146 



THE INTERNAL ENVIRONMENT OF THE BODY 



Part III 



The intimacy between bones and muscles is evident in the ridges and 

 grooves on the surface of dry bones, for these are special attachment places of 

 tendons. The latter are also attached to the large smooth areas of the pelvic 

 bones and shoulder blades (Fig. 9.16). 



Main Divisions of the Vertebrate Skeleton. All vertebrate skeletons consist 

 of two basic divisions: the axial skeleton, composed of skull and vertebral 

 column, and the appendicular skeleton, the shoulder and hip girdles and their 

 appendages (Fig. 9.15). Their parts correspond in relative position and 

 structure; they can be homologized more or less completely in all vertebrates. 



The Vertebrate Plan 



Early History. The lobe-fin fishes, probably ancestors of the land verte- 

 brates, must have tugged their bodies across oozing mud from one pool to 

 another, pulling with their front fins and pushing with the hind ones. Untold 

 generations later, their successors also pulled and pushed their bodies but by 

 limbs that bent at the joints and had small spreading bones at the ends that 

 got a foothold upon the earth (Fig. 9.11). After many more generations, the 

 limbs were held closer to the body and bent, the front ones backward and the 

 hind ones forward. In all the four-limbed vertebrates that have succeeded 

 them from early times into the present, the elbow, meeting place of the 

 humerus with the radius and ulna, has pointed backward, and the knee, the 



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Fig. 9.11. Diagrams illustrating the evolution of the limbs of the ancestors of 

 land vertebrates. A, front view of a probable early stage when the limbs projected 

 side wise and the body rested on the ground, an era when land vertebrates tugged 

 their bodies out of the water and through the muddy ooze. B, the body is lifted 

 from the ground and the limbs are bent outward at the knee joints. C, side view, 

 hypothetical condition; hind leg rotated so that the knee points forward; front leg 

 rotated backward so that the elbow points backward. D, side view, condition in 

 modern quadrupeds in which the radius crosses over the ulna when the forearm 

 rotates forward. E, front view of stage shown in D. (After DeBeer. Courtesy, 

 Walter and Sayles: Biology of the Vertebrates, ed. 3. New York, The Macmillan 

 Co., 1949.) 



