148 THE INTERNAL ENVIRONMENT OF THE BODY Part III 



three bones and the front and hind limbs likewise have three main bones 

 (Fig. 9.13). The same number and arrangement of bones occur typically in 

 the forefoot (or hand) as in the hind foot. Both are correlated with thevir uses 

 in swimming, running, flying, climbing, and burrowing. The feet of horses have 

 undergone striking modifications for running. In their wild state horses have 

 grazed over wide ranges of grassland and escaped their enemies by speed. 

 Their bodies are held relatively high by long slender legs. Through their evo- 

 lutionary history their toes have been reduced to one, the third, on each foot 

 and in readiness for flight they stand upon their hoofs, the nails of these single 

 toes (Fig. 9.12, 9.14). 



The Human Skeleton 



The human skeleton has no bones which are not represented by similar ones 

 in skeletons of other mammals. Nevertheless it has certain entirely unique 

 features: a round head, a chin, a broad chest, a triply curved backbone, and 

 most important, a bowl-shaped pelvis and an opposable thumb that fronts the 

 fingers (Fig. 9.15). 



Backbone. The 34 vertebrae of the human backbone are arranged like the 

 stones in a tower with the smallest cervicals (7) at the top, next the stronger 

 thoracic ones (12) jointed to the ribs, then the heavy lumbars (5). Beyond 

 this broad base of the tower are the fused pelvic vertebrae (5) forming the 

 sacrum which helps support the weight of the body and, finally, there is the 

 coccyx, the fused vertebrae (5) which are the remnants of the tail. 



A baby is born with a nearly straight backbone which gradually assumes its 



Clavicle 



CorQcoid 



Scapulo 



Humerus 



Femur 



Puhis 



Radius 



UIno 



Corpols — /o^^c 

 Metacorpals 

 Phalanges 



Ischium 



-- Fibula 



Torsals —f^^^ 



Metatarsals — / 

 Phalanges / 



Fig. 9.13. Diagrams showing the basic patterns of the girdles and appendages 

 of vertebrates and the similarity of arrangements in the fore and hind limbs. A, 

 forelimb and pectoral girdle; B, hindlimb and pelvic girdle. 



