VERTEBRATES: BACKBONE AND BRAIN 83 



ception of the number of experiments which were 

 tried in different groups before the definite mode of 

 forming a bony vertebra was attained. At the same 

 time the skull was developing in a somewhat similar 

 manner. But the skull is far more complex in origin 

 and undergoes far more numerous and important 

 changes than the simpler vertebral column. Into its 

 history we have no time to enter. 



And what shall we say of bone itself as a mere ma- 

 terial or tissue, with its admirable lightness, compact- 

 ness, and flawlessness. And every bone in our body 

 is a triumph of engineering architecture. No engineer 

 could better recognize the direction of strain and 

 stress, and arrange his rods and columns, arches and 

 buttresses, to suitably meet them, than these problems 

 are solved in the long bone of our thigh. And they 

 must be lengthened while the child is leaping upon 

 them. An engineer is justly proud if he can rebuild 

 or lengthen a bridge without delaying the passage of 

 a single train. But what would he say if you asked 

 him to rebuild a locomotive, while it was running even 

 twenty miles an hour ? And yet a similar problem 

 had to be solved in our bodies. 



But the vertebral column is not perfected by fish. 

 The vertebrae with few exceptions are hollow in front 

 and behind, biconcave ; and between each two verte- 

 brae there is a large cavity still occupied by the noto- 

 chord. Thus these vertebrae join one another by their 

 edges, like two shallow wine-glasses placed rim to rim. 

 Only gradually is the notochord crowded out so that 

 the vertebrae join by their whole adjacent surfaces. 

 Even in highest forms, for the sake of mobility, they 

 are united by washer-like disks of cartilage. Biconcave 



