XVI] OF RECIPROCAL DIAGRAMS 999 



times steeply, in a quadruped, is easily and obviously explained*. 

 For each vertebra tends to act as a "hinged lever," and its spine, 

 acted on by the tensions transmitted by the ligaments on either side, 

 takes up its position as the diagonal of the parallelogram of forces 

 to which it is exposed. 



It happens that in these comparatively simple types of cantilever 

 bridge the whole of the parabolic curvature is transferred to one 

 or other of the principal members, either the tension-member or 

 the compression-member as the case may be. But it is of course 

 equally permissible to have both members curved, in opposite 

 directions. This, though not exactly the case in the Forth Bridge, 

 is approximately so; for here the main compression-member is 

 curved or arched, and the main tension-member slopes downwards 

 on either side from its maximal height above the piers. In short, 

 the Forth Bridge (Fig. 470) is a nearer approach than either of 

 the other bridges which we have illustrated to the plan of the 

 quadrupedal skeleton; for the main compression-member almost 

 exactly recalls the form of the backbone, while the main tension- 

 meraber, though not so closely similar to the supraspinous and 

 nuchal ligaments, corresponds to the plan of these in a somewhat 

 simplified form. 



We may now pass without difficulty from the two-armed canti- 

 lever supported on a single pier, as it is in each separate section of the 

 Forth Bridge, or as we have imagined it to be in the fore-quarters 

 of a horse, to the condition which actually exists in a quadruped, 

 when a two-armed cantilever has its load distributed over two 

 separate piers. This is not precisely what an engineer calls a 

 "continuous" girder, for that term is applied to a girder which, 

 as a continuous structure, has three supports and crosses two or more 

 spans, while here there is only one. But nevertheless, this girder 



* The form and direction of the vertebral spines have been frequently and 

 elaborately described; cf. (e.g.) H. Gottlieb, Die Anticlinie der Wirbelsaule der 

 Saugethiere, Morphol. Jahrb. lxix, pp. 179-220, 1915, and many works quoted 

 therein. According to Morita, Ueber die Ursachen der Richtung und Gestalt der 

 thoracalen Dornfortsatze der 8augethierwirbelsaule {ibi cit. p. 201), various changes 

 take place in the direction or inclination of these processes in rabbits, after section 

 of the interspinous ligaments and muscles. These changes seem to be very much 

 what we should expect, on simple mechanical grounds. See also 0. Fischer, 

 Theoretische Grundlagen fur eine Mechanik der lebenden Korper, Leipzig, 1906, 

 pp. X, 372. 



