PECTORAL GIRDLE MAMMALS 



259 



that the spine may be nearly median, separating two fossae for muscles. 

 Many Edentates (fig. 272), Insectivores and Carnivores have a second 

 crest on the same surface. In swine the line between scapula and 

 suprascapula is pierced by foramina for the dorsal rami of spinal 

 nerves. The coracoid process is a small projection on the medial side 

 of the glenoid fossa and in the adult is fused with the scapula, as is 

 the coracoid in many lower Vertebrates. Myrmecophaga has a bar 

 from the process to the anterior margin of the scapula, enclosing a 

 foramen. 



The coracoid process arises from two centres in many mammals (Edentates, 

 Ungulates, Rodents, Sirenia, Carnivores and Primates) and for a time after birth 

 the two coracoidal bones retain 

 their individuality (fig. 276), the 

 anterior being excluded from the 

 glenoid fossa. Reference to these 

 two elements is made on page 245. 



The extent of develop- 

 ment of the clavicle is related 

 to the amount of freedom of 

 motion of the fore hmb, and 

 is greatest where there is a 

 variety of motion as in bats, chmbing mammals and man. There is 

 a tendency towards loss where only a pendulum motion occurs as 

 in walking or swimming fUngulates, whales, Sirenia and individuals 

 of other orders). Between full development and complete loss 

 are varying conditions (armadillos, Carnivores, etc.). When best 

 developed the clavicle forms a brace between shoulder blade and 

 sternum. Moles have a cuboidal clavicle, described as partly 

 cartilage, partly membrane in origin, and similar origin is stated 

 for other genera, so that in these it may be a composite bone as in 

 Anura, or it may be that the cartilage has no such morphological 

 significance. 



A pair of cartilages may intervene between the clavicles and the anterior end 

 of the sternum, often ossifying as separate bones which have been called remnants 

 of precoracoids, presterna, preclavia and omosternum, a variety of names indicat- 

 ing the uncertainty as to their morphology. 



Insectpvora have a long and narrow scapula, with spine and acromion 

 Httle developed in moles, the acromion bifurcate in shrews, one ramus articu- 

 lating with the clavicle. Other Insectivores have a more typical scapula. The 

 clavicle (possibly a compound bone) is usually present and in Talpids articulates 



Fig. 276. — Proximal end of scapula {s) and 

 coracoidal bones of 7 weeks Lepus (A), and (B) 

 young Brady pus (Howes, '93). c', anterior 

 coracoid, forming greater part of human cora- 

 coid; C-, posterior coracoid (metacoracoid) ; /, 

 coraco-scapular fenestra; I, ligament. 



