I o6 WHALES 



This can be seen clearly in Rorquals, and particularly in Dolphins, some 

 of which not only have an increased number of lumbar but also of caudal 

 vertebrae (Fig. 67). Generally, each of these vertebrae is so flattened 

 antero-posteriorly that their bodies resemble draughts counters. Such 

 vertebral bodies, with or without their spinous and transverse processes, 

 are often washed up. The greatest number of vertebrae is found in 

 White-sided Dolphins (93) ; the Common Dolphin has 75, the Porpoise 

 has 66 and the Blue Whale has 63, as compared with the dog's 50. If we 

 examine the history of Cetaceans, we shall find that the geologically 

 oldest types always have the smallest, and the more recent types the 

 greatest number of vertebrae. The advantage of this sequence of adjoining 

 short discs is probably not so much the possibility of increasing their 

 number within a given space as the consecjuent increase in the number of 

 cartilaginous pads between the vertebrae. As a result, the vertebral 

 column has proportionally more cartilage and less bone, and hence much 

 greater suppleness and elasticity. Bone which is primarily a means of 

 supporting the body is not so important in aquatic animals, most of whose 

 weight is borne by the water. 



The fact that Cetaceans lack a sacrum is obvious from the fact that the 

 pelvis has been reduced to a slender bone which, moreover, is quite 

 unattached to the v^ertebral column. However, because of the nerve roots 

 which emerge from the spinal cord behind certain vertebrae, we can still 

 say that these - otherwise quite ordinary - lumbar vertebrae correspond 

 to the sacral vertebrae of terrestrial mammals. If we regard them in this 

 way, it appears that the tail does not start directly behind these 'sacral' 

 vertebrae, but that a number of so-called post-sacral vertebrae are inter- 

 posed. The Porpoise, for instance, has six of these. 



Although it is difficult to tell superficially just where the lumbar region 

 ends and the tail begins, since, externally, the body has smoothly flowing- 

 lines, the distinction is quite easy to make on skeletons, for here every 

 caudal vertebra is provided with chevron bones (Figs. 21 and 226). 

 The first vertebra to have these bones at its anterior side can 

 therefore be distinguished as the first caudal vertebra. Chevron bones, 

 which occur in all caudal vertebrae with the exception of those in the 

 flukes, are so called because they strongly resemble the chevrons worn 

 by N.C.O.s. Chevron bones also occur in other mammals, but in the more 

 anterior caudal vertebrae only, where they are so small that they can be 

 located only with the greatest difficulty. The reason why they are so well 

 developed in Cetaceans is quite obvious: the muscles which raise the tail 

 are provided with adequate levers by the spinous processes on top of the 

 caudal vertebrae. However, no such levers are normally found on the 

 lower side of vertebrae, and the chevrons found here consequently help to 



