THE SKELETAL SYSTEM 



205 



on both anterior and posterior sides and with the centrum of the atlas 

 fused with the axis, as in mammals. 



The vertebral column of mammals shows little advance beyond that 

 of reptiles. A few Insectivora have intercentra in the lumbar region, — - 



CARTIU\GE 

 CELLS 





^d^-^ IW^ DEGENERATION 



OSTEOCLASTS 



OSTEOBLAST 



BONE 



Fig. 159. — Endochondral bone formation at the end of a long bone. Destruction of 

 cartilage is followed by the secretion of lime in the form of thin lamellae. Osteoblasts 

 then lay down bone upon these lamellae. In this way cancellous bone replaces cartilage. 

 (Redrawn after Dahlgren and Kepner.) 



a diplospondylous condition reminiscent of elasmobranchs. Parapophyses 

 are reduced to shallow pits for articulating the heads of the ribs. 



The human spine differs little from that of other mammals, except 

 that the tail is reduced to a coccyx with a few variable muscles attached. 

 Man's only distinctive feature is the sigmoidal curve, which bends his 

 spine in two directions, instead of one only as in other creatures. In 



