THE APPENDICULAR SKELETON 



61 



chiefly for the strong development of the scapula as opposed to 

 the weakness of the ventral, pectoral portion. In the rabbit the 

 clavicle is vestigial, and in many mammals, such as the ungulates, 

 it is entirely absent. In man, how- 

 ever, as well as in other mammals 

 in which the fore limb is capable of 

 much movement transversely in- 

 stead of only anteroposteriorly, the 

 clavicle is well developed and is ar- 

 ticulated at its ends with the scap- 

 ula and sternum. In this condition 

 it acts as a prop upon which the 

 well-developed pectoral muscles of 

 adduction move the limb. The 

 absence of direct articulation with 

 the vertebral column and the at- 

 tachment thereto by muscles and 

 ligaments provide a shock-absorb- 

 ing apparatus which is of evident 

 value in a quadruped, like the rab- 

 bit, w^here locomotion is largely of 

 a leaping character with the fore 

 limbs reaching the ground first 

 after each leap. 



Both pectoral and pelvic gir- 

 dles in terrestrial vertebrates con- 

 form more or less closely to a tri- 

 radiate shape if the two halves 



are considered individually. The corresponding portions may be 

 identified (Fig. 34), though in mammals the ventral portion in the 

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

 elements, those commonly present in vertebrates are the coracoid, 

 which forms the posterior ray, and the procoracoid, the anterior 

 ray, the latter 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 cora- 

 coid extension ventrally are identifiable in embryonic marsupials. 

 Adult marsupials and placentals show only a small hook-like 



Fig. 34. Plan of the anterior limb 

 skeleton in walking vertebrates, the 

 equivalent elements of the posterior limb 

 indicated in brackets: si, scapula (ilium), 

 pp, procoracoid (pubis) ; ci, coracoid 

 ( ischium) ; hf, humerus (femur) ; rt', 

 radius (tibia); uf, ulna (fibula); rt, 

 radial carpal (tibial tarsal) ; i, inter- 

 medium ;.uf, ulnar carpal (fibular tarsal); 

 c,c, centrals; 1-5, di.stal carpals (tarsals); 

 m. metacarpals (metatarsals) ; ph, pha- 

 langes of the digits. 



