XVI] IN AQUATIC ANIMALS 1007 



(7) An abnormal and very curious case is that of the sloth, which 

 hangs by hooked hands and feet, head downwards, from high 

 branches in the Brazilian forest. The vertebrae are unusually 

 numerous, they are all much alike one to another, and (as we might 

 well suppose) the whole pensile chain of vertebrae hangs in what 

 closely approximates to a catenary curve*. 



(8) We find a highly important corollary in the case of aquatic 

 animals. For here the eifect of gravity is neutralised; we have 

 neither piers nor cantilevers; and we find accordingly in all aquatic 

 mammals of whatsoever group — whales, seals or sea-cows — that 

 the high arched vertebral spines over the withers, or corresponding 

 structures over the hind-limbs, have both entirely disappeared. 



But in the whale or dolphin (and not less so in the aquatic bird), 

 stiffness must be ensured in order to enable the muscles to act against 

 the resistance of the water in the act of swimming; and accordingly 

 Nature must provide against bending-moments irrespective of 

 gravity. In the dolphin, at any rate as regards its tail-end, the 

 conditions will be not very different from those of a column or 

 beam with fixed ends, in which, under deflection, there will be two 

 points of contrary flexure, as at C, D, in Fig. 482. 



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

 represented by a continuous curve with its maximal elevation mid- 

 way between the points of inflection. 

 And correspondingly, in our dolphin, 

 we have a continuous 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 hgaments, wide apart from the compression- 

 member, which is as usual represented by the backbone itself. But 

 in our diagram we see that on the farther side of C and D we have a 

 negative curve of bending-moments, or bending-moments in a contrary 

 direction. Without enquiring how these stresses are precisely met 



♦ A hmvy cord, or a cord carrying equal weights for equal distances along its 

 line, hangs in a catenary: imagine it frozen and inverted, and we have an arch, 

 carrying Ihe same sort of load, and under compression only. On the other hand, 

 a flexible cable (itself of neghgible weight), carrying a uniform load along the line 

 of its horizontal projection, hangs in the form of a parabola. 



