XVI] ON STRENGTH AND FLEXIBILITY 1009 



well-nigh at equal angles, of 120°, with one another, giving the 

 greatest and most uniform strength of which such a system is 

 capable. On the other hand, in a flat fish, such as a plaice, where 

 from the natural mode of progression it is necessary that the back- 

 bone should be flexible in one direction while stiffened in another, 

 we find the whole outline of the fish comparable to that of a double 

 bowstring girder, the compression-member being (as usual) the 

 backbone itself, the tension-member on either side being constituted 

 by the interspinous ligaments and muscles, while the web or filUng 

 is very beautifully represented by the long and evenly graded neural 

 and haema) spines, which spring symmetrically up and down from 

 each individual vertebra. 



In the skeleton of the flat fishes, the web of the otherwise perfect 

 parabolic girder has to be cut away and encroached on to make room' 

 for the viscera. When the body is long and the vertebrae many, 

 as in the sole, the space required is small compared with the length 

 of the girder, and the strength of the latter is not much impaired. 

 In the shorter, rounder kinds with fewer vertebrae, like the turbot, 

 the visceral cavity is large compared with the length of the fish, 

 and its presence would seem to weaken the girder very seriously. 

 But Nature repairs the breach by framing in the hinder part of the 

 space with a strong curved bracket or angle-iron, which takes the 

 place very efl5.ciently of the bony struts which have been cut away. 



The main result at which we have now arrived, in regard to the 

 construction of the vertebral column and its associated parts, is 

 that we may look upon it as a certain type of girder, whose depth 

 is everywhere very nearly proportional to the height of the corre- 

 sponding ordinate in the diagram of moments: just as it is in a girder 

 designed by a modern engineer. In short, after the nineteenth or 

 twentieth century engineer has done his best in framing the design 

 of a big cantilever, he may find that some of his best ideas had, so 

 to speak, been anticipated ages again the fabric of the great saurians 

 and the larger mammals. 



But it is possible that the modern engineer might 4)e disposed to 

 criticise the skeleton girder at two or three points ; and in particular 

 he might think the girder, as we see it for instance in Diplodocus or 

 Stegosaurus, not deep enough for carrying the animal's enormous 



