THE SKELETAL SYSTEM 



241 



Evolution of the Free Extremities. Two contrasting types of free 

 extremity appear in vertebrates, the fins characteristic of fishes and the 

 toed appendages such as are found in the remaining classes from amphibia 

 to man. The conversion of the one into the other continues to be a vexed 



VERTEBRAL COLUMN 



Fig. 198. — The presence of rudimentary hind-limbs in the Python is evidence that 

 snakes have evolved from limbed ancestors. The presence of thoracic and lumbar 

 plexuses of spinal nerves points in the same direction. A, Ventral external aspect of 

 anal region of python. B, Skeleton in pelvic region, showing rudimentary femur and 

 ilium. (Redrawn after Romanes.) 



question of vertebrate morphology. Technically stated, the problem 

 has been to determine how the evolution of the ichthyopterygium into the 

 cheiropterygium has occurred. Interest has centered especially in the 

 transformation of the skeleton. 



VERTEBRAL MARGINS 



STERNAL EXTREMITY 



Fig. 199. — Human pelvic and pectoral girdles in lateral aspect. A is the pelvic girdle 

 of the right side and B-C the pectoral girdle of the same side. 



Primarily the fish fin like that of the fossil shark Cladoselache was 

 supported by radial cartilages which articulated with basalia, of which 

 one or more articulated with the girdle. In the pectoral fin of modern 

 elasmobranchs three basalia, propterygium, mesopterygium, and meta- 

 pterygium, connect the fin with the girdle. Morphologists, however, 



