144 



COMPARATIVE ANATOMY 



process), which is ossified from a separate centre, apparently repre- 

 senting an epicoracoid, while the coracoid proper may be occa- 

 sionally indicated by a small centre of ossification on the glenoid 

 margin of the scapula. 



Thus the scapula becomes freer from the rest of the skeleton, 

 and it alone serves to support the extremity ; it becomes at the 

 same time greatly broadened, and gives rise on its outer side, 

 in connection with the highly differentiated muscles of the limb, 

 to a strong ridge (spina scapulae), which extends downwards to 

 form the so-called acromion. The distal end of the clavicle usually 



FIG. 103. PECTORAL ARCH AND STERNUM or Ornithorhynchu-s paradoxus. 



c 1 , c 2 , c 3 , first, second, and third ribs; d, clavicle; e.c, epicoracoid; es l and es 2 , 

 prosternum (episternum) ; m.c, coracoid (metacoracoid) ; m.s, manubrium 

 sterni ; sc, scapula ; st, sternebra. 



becomes connected with the acromion, its proximal end articulat- 

 ing with the anterior edge of the sternum. 



In those Mammals in which the fore-limbs are capable of very 

 varied and free movements (Lemurs, certain Marsupials, many 

 Rodents and Insectivores, Bats, and Primates) the clavicles are 

 strongly developed. 1 In others (e.g. Ungulates, Cetaceans, Carni- 

 vores, most Edentates, Rodents, Marsupials) they may be en- 

 tirely wanting or only vestigial, and in the latter case their rela- 

 tions to the scapula become altered. 



1 The clavicle is primarily independent of the coraco-scapular portion of the 

 pectoral arch. Its original dermal character is retained in Monotremes, but in 

 all other Mammals it is developed on a cartilaginous basis. 



