procoracoid 



coracoid 



scapula 



procoracoid 



coracoid 



scapula 



THE ENDOSKELETON: GIRDLES, THE STERNUM, AND APPENDAGES 87 



situated along the dorsal border of the scapula and sometimes partially ossified, the epicora- 

 coids, cartilages anterior or medial to the coracoid (procoracoid) element of reptilian and mam- 

 malian pectoral girdles; and cartilages between the medial ends of the coracoids (also but 

 confusingly named epicoracoids in texts, and in this manual left unnamed). 



The pectoral girdle is still further complicated by the addition of membrane bones to the 

 cartilage bones and cartilages described above. As previously stated , such membrane bones are 

 homologous with the dermal plates of the exoskeleton 

 and have secondarily become associated with the 

 endoskeleton. It was stated in the introduction 

 to the endoskeleton that such membrane bones 

 occur in relation to certain parts of the endo- 

 skeleton, and we meet them here for the first 

 time, in association with the pectoral girdle. In 

 teleosts there are a number of such dermal 

 elements added to the pectoral girdle, as shown in 

 Figure 30; they form a sort of ventral incase- 

 ment of the girdle. Of these there persist in the 

 most primitive land vertebrates (extinct) five 

 elements, a median single inter clavicle, paired 

 lateral clavicles, and lateral to the clavicles paired 

 cleithra (singular, clei thrum) (see Fig. 29^). The 

 interclavicle (also called episternum) is situated 

 immediately ventral to the sternum, the clavicles 

 invest the procoracoids which are often concealed 

 within or dorsal to them, while the cleithra are 

 situated along the borders of the scapulae. The 

 cleithra are lost early in the evolution of land 

 vertebrates and do not occur in living forms, but 

 the interclavicles and clavicles are characteristic 

 of many reptiles, of birds, and of monotremes. In 

 placental mammals the interclavicle is missing, 

 and in some of them the clavicles are also reduced 

 or absent. There has thus occurred in the evolu- 

 tion of the pectoral girdle a gradual reduction in 

 the number of membrane bones, as will be seen by 

 reference to Figure 29. 



The pectoral girdle thus consists of a complex 

 of cartilage and membrane bones. The cartilage 

 bones may articulate ventrally with the sternum 

 but rarely have any articulation or fusion with 

 the vertebral column (exceptions: skates, ptero- 

 saurs). In this respect the pectoral girdle stands 



in marked contrast to the pelvic girdle, which, as we have seen, always has in land verte- 

 brates a firm articulation with the sacrum. In teleosts the membrane bones of the girdle 

 may articulate with the skull. The cartilage bones of the girdle always, and the membrane 

 bones never, take part in the articulation with the fore limb. This is one of the simplest ways 

 of distinguishing the two kinds of components in the girdle. 



The skeleton of the fore limb is composed of parts similar to those already described for the 

 hind limb. There is a proximal segment the upper arm or brachium, consisting of a single 



procoracoid 



coracoid 



scapula 



FIG. 28. Diagrams to illustrate two 

 theories of the homology of the cartilage bones 

 of the pectoral girdle of amniotes. A, carti- 

 lage bones of the primitive pectoral girdle; 

 there are three bones in each half of the 

 girdle, a condition found only in extinct 

 forms. B, one theory of the composition of 

 the pectoral girdle of present amniotes, show- 

 ing disappearance of the procoracoid and 

 retention of the coracoid. C, alternative 

 theory, showing retention of the procoracoid 

 (called, however, coracoid) and disappearance 

 of the coracoid. 



