268 HEAD AND NECK 



posterior aspects of the corresponding upper articular process 

 of the atlas, in a groove upon the upper surface of its posterior 

 arch (Figs. 20, 38). As soon as it has passed under cover 

 of the lateral margin of the posterior atlanto-occipital mem- 

 brane it becomes the fourth part. The fourth part turns 

 upwards, pierces the dura mater, and passes into the skull 

 through the foramen magnum, anterior to the uppermost 

 digitation of the ligamentum denticulatum ; then, turning 

 antero-medially, between the hypoglossal nerve above and 

 the first cervical nerve below, it passes to the anterior surface 

 of the medulla oblongata, and, as already stated, joins its 

 fellow of the opposite side at the lower border of the pons 

 (Figs. 37, 144). 



Relations. The relations of the first part have already 

 been sufficiently considered. The second part lies in and 

 between the transverse processes of the cervical vertebrae, 

 medial to the intertransverse muscles, lateral to the bodies 

 of the vertebrae, and anterior to the anterior rami of the 

 cervical nerves as they pass laterally. It is surrounded not 

 only by the sympathetic nerve plexus derived from the 

 inferior cervical ganglion, which accompanies all parts of 

 the artery, but also by a venous plexus which terminates, below, 

 as the vertebral vein or veins. The third part of the artery 

 lies on the posterior arch of the atlas in the anterior boundary 

 of the sub-occipital triangle. As it turns backwards, from the 

 foramen in the transverse process of the atlas, the anterior 

 ramus of the first cervical nerve lies to its medial side, 

 between it and the lateral mass of the atlas ; and, as it turns 

 medially, posterior to the upper articular facet of the atlas, 

 the trunk of the first cervical nerve lies below it, on the 

 posterior arch of the atlas, and the posterior ramus enters 

 the triangle from beneath its lower border. For the relations 

 of the fourth part see the preceding paragraph and pp. 

 117, 382. 



Branches. No branch of importance is given off from 

 the first part. The second part gives off lateral spinal 

 (p. 90) and muscular branches. The branches from the 

 third part are muscular twigs, and branches to anastomose 

 with twigs from the occipital and the deep cervical arteries. 

 The fourth part gives off a meningeal branch before it 

 perforates the dura mater and, afterwards, a series of branches 

 to the central nervous system (see pp. 382, 383). 



