36 



MANUAL OF ELEMENTARY ZOOLOGY 



The shoulder girdle or pectoral arch is a flat structure of 

 cartilage and bone embedded in the body-wall 

 of the forepart of the trunk, which it almost 

 encircles. It consists of two similar halves, one on each 

 side of the body, united below but separate above, where 

 they are bound by muscles to the backbone. Each half is 

 composed of an upper scapular portion or shoulder blade 

 and a lower coracoid portion. The uppermost part is a 

 broad, flat plate lying on the back known as the supra- 

 scapula. A great part of this consists of cartilage stiffened 

 by calcareous matter, but it has a narrow rim of plain 

 cartilage and a core of true bone 1 lies at its outer end, where 

 it joins the scapula, a narrower but stouter bone lying at 

 the side of the body. A forward projection from this 

 bone is known as the acromion process. To the lower end 

 of the scapula is attached the coracoid portion of the 



girdle. This is a plate of 

 cartilage and bone lying on 

 the under side of the body in 

 the breast region and pierced 

 by a wide oval space called 

 the coracoid fontanelle. Be- 

 hind the fontanelle lies the 

 stout coracoid bone; in front 

 is a narrow strip of calcified 

 cartilage, the precoracoid, con- 

 tinuous with another strip 

 known as the epicoracoid 

 which forms the inner border 

 of its half of the girdle and 

 lies against its fellow in the 

 middle line. This junction of the two halves of the girdle 

 is known as its symphysis. The only membrane bones in 

 the girdle are the clavicles. They are a pair of slender 

 structures which overlie the precoracoid cartilages. Each 

 sends forward a prolongation beside the acromion process. 

 At the junction of the scapula and coracoid bones is the 



1 The structure of bone has already been alluded to. It will be 

 more fully described together with that of cartilage in a later chapter. 

 Bone diners from cartilage not in the mere presence of calcareous 

 matter, but in structure and composition. 



Fig. 16. — The hyoid apparatus 

 of a frog. 



a.c, Anterior cornua ; b. t body ; 

 p.c, posterior cornua. 



