XVI] THE COMPARATIVE ANATOMY OP BRIDGES 691 



a part in relation to the whole body, analogous to that which a 

 little trabecula, or a little group of trabeculae, plays within the 

 bone itself: that is to say, in the normal distribution of forces 

 in the body, the bones tend to follow the lines of stress, and 

 especially the pressure-lines. To demonstrate this in a compre- 

 hensive way would doubtless be difficult ; for we should be dealing 

 with a framework of very great complexity, and should have to 

 take account of a great variety of conditions*. This framework 

 is complicated as we see it in the skeleton, where (as we have said) 

 it is only, or chiefly, the struts of the whole fabric which are 

 represented ; but to understand the mechanical structure in 

 detail, we should have to follow out the still more complex 

 arrangement of the ties, as represented by the muscles and 

 ligaments, and we should also require much detailed information 

 as to the weights of the various parts and as to the other forces 

 concerned. Without these latter data we can only treat the 

 question in a preliminary and imperfect way. But, to take once 

 again a small and simplified part of a big problem, let us think 

 of a quadruped (for instance, a horse) in a standing posture, and 

 see whether the methods and terminology of the engineer may not 

 help us, as they did in regard to the minute structure of the single 

 bone. 



Standing four-square upon its forelegs and hindlegs, with the 

 weight of the body suspended between, the quadruped at once 

 suggests to us the analogy of a bridge, carried by its two piers. 

 And if it occurs to us, as naturalists, that we never look at a 

 standing quadruped without contemplating a bridge, so, con- 

 versely, a similar idea has occurred to the engineer ; for Professor 

 Fidler, in this Treatise on Bridge-Construction, deals with the chief 

 descriptive part of his subject under the heading of "The Com- 

 parative Anatomy of Bridges." The designation is most just, for 

 in studying the various types of bridges we are studying a series 

 of well-planned skeletons "f ; and (at the cost of a little pedantry) 



* Oui' problem is analogous to Dr Thomas Young's problem of the best disposi- 

 tion of the timbers in a wooden ship {Phil. Trans. 1814, p. 303). He was not long 

 of finding that the forces which may act upon the fabric are very numerous and 

 very variable, and that the best mode of resisting them, or best structural arrange- 

 ment for ultimate strength, becomes an immensely complicated problem. 



j In like manner, Clerk Maxwell could not help employing the term "skeleton " 



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