The Skeletal System. 



6i 



are considered individually. The corresponding portions may he 

 identified (Fig. 32), though in mammals the ventral portion in the 

 case of the pectoral girdle is greatly reduced. Of the. ventral 

 elements, those commonlypresent in vertebrates are the coracoid, 



GROUND PATTERN '^^'^'^^ ^"'"'^^ ^^^ posterior ray, and the 



IN TERRESTRIAL procoracoid, the anterior ray, the latter 



VERTEBRATES. being partly covered in front by a 



derm splint, the clavicle. This condition 

 though not characteristic of mammals generally is still found in 

 monotremes, and rudiments of the coracoid extension ventrally are 



If ii i ' 



ff « *' 



Fig. 33. Homologies of the mammalian limb. A, fore foot, rabbit 

 B, tore foot, horse. C, human hand, r, radius; u, ulna; I-V, metacarpa 

 bones. \ 



identifiable in embryonic marsupials: Adult marsupials and 

 placentals show only a small hook-like coracoid process, together 

 with the clavicle in perfect or less perfect development. 



There is no more striking feature of homology than that shown 

 by the free extremities in respect of the different forms of vertebrates. 

 This is true homology because it concerns the resemblances, part 

 for part, in the anterior or in the posterior limb of any one verte- 

 brate as compared with the corresponding elements in the same 

 position in other forms. The froat limb of the rabbit (Fig. 33) is 

 slightly elongated, and semi-digitigrade, making it a more efficient 



