xvij OF RECIPROCAL DIAGRAMS 1001 



members — that is to say the height of the vertebral spines — must 

 be proportional to the bending-moment at each point along the 

 length of the girder. 



In the case of an animal carrying most oi his weight upon his 

 fore-legs, as the horse or the ox do, the bending-moment diagram 

 will be unsymmetrical, after the fashion of Fig. 478, the precise form 

 depending on the distribution of weights and distances. 



7a/7 Head 



Fig. 478. Stress -diagram of horse's backbone. 



On the other hand the Dinosaur, with his light head and enormous 

 tail would give us a moment-diagram with the opposite kind of 

 asymmetry, the greatest bending stress being now found over the 

 haunclies, at B (Fig. 479). A glance at the skeleton of Diplodocus 

 will shew us the high vertebral spines over the loins, in precise 

 correspondence with the requirements of this diagram: just as in 

 the horse, under the opposite conditions of load, the highest vertebral 

 spines are those of the withers, that is to say those of the posterior 

 cervical and anterior dorsal vertebrae. 



We have now not only dealt with the general resemblance, both 

 in structure and in function, of the quadrupedal backbone with its 

 associated ligaments to a double-armed cantilever girder, but we 

 have begun to see how the characters of the vertebral system must 

 differ in different quadrupeds, according to the conditions imposed 

 by the varying distribution of the load : and in particular how the 

 height of the vertebral spines which constitute the web will be 

 in a definite relation, as regards magnitude and position, to the 

 bending-moments induced thereby. We should require much de- 

 tailed information as to the actual weights of the several parts of 

 the body before we could follow out quantitatively the mechanical 

 efficiency of each type of skeleton; but in an approximate way 

 what we have already learnt will enable us to trace many interesting 

 correspondences between structure and function in this particular 

 part of comparative anatomy. We must, however, be careful to 

 note that the great cantilever system is not of necessity constituted 



