702 ON FORM AND MECHANICAL EFFICIENCY [ch. 



On the other hand the Dinosaur, with his Hght head and 

 enormous tail would give us a moment-diagram with the opposite 

 kind of asymmetry, the greatest bending stress being now found 

 oyer the haunches, at B (Fig. 348). A glance at the skeleton of 

 Diplodocus Carnegii will shew us the high vertebral spine3 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 



Tail Head 



Fig. 348. Stress-diagram of backbone of Dinosaur. 



conditions imposed by the varying distribution of the load : and 

 in particular how the height of the vertebral spines which con- 

 stitute the web will be in a definite relation, as regards magnitude 

 and position, to the bending-moments induced thereby. We 

 should require much detailed 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 by the vertical 

 column and its ligaments alone, but that the pelvis, firmly united 

 as it is to the sacral vertebrae, and stretching backwards far 

 beyond the acetabulum, becomes an intrinsic part of the system ; 

 and helping (as it does) to carry the load of the abdominal viscera, 



