54 ELEMENTARY ANATOMY. [LESS. 



27. In its marked peculiarities the AXIS of man agrees 

 with that of almost all Vertebrates above the Ichthyopsida, 

 where it is indistinguishable. As we have seen, however, it 

 may, as in many Cetaceans, not remain distinct ; and even 

 where it does remain distinct in them, the odontoid process 

 is absent or very short. 



In the coalescence of that process with the centrum, man 

 agrees with the immense majority of his class ; but in the 

 Duck-billed Platypus it remains distinct as an odontoid bone, 

 as it does also in many Reptiles (e.g. Crocodiles, Chelonians, 

 and Lizards), where, however, it remains closely connected 

 with the bodv of the axis. 



FIG. 68 ATLAS AXD Axis VERTEBRA OF A CHELOMAX REPTILE. 



ky, hypapophysis of atlas : t, transverse process ; z, prezygapophysis ; z, post- 

 zygapophysis ; s, neural spine : by, odontoid bone ; Aj", hypapophysis of 

 true centrum of axis. 



(from the College of Surgeons' Museum. ) 



The odontoid process may present a semi-cylindrical shape, 

 as in the Sheep. 



The slightly increased size of the neural spine of man's 

 axis compared with those of his other cervical vertebrae, is but 

 a faint indication of the great predominance it attains in many 

 Mammals arching forwards and backwards over three or 

 four vertebrae. Its apex may be trind, as in Mycetes and the 

 Potto. Single or double hypapophysial processes may be 

 developed, and the transverse process may be long and large, 

 as in the Monotremes, where it (as a short and wide rib) 

 remains for a considerable time distinct, and is attached to 

 two superimposed transverse processes. 



In its freedom the second vertebra varies in the way already 

 indicated in describing the atlas. 



28. The SIXTH cervical vertebra of man scarcely differs 

 from the "fifth, though the capitular root of the transverse pro- 

 cess is slightly wider. This widening is a feeble indication 

 of a marked and general mammalian condition, for (though 

 not in the highest Apes) this root is usually much enlarged, 



