430 HARRIS HAWTHORNE WILDER ON 



The Appendicular Skeleton. 



Shoulder Girdle. 



Tlie shoulder girdle consists of a pair of thin plates, almost entirely cartilaginous, 

 wrapped about the sides of the body near the anterior end of the trunk, and entirely dis- 

 connected from other parts of the body skeleton and from one another. At about the 

 middle of each plate is situated the glenoid fossa for the reception of the head of the 

 humerus, .and from this as a center there radiate three processes or lobes, one extending 

 dorsally and two ventrally. The dorsal or scapular extension is narrow at its origin but 

 broadens out towards its free end into a hatchet-shaped piece which extends as far dor- 

 sally as the transverse processes of the vertebrae. The narrow part of this extension 

 becomes ossified to form a scapula, in shape something like the diaph^'sis of a shaft bone, 

 with a constricted middle portion and two broadened ends; beyond this ossification the 

 hatchet-shaped piece remains as a cartilaginous suprascapula. Of the two ventral exten- 

 sions, the anterior, or procoracoid, is long and narrow and directed nearly anteriorly, while 

 the posterior, or coracoid, forms an almost circular flat plate closely applied to the myo- 

 tomic muscles on the ventral side of the thoracic region and extends so far beyond the 

 median line that the coracoid of one side considerably overlaps the other, the left being 

 usually the ventral or superfici.al one. 



The cartilage, which is thin in most regions, is considerably thickened about the 

 glenoid fossa both to strengthen the region and to alloAv sufficient depth for the reception 

 of the head of the humerus. Externally, the thickened portion forms a definite ridge or 

 lip nearly surrounding the fossa, being deficient only for a small space upon the antero- 

 medial aspect. 



Midway between the glenoid fossa and the re-entrant angle formed between procor- 

 acoid and coracoid is seen a small foramen coracoideum through which passes the supra- 

 coracoid nerve on its way to supply the muscles upon the ventral surface of' the shoulder 

 girdle. C. K. Hoffmann has pointed out that in the Aiiura this nerve lies in the interval 

 between the procoracoid and coracoid, while here it bores through the cartilage, and that 

 the result is brought about in the former case by a deepening of the incision between the 

 two elements in question far enough to include the region of the foramen. 



When the shoulder gii'dle is in its proper relationship to the body, the coracoid 

 extends from the second to the fourth myocomma, and hence the largest of the sternebra, 

 the one connected with the fourth myocomma, is situated exactly at the point at which 

 the posterior margins of the two coracoids diverge from one another, a relation precisely 

 similar to that of the sternal plate of the higher Urodela and of the arciferous Anura (e. g., 



