22 



ATLAS 



atlas, and articulates with the skull. The most remarkable 

 fact about this bone (shared, however, by lower Vertebrates) 

 is that its centrum is detached from it and attached to the 

 next vertebra, in connexion with which it will be referred 



FIG. 7. Hum.in atlas (young), showing de- 

 velopment, x f . as, Articular surface for 

 occiput ; g, groove for first spinal nerve 

 and vertebral artery ; i , inferior arch ; 

 t, transverse process. (From Flower's 

 Osteology.) 



FIG. 8. Inferior surface of atlas of 

 Dog. x ^. sn, Foramen for first 

 spinal nerve ; r, vertebrarterial 

 canal. (From Flower's Osteology.) 



FIG. 9. Atlas of Kangaroo. 



Parker and Haswell's Zoology.) 



to immediately. The whole bone thus gets a ring-like form, 

 and the salient processes of other vertebrae are but little de- 

 veloped, with the exception of 

 the transverse processes, which 

 are wide and wing - like. In 

 many Marsupials, such as the 

 Wombat and Kangaroo, the arch 

 of the atlas is open below, there 

 (From being no centre of ossification. 

 In others, such as Thylacinus, 

 there is a distinct nodule of bone in this situation not con- 

 crescent with the rest of the arch. 



The second vertebra, which is known as the axis or epi- 

 stropheus, is a compound structure, the anterior " odontoid process," 

 which fits into the ring of the atlas, being in reality the 

 detached centrum of that vertebra. 1 It is a curious fact about 

 that process that it has independently become spoon-shaped in 

 two divisions of Ungulates ; that it has become so seems to be 

 shown by the fact that in the earlier types of both it has the 

 simple peg-like form, which is the prevailing form. The cervical 



1 Its independence from the epistropheus is emphasised in Monotremes and 

 some Marsupials by its late fusion with that vertebra. 



