XVI] THE STRUCTURE OF BONE 975 



The Forth Bridge, from which the anatomist may learn many 

 a lesson, is built of tubes, which correspond even in detail to the 

 structure of a cylindrical branch or stem. The main diagonal struts 

 are tubes twelve feet in diameter, and withm the wall of each of 

 these lie six T-shaped "stiffeners," corresponding precisely to the 

 fibro- vascular bundles of Fig. 461 ; in the same great tubular struts 

 the tendency to "buckle" is resisted, just as m the jointed stem of 

 a bamboo, by "stiffening rings," or perforated diaphragms set 

 twenty feet apart within the tube. We may draw one more curious, 

 albeit parenthetic, comparison. An engineering construction, no 

 less than the skeleton of plant or animal, has to grow; but the 

 living thing is in a sense complete during every phase of its existence 

 while the engineer is often hard put to it to ensure sufficient strength 

 in his unfinished and imperfect structure. The young twig stands 

 more upright than the old, and between winter and summer the 

 weight of leafage affects all the curving outlines of the tree. A 

 slight upward curvature, a matter of a few inches, was deliberately 

 given to the great diagonal tubes of the bridge during their piecemeal 

 construction; and it was a triumph of engineering foresight to see 

 how, like the twig, as length and weight increased, they at last came 

 straight and true. 



Let us now come, at last, to the mechanical structure of bone, 

 of which we find a well-known and classical illustration in the 

 various bones of the human leg. In the case of the tibia, the bone 

 is somewhat widened out aboVe, and its hollow shaft is capped by 

 an almost flattened roof, on which the weight of the body directly 

 rests. It is obvious that, under these circumstances, the engineer 

 would find it necessary to devise means for supporting this flat roof, 

 and for distributing the vertical pressures which impinge upon it 

 to the cylindrical walls of the shaft. 



In the long wing-bones of a bird the hollow of the bone is empty, 

 save for a thin layer of living tissue lining the cylinder of bone; 

 but in our own bones, and all weight-carrying bones in general, the 

 hollow space is filled with marrow, blood-vessels and other tissues; 

 and amidst these living tissues lies a fine lattice-work of little 

 interlaced "trabeculae" of bone, forming the so-called "cancellous 

 tissue." The older anatomists were content to describe this can- 



