THE BONES OF THE TRUNK 17 



nerv'es, do not correspond in position with the articular processes 

 of succeeding vertebrae (with the exception of the superior pair 

 of the axis). They occupy a position corresponding with the 

 pedicular portions of the bodies of vertebrae, and in this way 

 the superincumbent weight is transmitted to the vertebral 

 bodies. 



The transverse and costal processes spring from the side of each 

 lateral mass, and, external to the cos to- transverse foramen, the 

 costo- transverse lamella and the anterior and posterior tubercles 

 are more or less fused into one long irregular mass, though the 

 posterior tubercle usually remains conspicuous. The upper surface 

 of this mass at its front part gives origin to the rectus capitis 

 lateralis, and at its back part to the obliquus capitis superior, 

 whilst the lower surface at its back part gives insertion to the 

 obliquus capitis inferior. The costo - transverse foramen is of 

 large size in order to guard against the vertebral artery being 

 compressed during the rotatory movements of the bone upon the 

 axis. 



The ring of the atlas, in the recent state, is divided into two com- 

 partments by the transverse ligament. The anterior small division 

 is called the odontoid compartment, and it lodges the odontoid pro- 

 cess of the axis. The posterior large division represents the neural 

 foramen of other vertebrae, and it lodges the spinal cord with its 

 membranes. 



Varieties. — (i) The posterior arch may be incomplete at the centre, the 

 deficiency being bridged over by fibrous tissue. (2) The costal process may 

 be incomplete, the deficiency in the costo-transverse foramen being filled b\' 

 fibrous tissue. (3) There is sometimes an additional small foramen on either 

 side, a httle behind the costo-transverse foramen, for the peissage of the sub- 

 occipital radicles of the vertebral plexus of veins. 



The Axis. — The axis is the second cer\'ical vertebra, and is 

 so named because its odontoid process, which is the distinctive 

 character of the bone, forms a pivot on which the atlas, support- 

 ing the head, rotates. From the presence of this process the axis 

 is sometimes called the vertebra dentata. 



The odontoid process {processus dentatus) springs from the 

 superior surface of the body, and represents the body of the atlas. 

 It is constricted and somewhat circiilar close to the body, this part 

 being called the 7teck. Above this it expands into a head, which 

 tapers off at either side by two sloping surfaces, forming by their 

 convergence an antero-posterior ridge, known as the summit. 

 The anterior surface presents a circular convex facet, called the 

 atlantal facet, for articulation with the odontoid facet on the posterior 

 surface of the anterior arch of the atlas. The posterior surface 

 presents a shallow transverse groove for the play of the transverse 

 Ugament of the atlas. The lateral sloping surfaces on either side 

 of the summit give attachment to the lateral odontoid ligaments, 

 whilst the summit itself gives attachment to the middle odontoid 

 ligament. 



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