XVI] IN THE SKELETON OF QUADRUPEDS 705 



position of the heavy tusks of the elephant. In both eases the 

 dorsal spines are large, but they do not strike us as exceptionally 

 so; but in both cases, and especially in the elephant, they slope 

 backwards in a marked degree. Each spine, as already explained, 

 must in all cases assume the position of the diagonal in the 

 parallelogram of forces defined by the tensions acting on it at 

 its extremity; for it constitutes a "hinged lever," by which the 

 bending-moments on either side are automatically balanced ; and 

 it is plain that the more the spine slopes backwards the more it 

 indicates a relatively large strain thrown upon the great ligament 

 of the neck, and a relief of strain upon the more directly acting, 

 but weaker, ligaments of the back and loins. In both cases, the 

 bending-moments would seem to be more evenly distributed over 

 the region of the back than, for instance, in the stag, with its 

 light hind-quarters and heavy load of antlers: and in both cases 

 the high "girder" is considerably prolonged, by an extension of 

 the tall spines backwards in the direction of the loins. When 

 we come to such a case as the mammoth, with its immensely 

 heavy and immensely elongated tusks, we perceive at once that 

 the bending-moments over the fore-legs are now very severe; 

 and we see also that the dorsal spines in this region are much 

 more conspicuously elevated than in the ordinary elephant. 



(3) In the case of the giraffe we have, without doubt, a veiy 

 heavy load upon the fore-legs, though no weighings are at hand 

 to define the ratio ; but as far as possible this disproportionate 

 load would seem to be relieved, by help of a downward as well 

 as backward thrust, through the sloping back, to the unusually 

 low hind-quarters. The dorsal spines of the vertebrae are verjr 

 high and strong, and the whole girder-system very perfectly 

 formed. The elevated, rather than protruding position of the 

 head lessens the anterior bending-moment as far as possible ; but 

 it leads to a strong compressional stress transmitted almost 

 directly downwards through the neck : in correlation with which 

 we observe that the bodies of the cervical vertebrae are excepr 

 tionally large and strong and steadily increase in size and strength 

 from the head downwards. 



(4) In the kangaroo, the fore-limbs are entirely relieved of 

 their load, and accordingly the tall spines over the withers, which 



T. G. 45 



