26 Anatomy of Skeleton 



process, and does not extend laterally beyond it as in the lower segments, partly 

 because the vein here is smaller, but mainly owing to the breadth ot the Atlas : the tip 

 of the process projects beyond the vein, and is crossed by the spinal accessory nerve as 

 this leaves the sheath after passing behind or in front of the vein, while the posterior 

 belly of the Digastric covers all the structures and separates them from the Sterno- 

 mastoid. In this way the spinal accessory nerve passes on to the Levator anguli 

 scapulas under cover of the Digastric, and when it emerges from under the lower 

 border of that muscle it is lying on the Levator directly under cover of the Sterno- 

 mastoid (Fig. 22). 



The transverse process can be felt deeply on pressure through the Sterno-mastoid, 

 below the mastoid process and a little behind the mandible. 



The relation of the vertebral artery, as it turns round the articular process, to the 

 nerve differs from that obtaining in the other segments : this is probably owing to 

 the different value of the artery, which should be looked on here as an enlarged spinal 

 branch, not comparable with the direct artery below this level. As the vessel runs 

 from the transverse process to the posterior arch, it overhangs the outer border of 

 the bone, which is notched here by a vein passing to the vertebral system. This notch 

 is occasionally converted into a foramen by ossification in the fibrous tissue that 

 bridges it in and supports the artery. 



The same conversion may occur in the notch that marks the end of the arterial 

 groove, behind the articular mass : this is usually bridged by a ligament, the " oblique 

 ligament of the atlas" attached in front to the overhanging edge of the mass and 

 extending back to the outer extremity of the ridge on the posterior arch that marks the 

 line of the posterior occipito-atloid ligament. 



The upper articular facet shows a tendency to division by a constriction of its 

 area. Opposite this constriction there is a vascular pit on the upper surface of the 

 internal protuberance caused by the lower articular surface : the vascular spot has 

 the tubercle for the transverse ligament in front of it, while a small projection for the 

 accessory atlo-axoid ligament may be seen below and internal and somewhat behind it. 



On the under surface of the bone may be seen the line for the thin Lig. subflavum, 

 evidently passing into the fibrous tissue that bridges over the venous notch in the bone. 

 On the under side of the transverse process is the concavity for the Inferior Oblique, 

 bounded by the ridge for Splenius colli, and a roughness on the anterior limb of the 

 process marks the attachment of the intertransverse muscle. The markings of Longus 

 colli and anterior common ligament are well seen from below. 



The anterior surface of the posterior arch is smooth and is separated from the 

 dura mater by a layer of soft vascular tissue : the front wall of the neural canal is 

 formed by the ligaments that cover the back surface of the odontoid process. 



DORSAL VERTEBRA. 



The distinguishing mark of this series is the presence of costal facets on the bodies. 

 Other general characters, as seen in a middle vertebra in the series, are as follows : 



Body somewhat triangular or heart-shaped viewed from above, and slightly deeper 

 behind than in front, carrying an upper and a lower costal demi-facet on each lateral 

 aspect, which are covered with fibro-cartilage in the recent state. 



Upper intervertebral notches very shallow or even absent, lower notches corre- 

 spondingly deep. 



