1008 ON FORM AND MECHANICAL EFFICIENCY [ch. 



towards the dolphin's head (where the coalesced cervical vertebrae 

 suggest themselves as a partial explanation), we see at once that 

 towards the tail they are met by the strong series of chevron-bones, 

 which in the caudal region, where tall dorsal spines are no longer 

 needed, take their place below the vertebrae, in precise correspondence 

 with the bending-moment diagram. In many cases other than these 

 aquatic ones, when we have to deal with animals with long and 

 heavy tails (like the Iguanodon and the kangaroo of which we have 

 already spoken), w^e are apt to meet with similar, though usually 

 shorter chevron-bones; and in all these cases we may see without 

 difficulty that a negative bending-moment in the vertical direction 

 has to be resisted or controlled. 



In the dolphin we may find an illustration of the fact that not 

 only is it necessary to provide for rigidity in the vertical direction 

 but often also in the horizontal, where a tendency to bending must be 

 resisted on either side. This function is effected in part by the ribs 

 with their associated muscles, but they extend but a little way and 



their efficacy for this purpose can 

 be but small. We have, however, 

 behind the region of the ribs and 

 on either side of the backbone 

 a strong series of elongated and 

 flattened transverse processes, 

 forming a web for the support 

 of a tension-member in the usual 

 form of Hgament, and so playing 

 a part precisely analogous to that 

 performed by the dorsal spines in 

 the same animal. In an ordinary 

 fish, such as a cod or a haddock, 

 we see precisely the same thing: 

 the backbone is stiffened by the indispensable help of its three series 

 of ligament-connected processes, the dorsal and the two transverse 

 series ; but there are no such stiffeners in the eel. When we come to 

 the region of the tail, where rigidity gives place to lateral flexibility, 

 the three stiffeners give place to two — the dorsal and haemal s|)ines 

 of the caudal vertebrae. And here we see that the three series of 

 processes, or struts, tend (when all three are present) to be arranged 



Fig. 483. a, dorsal and b, caudal 

 vertebrae of haddock. 



