XVI] THE SKELETON AS A BRIDGE 695 



a good deal more than what is carried on his hind feet, say about 

 three-fifths of the whole weight of the animal. 



The great (or anterior) cantilever then, in the horse, is con- 

 stituted by the heavy head and still heavier neck on one side of 

 the pier which is represented by the fore-legs, and by the dorsal 

 vertebrae carrying a large part of the weight of the trunk upon 

 the other side; and this weight is so balanced over the fore-legs 

 that the cantilever, while "anchored" to -the other parts of the 

 structure, transmits but little of its weight to the hind-legs, and 

 the amount so transmitted will vary with the position of the 

 head and with the position of any artificial load*. Under certain 

 conditions, as when the head is thrust well forward, it is evident 

 that the hind-legs will be actually relieved of a portion of the 

 comparatively small load which is their normal share. 



Our problem now is to discover, in a rough and approximate 

 way, some of the structural details which the balanced load upon 

 the double cantilever will impress upon the fabric. 



Working by the methods of graphic statics, the engineer's 

 task is, in theory, one of great simplicity. He begins by drawing 

 in outline the structure which he desires to erect; he calculates 

 the stresses and bending-moments necessitated by the dimensions 

 and load on the structure ; he draws a new diagram representing 

 these forces, and he designs and builds his fabric on the lines of this 

 statical diagram. He does, in short, precisely what we have seen 

 nature doing in the case of the bone. For if we had begun, as 

 it were, by blocking out the femur roughly, and considering its 

 position and dimensions, its means of support and the load which 

 it has to bear, we could have proceeded at once to draw the system 

 of stress-lines which must occupy the field of force: and to 

 precisely these stress-lines has nature kept in the building of the 

 bone, down to the minute arrangement of its trabeculae. 



The essential function of a bridge is to stretch across a certain 

 span, and carry a certain definite load ; and this being so, the 



* When the jockey crouches over the neck of his rac'e-horse, and when Tod 

 Sloan introduced the "American seat," the object m both cases is to reUeve the 

 hind-legs of weight, and so leave them free for the work of propulsion. Never- 

 theless, we must not exaggerate the share taken by the hind-limbs in this latter 

 duty; cf. Stillman, The Horse in Motion, p. 69, 188?. 



