SOME INTERNAL STRUCTURES 41 



Cetacea these same vertebrae are very numerous. 

 But in I nia there are only three, a number which 

 is repeated in the Manatee. In this connection it 

 is interesting to recall the fact that in Rhytina, the 

 most "cetacean" of the Sirenia, the lumbar region has 

 increased to six vertebrae. As the pelvis (see p. 25) 

 is so rudimentary a structure it is not surprising to 

 find that there is no sacrum ; no lumbar vertebrae 

 are fused to make the complex and firm mass of bone 

 which in terrestrial creatures supports the arch of the 

 hind limbs. 



As there is no sacrum it would seem at first a little 

 difficult to define the commencement of the caudal 

 series of vertebrae. Practically there is a difficulty, 

 owing to the frequent incompleteness of skeletons in 

 museums. But theoretically there is none, since the 

 first caudal is provided below with a V-shaped ap- 

 pendage of bone, the intercentrum or chevron bone. 

 Professor Delage has also pointed out that in Bal<znop- 

 tera umsculus at any rate the lumbar series is defined 

 by the termination opposite to the last one of the' 

 abdominal cavity. 



In terrestrial mammals there is not, as a rule, much 

 "give" in the backbone. They cannot "wriggle" 

 their bodies to any great extent. The reason for 

 this is clearly the desirability of a firm support for 

 the limbs by which locomotion is affected. This is 

 brought about not only by the fusion of vertebrae in 

 the region of attachment of the hind limbs to form 

 the sacrum already mentioned, but elsewhere in the 

 series the successive vertebrae are locked together by 



