22 



ATLAS 



atlas, and articulates with the skull. The most remarkable 

 fact about tliis l)one (sh;ire(l. however, hj 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. — Huniau atlas (youug), showing de- 

 velopment. X |. «.y. Articular surface for 

 occiput ; g, gi-oove for tirst spinal nerve 

 and vertebral artery : / </, inferior arch : 

 t, transverse process. (From Flower's 

 OstealoiJii.) 



Fi<;. S. — Inferior surface of atlas of 

 Dog. X ^. SH, Foi-amen for tirst 

 spinal nerve ; r, vertelirarterial 

 canal. (From Flower's Oste<ilo<i!l.) 



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 Kangart)0, the arch 

 of the atlas is open below, thej-e 

 (From lieing' no centre of ossification. 

 In others, such as Thylaciims, 

 there is a distinct nodule of ])one 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 verteljra.^ 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 l)y the fact tbat in the earlier types of both it has the 

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



^ Its iudependeiice from the epistropheus is em]i]iasise(l in Monotremes ami 

 some Marsupials by its late fusion with that vertebra. 



