820 



SUBCL AVIAN ARTERIES. 



aspect of \vhich it usually arises. There is 

 no example on record where this branch 

 arose either between or beneath the scaleni 

 muscles. 



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. 



Relations. From 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 

 regularly by 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. 



Basilar artery (Artcre meso-cepliaTique, 

 Chaussier). This artery, larger than either 

 of the vertebral arteries, is yet much less capa- 

 cious than the two vessels conjointly which 

 unite to form it ; its length corresponds 

 accurately to the longitudinal measurement 

 of the pons varolii ; it runs along the median 

 depression of the pons, lodged between that 

 body and the upper surface of the basilar 

 process of the occipital bone, and preserved 

 from pressure by the double concavity of the 

 surfaces between which it is interposed. 



At its commencement the basilar artery 

 separates from each other the sixth nerves of 

 the opposite sides ; the arachnoid membrane 

 and the dura mater, with the transverse sinus of 

 Haller, are interposed between the artery 

 and the bone, whilst the pia mater alone 

 intervenes between it and the pons. At the 

 antero-superior edge of the pons the basilar 

 artery usually terminates in four branches, 

 two for the cerebrum and two for the cere- 

 bellum. 



Branches of the vertebral artery. 1. 

 Branches to the praevertebral muscles, which 

 anastomose with the cervicalis a^cendens and 

 superficialis colli arteries. 



2. Numerous small branches, which enter 

 the spinal canal through the intervertebral 



