery.
Within the cranium, the vertebral arteries, corresponding at first to the lateral aspects of the upper portion of the medulla oblongata, approximate to each other more and more as they ascend, and ultimately unite at an acute angle, opposite the inferior edge of the pons varolii ; in this manner the basilar trunk is formed.