xvi] IN AQUATIC ANIMALS 709 



Here, between C and D we have a varying bending-moment, 

 represented by a continuous curve with its maximal elevation 

 midway between the points of inflexion. And correspondingl}'-, 

 in our dolphin, we have a con- 

 tinuous series of high dorsal 

 spines, rising to a maximum 

 about the middle of the animal's 

 body, and falling to nil at some ''' 



distance from the end of the tail. It is their business (as 

 usual) to keep the tension-member, represented by the strong 

 supraspinous ligaments, wide apart from the compression-member, 

 which is as usual represented by the backbone itself. But in 

 our diagram we see that on the further side of C and D we 

 have a negative curve of bending-moments, or bending-moments 

 in a contrary direction. Without inquiring how these stresses 

 are precisely met 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 beloiv 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), 

 we 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 is there to be resisted. 



In the dolphin we may find a good illustration of the fact 

 that not only is it necessary to provide for rigidity in the vertical 

 direction, but 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 trans- 

 verse processes, forming a web for the support of a tension-member 

 in the usual form of ligament, and so playing a part precisely 

 analogous to that performed by the dorsal spines in the same 



