EXISTING ORGANISMS— ANATOMY 



79 



birds, and the broadening of the ilia in man to form the basin- 

 shaped pelvis correlated with his erect posture, the three bones 

 retain characteristic fundamental relations. 



The Pectoral Girdle. This girdle is further modified. The 

 scapula remains in Amphibia, surmounted by the cartilaginous 

 supra-scapula, as part of the girdle extending dorsad from the 

 anterior limb. The coracoid develops a pair of distinct bones, 

 corresponding to the ischia of the pelvic girdle, while anterior 

 cartilaginous strips, the procoracoids, separated from these bones 

 by the coracoid foramen, correspond to the pubes. Three mem- 

 brane bones, the clav- 

 icle, interclavicle and 

 cleithrum, are associ- 

 ated with the cartilag- 

 inous parts, but the 

 two last are rare above 

 the fishes (Fig. 54). 

 A median element, the 

 interclavicle, is some- 

 times present between 

 the ends of the clav- 

 icles, and epicoracoid 

 cartilages may join the 

 clavicles and coracoids. 



Fig. 54. — Diagram of the shoulder girdle of a 

 primitive reptile, showing the complete series of 

 elements found in the vertebrates above the 

 fishes. Dermal bones: CLTH, cleithrum; CL, 

 clavicle; IC, interclavicle; Cartilaginous ele- 

 ments: PC, procoracoid; C, coracoid; S, scapula. 

 (From Wilder's History of the Human Body, 

 with the permission of Mrs. H. H. Wilder and 

 Henry Holt and Company.) 



The coracoid and scapula at least take 

 part in the formation of the glenoid cavity, in which the fore 

 limb is articulated. The ventral parts are complicated by associa- 

 tion with the median epicoracoid cartilages and with the sternum. 

 Of these bones the dorsal scapula persists as a more or less blade- 

 like bone, the shoulder blade of man. All ventral parts are found 

 in reptiles and both coracoid and clavicle in birds, but only the 

 clavicle in man. In the primitive Prototheria all parts of the 

 primitive girdle are found, and in some of the higher mammals 

 the scapula is the only bone present (Fig. 55). 



The Pentadactyl Appendage. The appendages of terrestrial 

 forms, both pectoral and pelvic, are invariably attached to their 

 respective girdles by a single bone, the humerus of the anterior 

 limb and femur of the posterior. To these are articulated two 

 bones, the inner radius and tibia, and the outer ulna and fibula 

 of the anterior and posterior limbs respectively. These are fol- 

 lowed by a series of small bones, the carpals and tarsals, which 



