THE SKELETAL SYSTEM 



i8i 



In mammals, the coracoid is reduced to a process fused to the scapula. 

 In man, in addition to the coracoid process, a remnant of the coracoid bone 

 survives in the coracoid ligament which extends from the coracoid process 

 to the sternum, and in which occasional pieces of cartilage are found as 

 rudiments of the coracoid. The clavicle has supplanted the precoracoid, 

 remnants of which, however, usually occur within the clavicle. See Fig. 

 170. 



The mammalian hip bone differs Httle from that of reptiles. The 

 number of sacral vertebrae to which the coxal bone is attached increases in 

 mammals. In man there are five sacral vertebrae, to three of which the 

 hip bone is attached. 



Evolution of the Free Extremities. Two contrasting types of free 

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



VLRTEBRAL MARGIt 



5-STERNAL EXTREMITY 



Fig. 171. — 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. 



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

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

 cjuestion 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. 



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 (Fig. 172^), connect the fin with the girdle. Morphologists, 

 however, disagree as to the skeleton of the primitive extremity, the 

 archipterygium. While some suppose it to have been uniserial, i.e., the 

 radial cartilages were limited to one side of the basal cartilages or axis as 



