52 ANATOMY OF THE RABBIT 



the ventral part, or centrum, is massive and the dorsal arch and 



processes are thinner and weaker. 



Any vertebra of the rabbit may be 

 compared with the corresponding vertebra 

 of any mammal or in a general way with 

 those of any vertebrate, and will be found 

 to show resemblances and differences corre- 

 sponding with those of the precise functions 

 performed by the bones compared. The 

 fifth lumbar vertebra of the rabbit, for 

 example, would be found distinguished by 

 Fig. 28. Lumbar vertebra of the great development of its proccsscs, 



four-day-old rabbit. Cartilage -it r i i 



dotted, bone shaded. X5. smcc the latter support powcrful muscles 



used in leaping. The corresponding human 

 vertebra, or the third as functionally more nearly equivalent, is 

 weak in muscular expansion, but its body is massive for purposes 

 of axial support. A corresponding vertebra of the bear will be 

 found more or less intermediate between the two types (Fig. 27). 

 The two most cephalic vertebrae are very much modified to 

 provide for the movements and the support of the head. The 

 centrum of the first vertebra, or atlas, has lost its connection with 

 the rest of this part and has fused with the anterior end of the centrum 

 of the second vertebra, the epistropheus, to form a pivot round which 

 the ring-like atlas can rotate. The ventral part of the adult atlas is 

 derived from a small element, the intercentrum, which in other 

 vertebrae has disappeared. These changes occurred in early 

 reptiles but reach their most perfect expression in mammals. 



The Notochord 



The axial line of the vertebrae passes through the centres of the 

 bodies, the position occupied in the embryo by the notochord 

 (Fig. 22). Some of the lower aquatic chordates, such as lampreys, 

 exhibit the notochord in both young and adult conditions, and 

 show little indication of the development of the elements of verte- 

 brae. Others, of slightly more advanced grade, such as sharks 

 (Fig. 29), show the notochord surviving more or less to the adult 

 condition with the vertebral elements developed round about it. 



