THE LIMBS. 



301 



segment of the pelvic girdle becomes fixed to the costal processes • 

 the corresponding part of the scapula remains free. 



In the typical reptilian shoulder girdle, as in the pelvic 

 (Fig. 242), two elements are formed in the ventral part of 

 the arch — a posterior part — the coracoid, corresponding to the 

 ischium, and an anterior — the pre-coracoid, corresponding to 

 the pubes. Both elements reach the ventral median line 

 in which the presternum is developed. In Ornithorynehus 

 the coracoid element is represented by two bones — the cora- 

 coid and epi-coracoid. The coracoid helps to form the glenoid 

 cavity at its dorsal extremity ; its ventral articulates with 

 the presternum. In man and all higher mammals, in which 

 mobility of the fore limb is of advantage for speed and free move- 

 ment, the coracoid element is much reduced. It forms merely a 

 process on the scapula, which it joins in man about the 15th year. 

 It still enters into the formation of the glenoid cavity, the articular 

 part (supra-glenoid) having a separate centre of ossification which 

 appears in the 12 th year. It is possible that the costo-coracoid 

 ligament may be derived from the ventral part of the coracoid 



post. asp. manub. 



inter-clau. lig. (inter-clauicle) 

 ^represent pre-or epi-coracoid 



ytnemb. clauicle (primitive clau.) 

 ,cartilage 



acrom. 



coraco-clav. ligs. 

 coracoid 

 glenoid 



right scap. fr. behind 



Fig. 245.— The Parts in the Shoulder Girdle of a human foetus which correspond 

 with those of Ornithorynehus. 



element — the part which articulates with the sternum in the 

 duckbill. The pre-coracoid in the shoulder girdle of the lizard 

 corresponds to the pubic element in the pelvis. The pre-coracoid, 

 which, like all the primitive elements of the pelvic and shoulder 



