990 ON FOKM AND MECHANICAL EFFICIENCY [ch. 



we might go even .further, and study (after the fashion of the 

 anatomist) the "osteology" and "desmology" of the structure, that 

 is to say the bones which are represented by "struts," and the 

 Hgaments, etc., which are represented by "ties." Furthermore 

 after the methods of the comparative anatomist, we may classify 

 the families, genera and species of bridges according to their dis- 

 tinctive mechanical features, which correspond to certain definite 

 conditions and functions. 



Fig. 468. Skeleton of an American bison. (An unusually weU-mounted skeleton, 

 of American workmanship, now in the Anatomical Museum of Edinburgh 

 University.) 



In more ways than one, the quadrupedal bridge is a remarkable 

 one; and perhaps its most remarkable peculiarity is that it is a 

 jointed and flexible bridge, remaining in equilibrium under con- 

 siderable and sometimes great modifications of its curvature, such 

 as we see, for instance, when a cat humps or flattens her back. 

 The fact thsit flexibility is an essential feature in the quadrupedal 



draw it as a skeleton, in which the different points are joined by straight lines, 

 and we may indicate by numbers attached to these lines the tensions or com- 

 pressions in the corresponding pieces of the frame" {Trans. R.S.E. xxvi, p. 1, 

 1870). It follows that the diagram so constructed represents a "diagram of 

 forces," in this limited sense that it is geometrical as regards the position and 

 direction of the forces, but arithmetical as regards their magnitude. It is to just 

 such a diagram that the animal's skeleton tends to approximate. 



