THE SKELETON IN MAMMALIA. VERTEBRAL COLUMN. 451 



In most cases the neural spines converge more or less to a 

 centre of motion, and this is especially marked in some of the 

 Lemurs ; it does not occur in man and the anthropoid apes. 



SACRAL AND CAUDAL VERTEBRAE. 



At the posterior end of the trunk in all mammals a 

 certain number of vertebrae are found fused together forming 

 the sacrum. But of these only two or three answer to the 

 definition of true sacral vertebrae in being united to the ilia 

 by small ribs. The others which belong to the caudal series 

 may be called pseudosacral vertebrae. In different individuals 

 of the same species it sometimes happens that different 

 vertebrae are attached to the pelvis and form the sacrum. 

 Sometimes even different vertebrae are attached to the pelvis 

 at successive periods in the life history of the individual. This 

 is owing to a shifting of the pelvis and has been especially 

 well seen in man. In young human embryos the pelvis is 

 at a certain stage attached to vertebra 30, but as develop- 

 ment goes on it becomes progressively attached to the twenty- 

 ninth, twenty-eighth, twenty-seventh, twenty-sixth and twenty- 

 fifth vertebrae. As the attachment to these anterior vertebrae 

 is gained, the attachment to the posterior ones becomes 

 lost, so that in the adult the pelvis is generally attached 

 to vertebrae 25 and 26. But there are no absolutely pre- 

 determined sacral vertebrae, as sometimes the pelvis does not 

 reach vertebra 25, remaining attached to vertebrae 26 and 27; 

 sometimes it becomes attached even to vertebra 24. This 

 shifting of the pelvis is seen in Choloepus in a more marked 

 degree even than in man. 



Of the MONOTREMATA, Omithorhynclius has two sacral verte- 

 brae ankylosed together, while Echidna has three or four 1 . 



In MARSUPIALIA as a rule only one vertebra is directly 

 united to the ilia, but one or two more are commonly fused 

 to the first. In the Wombats there may be as many as four 



1 G. B. Howes, Journ. of Anat. and Phys. xxvu., p. 544. 



292 



