THE BONES OF THE TRUNK 1 5 



are as follows : an antador arch : a posterior arch ; two lateral 

 masses ; and a ring. 



The anterior arch is a curved plate of bone which connects the 

 antero-intemal parts of the lateral masses. It is compressed from 

 before backwards, convex in front, and concave behind. The 

 anUricr surface presents at its centre a conical prominence, called 

 the anterior tubercle. This gives attachment at either side to a 

 portion of the longus colh muscle, and its central port receives 

 the accessory ligament. The posterior sitrface presents at its 

 centre a circular concave facet, called the odontoid facet, for articu- 

 lation with the anterior surface of the odontoid process of the 

 axis. The upper border gives attachment to the anterioar occipito- 



Tiaisvcrse Process 



Vertebrarteral ' 

 Gfoove 



PostericH- Tafaode 



Fig. 8- — ^The Atlas (Scpekiob View). 



(The Verteixartetial Groove cm this bone was ccmveited into a Foramen on 



botli ades.) 



atlantal l^ami^it, and the lower to the anterior atlanto-axial liga- 

 ment. 

 The posterior areh is smaUy homoIogoQs with the laminae of 



other vertebrae. It springs at either side from the back part of a 

 lateral mass, from which it sweeps backwards and inwards. The 

 part close to the lateral mass at either side is flattened from above 

 downwards. It presents on its upper surface a shallow depression, 

 called the vertebrarterial groove, which lodges the vertebral artery 

 and suboccipital nerve. This groove is sometimes converted into 

 a foramen on one or both sides by a spiculum of bone extending 

 from the back part of the superior articular process to the posterior 

 arch behind the groove. The vertebrarterial groove is serially 

 homologous with the superior vertebral notch of other vertebrae, 

 but, unlike them, it hes behind the superior articular process. The 

 inferior surface of the posterior arch, behind each lateral mass. 



