THE ENDOSKELETON 



129 



>9th on one side and the 2Oth on the other, or to the i8th on 

 one side and the iQth on the other. 



In these low forms this sacral vertebra shows no special 

 modification save that it may be slightly stouter than its fel- 

 lows, and have longer transverse processes and stouter ribs, 

 but as the posterior limbs increase in size and functional power 

 their girdle increases with them and may form similar attach- 

 ment to two or more adjacent vertebrae, which may become 

 more and more modified and form a more or less complete 



^=5^ \y ''<^? 



FIG. 34. Variations in the composition of the human sacrum. [After 

 GEGENBAUR.] 



fusion into a single piece, the sacrum. This anchylosis of 

 adjacent sacral vertebrae is the most complete in birds and in 

 Man, and for the same reason, namely, the employment of 

 the hind limbs alone for the support of the body ; although in 

 the two cases the number and arrangement of the associated 

 parts differs very considerably. 



Variation in the sacral region is not confined to the lower 

 forms, although it is more frequent in these latter (e. g., 

 Necturus) and becomes relatively stable in the higher and 

 more specialized classes. As shown in Fig. 34, there is varia- 

 tion both in the point of attachment of the hip bones and in 

 the number of vertebrae involved in the composition of the 

 human sacrum, and similar variations have been noted in other 

 mammals. These, like the variation in the number of ribs and 

 in the groups of vertebrae, not infrequent in the human subject, 

 should serve to dispel the idea that the body of man or any 



