42 A BOOK OF WIfALES 



special joints, which, allowing of a certain amount 

 of movement, curtail that movement within very 

 narrow bounds. In some Edentate animals (Ant- 

 eater, Sloth) these usual joints are increased by the 

 presence of supplementary articulations between suc- 

 cessive vertebrre, which renders the backbone of 

 the creatures in question a much more rigid rod than 

 it is in the majority of mammals. Now to the 

 whale an eminently flexible backbone is obviously a 

 desideratum. It moves mainly by powerful strokes 

 of the tail and of the hind part of the body generally. 

 Hence we find that the interlocking joints, the 

 zygapophyses as they are technically termed, are 

 much reduced, and indeed do not exist at all in the 

 hinder part of the series, where their presence would 

 interfere with the necessary undulations of body by 

 means of which the whale forces its way through the 

 water. Furthermore, a large development of the 

 discs of fibrous tissue which lie between the centra 

 of the vertebrae adds efficiency to this important 

 part of the whale's skeleton. It is interesting to note 

 that in Ptatanista, so frequently referred to as an 

 archaic type of Cetacean, the interlocking of the 

 vertebrae is much more marked than in other forms. 



THE STERNUM 



All whales possess a sternum or breast-bone. But 

 the form of this bone, or series of bones as it actually 

 is in many forms, varies (see Figs. 9, 10) ; and the 

 variations concern us in the present chapter, inas- 



