Chap. 9 PROTECTION, SUPPORT, AND MOVEMENT SKELETONS 147 



meeting joint of the femur with the tibia and fibula, has pointed forward 

 (Figs. 9.11, 9.15). 



The Bridge. The plan of the vertebrate body is like the layout of a single- 

 span bridge. The piers of the bridge are the front and hind limbs attached to 

 their respective girdles and the arched span is the backbone. This metaphor 

 drawn by D'Arcy Thompson has been developed effectively by W. K. Gregory 

 in The Bridge that Walks with photographs of skeletons of fossil and present- 

 day vertebrates that illustrate the theme (Fig. 9.12). In its long history the 

 bridge plan of the vertebrate skeleton has admitted hundreds of variations 

 without departing from its unique character and basic simplicity. It persists 

 under many guises and ways of making a living, in burrowing ground moles, 

 swimming muskrats, and climbing squirrels, in elephants that are sure-footed 

 and ponderous, deer that are light and agile, cats that hunt their prey, and 

 cattle that forage on grass. 



Paired Appendages and Locomotion. Paired appendages attached to carti- 

 laginous or bony girdles are typical of vertebrates. The basic pattern of these 

 structures underlies great modifications, especially in amphibians and birds. 

 In this pattern the pelvic or hip girdle is attached directly to the axial skeleton, 

 the pectoral or shoulder girdle indirectly by muscles. Each girdle is formed of 



Fig. 9.12. Skeletons of a giant Percheron horse and a Shetland pony, the latter 

 in grazing position. Both show the bridge-like plan of the vertebrate body, the 

 front and hind limbs and their girdles taking the place of supporting piers, and 

 the backbone that of a connecting span. The neck has been compared to the arm 

 of a steam shovel; in the pony the steam shovel is in action. (Skeleton mounted 

 by S. H. Chubb. Photograph, courtesy, American Museum of Natural History.) 



